March 6, 2019 • 43 mins
How do car brakes work? From the earliest wooden block brakes to anti-lock brake systems, we learn about the science of coming to a stop.
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Episode Transcript
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Speaker 1 (00:04):
Get in touch with technology with tech Stuff from how
stuff works dot com. Hey there, and welcome to tech Stuff.
I'm your host, Jonathan Strickland. I'm an executive producer with
How Stuff Works, and I heart radio and I love
all things tech. And you know, we've been going pretty
fast with tech stuff over the past I don't know, decade,
(00:27):
So I felt maybe it was time to apply the
brakes a little bit. I don't mean to slow down
the show or stop it, but rather to take time
to talk about how breaks work and the science surrounding breaks,
because puns are sort of a thing I do, but seriously,
I thought this would give us a chance to talk
(00:47):
about mechanical and hydraulic systems as well as the science
that's behind breaking, the physics that are involved. So first,
let's talk about what makes breaks no necessary in the
first place, which is pretty obvious stuff, but it leads
into a discussion about physical forces that guided the whole
evolution of breaks. So, a body in motion has momentum.
(01:14):
Momentum is a quantity of movement. It depends upon two things,
the mass of the object that's in motion and how
quickly that mass is moving. So really comes down to
how much of the stuff is there and how fast
is it going. A simple equation would be momentum equals
mass times velocity. So if you have something with a
(01:38):
low mass, like a bullet, but it's traveling at a
high velocity, it has a pretty good amount of momentum.
If you have something that's really really huge, like a glacier,
but it's moving incredibly slowly, it still has a lot
of momentum, because momentum depends upon both the mass and
the velocity, not just one or the other. Now, if
(01:59):
you have something that's of a pretty decent size, like
a vehicle, and it's moving at a pretty good speed,
then that has got a lot of momentum too. And
that's a problem, right If you've got a mass that's
moving at a pretty good speed and you need to
stop that mass, you have to figure out how do
you offset that momentum, How do you convert this movement
(02:22):
energy into something else. So momentum is also a vector
quantity vectors and physics have not just a magnitude but
also a direction. Acceleration is the same. It has a
magnitude and a direction. So to fully describe momentum, we
need not just how many kilograms of mass are traveling
(02:43):
at a speed typically in meters per second, but also
the direction of travel. Now, a body and motion has
kinetic energy as well. So to reduce momentum and stop
a body in motion, you need to convert that kinetic energy,
that energy of movement in to something else. Because we
have to remember the laws of conservation, the laws of
(03:06):
the universe. We we energy cannot be created, it cannot
be destroyed. We cannot just make energy, we cannot destroy energy.
We can convert it from one form into another. So
breaks do this by converting the kinetic energy of an
object in motion into heat through friction. And you've probably
(03:26):
heard the term waste heat. Now what we mean by
that is that we've got some sort of system. It
can be pretty much any given mechanical system in particular,
but really other systems as well, biological systems. Uh So,
it could be pretty much anything. And in this system,
some of the energy that we're depending on is going
(03:47):
not to whatever it is we're trying to do, but
rather into generating heat. So with a bicycle, some of
the energy we're putting forth by peddling is not going
directly to making us move we're losing it in the
form of friction, and thus heat um really the heat
that's generated from friction. That's the best way of putting it.
(04:08):
The heat represents energy that we could not otherwise exploit
for whatever it is we're trying to do. So here's
another example, with an electrical generator. We might say that
the friction of the moving parts inside that electrical generator
means that some of the kinetic energy we would otherwise
use to create more electricity is instead converting to heat,
(04:28):
and that heat isn't being captured in any meaningful way.
So the work we did two direct this kinetic energy
to the generator was wasted. Some of that work we
weren't able to get efficiency. So no system is perfect.
Any system with moving parts is going to have friction
to deal with. And there are a lot of super
(04:50):
smart materials scientists out there who have worked really hard
to develop stuff that generates very little friction, and that
is in an effort to increase efficiency and minim eyes
waist heat. But when it comes to breaks, we want
that friction. That's what is stopping an object. So we
need something that is really good at creating this friction.
(05:11):
We want to convert that kinetic energy into heat and
thus decrease the momentum of a fast moving massive object,
perhaps even bringing that object to a complete stop, not
just slowing it down, but stopping it. So in the
very early days of break systems, even before there were cars,
you were looking at a pretty simple device. And this
(05:33):
would be in the horse drawn carriage era. You've got
a carriage that's being pulled by horses. Even when the
horses start to slow down, you know you still have
the momentum of the actual carriage itself. You need to
slow down the carriage uh to to come to a stop.
So the break was typically a lever, and the long
end of the lever was extended up towards the driver.
(05:55):
That was the handle side, So the longside of the
lever is the handle. On the short end of the
ever was a wooden block. The wooden block was when
you pull, the lever, would make contact, typically with the
driver's side front wheel of the carriage to create that
source of friction and convert the kinetic energy of the
turning wheel into heat and thus slow and eventually stop
(06:18):
the carriage. A lever is one of the classic simple
machines it's a bar that rotates around the fulcrum, and
a fulcrum is just a fixed point. So the length
of the lever on either side of the fulcrum determines
the amount of effort or force exerted or acquired per side.
In a classic class one lever, lever makes certain types
(06:40):
of work, such as lifting, easier by reducing the workload
that is needed to affect a change to to create
that lift. There are three classes of levers, and I
just mentioned class one lever. That's the one where you've
got a force that's applied on one end of the lever,
you have a load the thing you're trying to move
(07:00):
or activate on the other end of the lever, and
the fulcrum is somewhere in between those two. If it's
in the center of the bar, then the fulcrum ends
up balancing loads on either side. If you put equal
loads on either side, it should just balance out, kind
of like you know, just a scale if you think
of it that way. So a see saw or a
(07:21):
teeter totter would be an example of a class one lever.
If two people weigh the same on a seesaw, it
balances out. But if you put a heavier person on
one end and a lighter person on the other. The
heavier person sinks down the lighter person goes up into
the air. But if the heavier person were to scoot
forward on the seesaw to decrease the space between the
(07:42):
heavier person and the fulcrumb, then you would eventually balance out.
And if the heavier person kept scooting forward to get
closer to the fulcrum, the lighter person would eventually sink
to the bottom and the heavier person would be up
in the air. So by decreasing the disc dins between
the load and the fulcrum, you can increase the the
(08:06):
lifting power essentially, or the really you're decreasing the amount
of force needed to lift that load, if you're really
thinking about it that way, you're creating a mechanical advantage.
But there are two other classes of levers. I should
probably cover those because some of them actually factor into
breaks as well later down the line. Class two levers
put the fulcrum on one end of a bar, so
(08:29):
instead of it being in the middle, somewhere the fulcrum
the fixed point is actually on one end of the bar.
The load is somewhere in the middle of the bar
and the force is at the far into the bar.
So a wheelbarrow is this type of lever, because these
can be kind of difficult to imagine otherwise. But if
you think about the wheel on a wheelbarrow is a fulcrum,
(08:49):
and you put a load of stuff into the wheelbarrow itself,
and then you lift the other side of the wheelbarrow
by the handles. So the closer the load is to
the fulcrum, the more the mechanical advantage you will have
uh and the less you will the less effort you'll
need to lift the handles for a given load. Or
you can think of it another way, the heavier the
(09:10):
load you will be able to manage as long as
it's closer to the fulcrum. A class three lever has
the fulcrum on one end, the load at the other end,
and the lifting force is in the middle. Now, these
leavers don't provide mechanical advantage, but they can increase the
speed at which a force moves a load. A baseball
bat would be an example of this, or even your
(09:32):
your own forearm would be an example of this. But
we're gonna leave the off from here because class three
leavers don't really factor into the discussion for breaking, So
the wooden block break is a Class one lever. On
one end is the wooden block, its position near the
carriage wheel. A little bit further up from the wooden
block is the fulcrum, the fixed point closer to the
(09:53):
wooden block than to the handle, and the other end
of the lever, the long end has the handle, so
the handle must travel a radar distance than the wooden block.
When you pull back on the brake um, you are
pulling the breakback much further than the wooden block has
to travel to make contact with the the carriage wheel.
But by increasing that distance, you reduce the amount of
(10:15):
work you need to do on the force end to
get a result on the load end. Uh. This is
important because if you're going pretty fast and you need
to slow down that wheel, you need to exert a
good deal of force to create enough friction and convert
that kinetic energy into heat. You couldn't easily do that
if you were, let's say, just holding a wooden block
(10:37):
and just trying to push the wooden block directly against
the carriage wheel. You probably wouldn't be able to do
that with enough force. To make a difference in a
sufficient amount of time. By putting it on this lever,
you can increase the amount of force you're exerting on
the wheel itself on the carriage wheel through pressure with
this wooden block, then you would if you were to
(10:58):
do it directly. So the lever makes the job easier.
Very important part of physics in general. The brake system
made the transition from horse drawn carriages to some of
the earliest automobiles in the nineteenth century, and it was
the same sort of thing. They were using wooden blocks.
The wheels on these early automobiles were often metal. There
(11:20):
was no rubber tires yet, and they worked reasonably well
under certain conditions, namely that the vehicles were traveling at
slower speeds were talking below twenty miles per hour or
below thirty two kilometers per hour, and traffic was pretty
light in those days, so there weren't a lot of
incidents where you would need to break quickly due to
(11:42):
an increased amount of traffic on the streets. But you
also weren't necessarily uh, it wasn't necessarily a good idea
to go, like driving a car up a steep hill,
for example, because your breaking system was pretty primitive. The
only way you would prevent yourself from rolling backwards was
either by applying more acceleration to overcome the force of
(12:05):
gravity that's pulling you down, or to hold a break
real strong against that wheel so that you're not slipping backward. Uh.
This also was probably a good thing. Like that, It's
probably a good thing you weren't going very fast, not
just because the brakes were primitive, but because those wheels
were metal. You would feel it when you would run
(12:27):
over bumps in the road, of which there were many.
It was very similar to if you remember my episodes
about the history of the bicycle, the bone shaker, that
it was a nickname for a type of early bicycle
that was really uncomfortable to ride. It was a very uh,
stiff ride, you might say. But it was clear the
(12:48):
lever approach wasn't going to remain sufficient as cars were
getting faster and heavier and traffic was increasing. You could
stop a faster car with a lever breake, but it
would take longer as you converted that kinetic energy into
heat through friction, so you didn't come to a stop
in as reasonable amount of space. You know, and an
(13:11):
increase in traffic, you had a decreased the email of
time you had in order to slow down to avoid collisions.
So something had to change. A couple of people came
up with some very clever alternatives. One was a proposal
that was made. It was actually not just proposed, it
was developed. It was the creation of Elmer Ambrose Sperry
(13:33):
of Cleveland, Ohio. Sperry's solution involved using disc brakes. Now
I'm going to cover modern disc brakes a little later
in this episode, but it's good to just go ahead
and then explain what the general idea was because it
remained the same even later on. So let's say you've
got a wheel, and you've got that hub of the wheel.
This is part that turns on the axle. Attached to that,
(13:57):
you have a disk that is really just part of
the hub of the wheel, but it's separate from the tire.
So you've got this free standing disc. Positioned over this
disc is a set of calipers, so that either side
of the caliper are on either side of the disk,
and it can pinch down on the disk, just like
(14:18):
your fingers would pinch down if you had let's say
a spinning frisbee between two fingers and you rut uh.
You know, your friend is holding their two fingers together,
and the frisbee is spinning around on the axle of
those two fingers. If you brought your fingers like calipers,
right on the gadge and then pinched, you could stop
the frisbee and its spin. That's the same idea as
(14:41):
this set of disc brakes the the In a sense,
it's working very similar to the way the wooden block
that would press directly against the wheel itself works, except
instead of of hitting the wheel, you're hitting this disc
that's attached to the wheel. Um. Pretty clever. And Sperry
was working on an early electric vehicle. You may remember
(15:02):
in some of my previous episodes, I've talked about how
electric cars actually pre date internal combustion engine cars. The
earliest automobiles were electric vehicles, not not gasoline powered vehicles.
So he's working on this electric car, and he was
actually using electro magnetism to close those calipers, to shut them,
(15:23):
to attract the pincher to the disk and have it
clamped tight enough to start to break the wheel. Uh.
Springs that were attached to the calipers would create the
force that would allow the calipers to open up again
once the electro magnetic field went away. So if you
were to step on a brake in Sperry's design, you
(15:45):
would cause a current to flow through the braking system,
thus generating the electromagnetic field and forcing the calipers closed
and breaking the disk, breaking b R A K I,
n G. The disk. Letting the foot pedal brake would
interrupt the current, so the field would dissipate, the springs
(16:05):
on the caliper would pull the caliper open again. It
was a very ingenious creation, but Sperry's invention didn't catch
on in the States. A similar idea would take off
in Europe, however, and I'll talk about a different breaking
system that would see early success in the United States,
and that early success would extend all the way into
the modern day. But I'll do that in just a second. First,
(16:29):
let's take a quick break. So right around the same
time Sperry was working on the disc brake solution, Gottlieb
Daimler was developing a different approach called the drum brake.
(16:49):
Daimler's idea was to attach a drum mounted on an
axle and to wrap a cable around that drum and
pulling the brake would create tension on the cable, tightening
it around the drum and creating the friction needed to
slow and then stop a vehicle. So if you can
think of maybe a spinning axle and then wrapping a
(17:10):
belt around it and then pulling the belt, really taught
so that that creates that friction and then slowing the
axle down. It's similar to that. So you have this
dedicated drum for this purpose. UH. This was an idea
that will Helm Maybach used in nineteen o one in
some early Mercedes designs, but it was Louis Renault who
(17:31):
defined the drum break as a standard in vehicles. It
was Renault's UH design that really took off. That being said,
there are also a ton of different mechanics and engineers
working on this problem, and so the idea may have
been developing in parallel around the world. Renault gets the
credit for making this the first really practical drum break,
(17:54):
but lots of people were working on this because the
automobile was a rising technology at the time. Now, the
drum brake is a pretty clever invention, and a modernized
version is still used in many vehicles today, though typically
only for the rear wheels for most passenger vehicles. It
was also the dominant form of breaking systems in the
(18:15):
United States until the nineteen seventies, though in Europe things
were a little different. The original drum brakes weren't great
on flat surfaces. These drum brakes that had a a
a belt or a strip of metal or something wrapped
around a drum that would then tighten to slow things down,
(18:36):
those worked fine on flat surfaces. A break race in
nineteen o two pitted a horse drawn coach that had
a lever style break against a Victoria horseless carriage that
had an internal drum brake, and a custom vehicle that
was created by a guy named Ransom E. Olds And
he called it the Oldsmobile. Yep, that's where that comes from. So,
(19:01):
like I said, the coach had a tire break, the
horseless carriage had an internal drum break, which I'll talk
about in a second, and the Oldsmobile had a different
drum brake design that used a band of stainless steel
wrapped around the drum. So pressing on the brake pedal
rather than pulling a lever would contract this band. So
that would grip the drum more tightly and thus create
(19:23):
the friction. The oldsmobile proved it could stop in less
time and thus travel the least distance while breaking than
the other two vehicles. So this meant that for a while,
external breaks, in which the breaking mechanism is on the
outside of the drum, were more popular, but both external
and internal drum breaking systems existed at the same time.
(19:45):
So what was an internal drum break system. Well, in
that case, you have the drum assembly that's mounted on
a wheel or an axle, So just think of it's
almost like a pot, right, It's just mounted on there
and and everything is inside this pot. So you have
these extendable parts inside the drum. They're called shoes. And
(20:08):
these extendable shoes are anchored with respect to the car,
so they are not rotating. They are stationary with respect
to the chassis of the vehicle. So the drum rotates
around these shoes as as the cars in motion. The
shoes have breaking material, so a material meant to generate
(20:30):
friction coding the surface of the shoe itself the stopping surface.
So when you engage the brake, when you step on
the brake, pedal A system extends these shoes on the
inside of the drum, so that that breaking surface is
rubbing up against the inside edge of that rotating drum.
(20:52):
So it's the same effect that you were getting before,
this idea of pressing a surface against a moving object
in order to generate friction and convert kinetic energy into heat.
It's just in this case it's happening on the inside
of a rotating surface, not on the outside of the
rotating surface. Uh. The this is really interesting stuff. It's
(21:16):
a little difficult to envision just from my audio, I imagine,
but if you really want to look into this and
see some diagrams and some animations and things like that,
How Stuff Works has several articles about breaks, including how
drum brakes work, and that's incredibly useful. So if you
want to check in on a visual aid, I highly
(21:37):
recommend that I don't write for How Stuff Works anymore,
but I still respect the heck out all the articles
that are on that site. They are incredibly useful, especially
for stuff like this, to get an understanding of these
mechanical systems. So in addition, uh, these brakes would typically
have some sort of cable or belt wrapped around the drum.
So you would have a lot of cars that would
(21:59):
have kind of a high red system where they have
both external and internal breaking UH systems incorporated into the
same overall brake system and create more friction more more efficiently,
so that you could convert that kinetic energy into heat
and stop faster. Now, early on, the external drum brake
(22:19):
systems were believed to be superior. They could stop a
car faster and less time less distance than internal ones,
but they did have some big drawbacks. One of those
was that the brakes would tend to unwind if you
try to stop halfway up a hill. So you you're driving,
you got a hill. Let's say you're in San Francisco.
That's a great example. You're in San Francisco, you hit
(22:42):
a hill, you're driving up the hill, and then you're
coming up to an intersection where you have to stop. Well,
that was an issue because the brakes would unwind. At
that point. Gravity would create a backward pull on the
car to pull it back down the hill, and that
would cause the bands wrapped around the drum to unwind
(23:03):
in the car would actually start to roll backwards. For
that reason, early motorists would sometimes carry wedged blocks called chalks,
just like you would have for an airplane, and you
would actually there's footage of people who were driving cars
around that time who would jump out of their car
with these wooden blocks, run behind the cars it's starting
(23:24):
to roll backward, and try to wedge those blocks behind
the wheels so that the car wasn't rolling backward anymore.
The external brakes also had no protection against dirt and
other debris, so they would get dirty and they would
wear down relatively quickly, which would necessitate frequent maintenance or
replacement of those brakes. The internal drum braking systems were
(23:45):
protected from debris and dirt right because they're inside the drum,
so that stuff wasn't getting to them. Although as you
use the drum brakes over and over over again, they
do develop dust inside the drum itself because it's actually
wearing away both the brake pads and the inside of
(24:07):
the drum. You know that friction is slowly grinding away
some of that material. But also because of the design,
the shoes inside a drum could maintain pressure for as
long as the brake was engaged, which meant there was
no need to worry about rolling backward down the hill.
If you had the brake pressed, then you know it
wasn't unwinding. If it was an internal drum brake, the
(24:29):
brakes would last longer than external breakes, but they weren't
as effective at stopping the vehicle as quickly, so for
that reason, some car manufacturers chose to employ both types
of brakes on the rear wheels of vehicles, in particular,
so that both an external and internal drum brake system
could work together from the same press of a brake pedal.
In those early systems, everything was purely mechanical. We haven't
(24:53):
gotten to the hydraulic sections yet, so that meant that
it was using stuff like cables and rods and levers
to do mechanical work and to move these different elements
to where they needed to be. Eventually that gave way
to hydraulic systems, but will have a different type of
break to talk about first before we get into hydraulics.
(25:14):
So that means we need to get back to Sperry's
disc brakes. His implementation didn't get much traction, and yes
that's a pun, but others began to work on similar designs,
possibly with complete independence and without knowledge of Sperry's work, because,
like I said, a lot of people were working on
this at the same time. Uh. One of those was
(25:36):
an inventor named F. W. Lanchester in the UK who
received a patent in nineteen o two for a mechanical
approach to the disc brake design, so instead of the
electromagnetic approach, the calipers would be controlled by a cable.
The cable, when pulled taught, would force the calipers closed.
But lanchester solution wasn't ideal. The disc mounted to the
(26:00):
wheel hub was made of metal, which makes sense, but
the brake pads that were actually on the caliper were
made out of copper, So when you were applying a break,
that meant that you were applying to copper pads on
either side of a rapidly spinning metal disc. And as
you might imagine, this created no small amount of noise.
(26:20):
From what I understand, it was the type of high pitched,
screeching noise that was not that different from what you
might get with fingernails dragged down a chalkboard, so not
a pleasant sound. In addition, the brake pads would wear
down very quickly and had the same issues with dart
and debris that the external drum systems had, so lanchester
solution was, like Sperry's, largely unimplemented. A dude named Fruit
(26:46):
made the next big contribution. His name is Herbert Fruit.
He was an English engineer who took the brake pads
and lined them with a substance that would cut back
on that noise. It was a long lasting material that
could withstand a lot of abuse, and it would become
a common brake pad material for both disc brakes and
drum brakes, and still is used in a lot of
(27:08):
breaks today. That material was asbestos. Yikes, So asbestos was
long considered a truly remarkable substance with a ton of
practical applications. It's actually a group of silicate minerals, it's
not just one, and this group all have similar traits
(27:28):
their fibrous which means you can actually draw the stuff
out and create a material that's similar in consistency to
cotton balls in a way. Its heat resistant, it's an
electrical insulator, and it holds up against a lot of
otherwise corrosive chemicals, so it can make other stuff stronger
when you mix the substances together, and it was a
common additive for everything from cement to paper. But what
(27:52):
was not known for a very long time was that
asbestos is actually incredibly toxic. Now, as I said, it's fibrous,
and so they are these tiny asbestos fibers in the
mineral and these can easily be swallowed or inhaled without
your knowledge. Their microscopic in size, so you're not able
to spot them. And due to those qualities I mentioned earlier,
(28:15):
the fact that the fibers are so resistant to so
many different things, it also means that they can last
indefinitely inside a person's body and there's no real way
to flush them out. They don't dissolve, and these trapped
fibers can cause inflammation and even much worse problems genetic
damage to cells. Uh they can lead to development of cancer.
(28:38):
The illnesses take a really long time to develop, too,
like between twenty to fifty years, which is one of
the reasons asbestos remained in popular use for so long.
It took ages to figure out that it was hazardous
in the first place because the effects took so long
to manifest. Now, nearly all the auto manufacturers in the
(28:58):
United States stopped making asbestos brake pads in the ninety nineties,
but there are a lot of aftermarket companies that continue
to do so, largely manufacturing in places like China and
India and Mexico. So to this day, there are aftermarket
brake pads that have potentially dangerous amounts of asbestos in them. Now,
(29:19):
that probably doesn't pose a huge health hazard to the
average driver, but it is a concern for mechanics. For
people who work on break systems regularly, that's something that
they should actively be concerned about and protect themselves against.
Because there's no telling if someone's getting cheap break pads
(29:40):
from overseas, because you know there they are much less
expensive than buying them here Domestically, there's a chance that
one of the materials in there is asbestos and it
could be in a concentration high enough for it to
be a danger. So just to be aware if you
happen to be someone who works on such things, just
you know, you need to just be care full. We're
(30:01):
masks and stuff. Now. I've got a lot more to
say about breaks, including the introduction of hydraulics and modern
breaking systems, but first let's take another quick break. Now,
it didn't take long for engineers to look into hydraulics
(30:21):
to work with car brakes. Cars were getting heavier and faster,
which meant the brake systems needed to be up to
the task of bringing these increasingly speedy hunks of metal
to a stop. In nineteen eighteen, Malcolm Lockheed actually law
feed at the time, began to experiment with hydraulics. So
what exactly is a hydraulic system and why is it important? Well,
(30:44):
hydraulics is a branch of science that is really about
the practical applications of fluids, typically liquids. It's largely, but
not exclusively, about how those fluids moved through systems like pipes, channels,
and tanks. Blaze Pascal and Daniel Bernoulli first worked out
(31:04):
the basic principles of fluid dynamics and hydraulic power. But
people had been making practical use of fluids for some
time already, So this was one of those things where
people had figured out that they could could use fluids
to do work, but no one had quite worked out
the science behind it yet until Pascal and Bernoulli came along.
Pascal figured out that a pressure in an incompressible liquid
(31:27):
transmits equally in all directions, and this law ends up
being incredibly useful if you want to leverage fluids to
do work. Bernoulli's last data that energy and a fluid
remains constant, but that changing things like the diameter of
a pipe will change the pressure in a system. The
energy remains the same, but the flow slows down as
it encounters a larger diameter, and the surface area of
(31:48):
the fluid presses against is increased. And effectively, what that
means is you can create mechanical advantage through hydraulic systems.
So if we have a closed system with income rescible fluid,
so you if you push against the fluid, you can't
compress it to a smaller form. It's going to push
against all areas equally. We can actually transfer force from
(32:11):
one side of a system to another, with the liquid
acting as the carrier for that force. And by changing
the sizes of cylinders and pistons, you can also amplify
force by trading force for distance, kind of similar to
what we were doing when we were talking about levers.
So let's say we've got two pistons connected in a
(32:33):
hydraulic system. It's much easier to understand if we take
a concrete example, or at least a hypothetical example. So
we've got piston one. Piston one is two inches in
diameter or one inch radius. That's a five point eight
centimeters in diameter. Piston two is six inches in diameter
(32:53):
or fifteen point to four centimeters in diameter. So then
we have to figure out the area of these two
p stance and area of a circle is pie times
the radius squared, So piston one has the radius of
one inch. One inch squared is one one times pie
is three point one four, etcetera, etcetera. So we just
(33:15):
will simplify to say three point one four is the
area of our first piston. Our second piston has an
area of twenty eight point two six because it's much larger.
So that means piston two is nine times the size
of piston one in area. If we apply a force
(33:36):
to piston one in this closed system where we have
liquid acting as the uh the transmission force between one
piston and the other. So we push a piston one down,
we're gonna get nine times that force on piston two.
So if we push down on piston one with one
pounds of force, it makes piston two go up. Nine
(34:00):
hundred pounds of force because piston two is nine times
the size of piston one. However, there is a tradeoff.
That tradeoff is in the distance traveled by the each piston.
Piston two will travel one ninth the distance of piston one.
So in order to make piston two rise up one inch,
you would have to push piston one down nine inches
(34:23):
push down piston one nine inches into a cylinder at
one dred pounds of pressure. Piston two will lift up
one inch with nine hundred pounds of pressure, so you
amplify the force you decrease the distance. Lackeed's method, in
which hydraulic pressure would create the force that would push
a brake shoe against the brake drums, wasn't embraced immediately.
(34:46):
According to Popular Mechanics, the first passenger car to have
four wheel hydraulic brakes was the Model A Dusenberg in
nineteen twenty one. A decade later, a handful of car
manufacturers were using hydraulics and the brake systems, but the
rest were still relying on cable brakes. Ford would be
the last of the major manufacturers to make the switch
(35:07):
to hydraulics, and that happened in nineteen thirty nine. Going
back just a bit in n another advance helped make
hydraulic disc brakes practical. It was the power assist technology,
and that would reduce the physical effort a driver would
have to exert to apply the brakes. So if you
didn't have power assist, you would find that you have
(35:28):
to push that brake pedal really hard in order to stop.
So instead of having to really stomp on the brake pedal,
the driver just uses a little effort and the car
itself would help to do the rest. The N Pierce
arrow used vacuum that was generated by the inlet manifold
of the engine to offset the physical force required by
the driver. Diesel powered cars, by the way, actually require
(35:51):
a secondary vacuum pump to generate the vacuum necessary for
the power assiste because their engines don't work the same
way anyway. Describing how all this works is tri key
without using visual aids. It involves a diaphragm that initially
has a partial vacuum on either side of the diaphragm,
but when you press the brake pedal, it opens up
a valve on the vacuum booster side. Of the diaphragm
(36:13):
and increases the pressure on that side and thus gives
the boost to the driver who's pushing down on the brake.
To understand this, I really recommend looking at the article
how power brakes work on the House Stuff Works site,
because like I said, it's really hard to describe just
in audio alone and haven't make any sense. But the
point is this was one of those necessary features to
(36:36):
make hydraulic brakes practical to remove some of that effort
that was required in order to push down on the brake.
Oh and and hey, remember when I talked about the
lever at the top of the episode and how the
distances between a force of fulcrum and a load can
affect how much force you apply on the system. The
same is true with brake pedals. Brake pedals are actually levers,
their class two levers, so the distance the pedal has
(36:59):
to travel tends to be much greater than the distance
from the pedal cylinder to the pivot, so the force
of the pedal will be multiplied at the point of
the cylinder. So these are all both These are all
like mechanical ways to make it easier to actually apply
the brakes physically easier. Typically, a hydraulic brake system has
(37:20):
one master cylinder, so this is the one that's actually
controlled by your brake pedal. The pedal cylinder with a
piston that connects via a rod to the brake pedal,
so it's like a direct line from the brake pedal
through a rod to the the actual piston for the
master cylinder. There's usually some more complicated stuff in there now,
(37:43):
especially for modern vehicles, but this is the basic idea.
Each wheel on a car tends to have its own
secondary cylinder, sometimes called a slave cylinder, the master cylinder
and the slave cylinders. The master cylinder tends to be
smaller than the slave cylinders, so we get that force
amplification effect I just talked about. This is what allows
(38:04):
the brakes to apply enough force to slow down a
massive vehicle traveling at high speeds, just from a human
stepping down on a brake pedal. Alright, so we've covered
drum brakes, which you can still find on many car
models on the rear wheels, and we've covered disc brakes,
which were adopted quickly in Europe and later in America
and tend to be the brake system used for front wheels.
(38:26):
Both systems now rely on hydraulics to transmit force from
the brake pedal to the brakes attached to the respective
wheels to those those UH brake shoes or the brake calipers.
There's also the emergency brake, which may have a physical
cable attached to a brake shoe UH that in turn
is attached to one or more wheels. And there's one
(38:49):
other advance I feel like I should cover, and that's
anti lock braking systems or a B s. What are
they and how do they work well? A skidding wheel
has less traction and a non skidding wheel, and a
skidding wheel is one in which the patch of tire
in contact with the road is sliding relative to that
road and the wheel the tire itself is not rotating anymore.
(39:11):
It's locked. So if you can break a car without
locking the tire, like without locking the wheels down entirely,
but rather applying pressure so that the wheels are slowed
to a stop, you're in better shape. You're not gonna
skid out and have a terrible accident, or at least
you won't have a terrible accident in that way. This
is particularly important on slippery road conditions, and that's what
(39:32):
a b S or anti lock brake systems do so.
And a b S has four main additional components on
top of the regular brake system. You have speed sensors
that's what's monitoring the speed of rotation of the wheels.
You have a set of valves, you have pump, and
you have a controller. The sensors are pretty self explanatory,
(39:54):
Like I said, they monitor the wheels. They live for
signs that the wheel is about to lock into position,
which means the wheel would actually stop spinning entirely. The
sensors may be located at each wheel, or it might
be located at the differential. The valves are meant to
control break pressure from the master cylinder, so the master
cylinders providing the pressure for the overall brake system. A
(40:17):
valve can be open and thus send pressure onto the
brake system as per normal. So in other words, it's
it's almost as if the valve is not even there.
It could be closed and block the hydraulics from going
to the break from the master cylinder. This would be
for each individual wheel would have its own so it's
not like one for all four wheels. It's one for
(40:39):
each wheel, so in that case, the hydraulic fluid would
essentially bypass that wheels brake system, and then the valve
has a third position to release some of the pressure
from the brake system. Now, because there's a pressure release system,
the a B S needs a pump to build pressure
up again, and the controller is essentially the brains, it's
in charge of the whole thing. The controller sends signals
(41:01):
to decrease or increase break pressure to individual wheels to
avoid lock up, while still allowing for deceleration. The hydraulic
system begins to pulse a bit as this happens, which
can feel a little weird if you if you're not
used to it, and those that pulse can be fast.
It can be like fifteen times per second with some vehicles.
(41:21):
A BS doesn't magically make cars safer in all conditions,
but they do come in really handy, as I said,
in those slippery road conditions, so they do have a
real benefit. But that does not mean that a BS
is magically going to make every driver safer. There are
possible problems that you can encounter. So, for one example,
(41:43):
when you're breaking with a system that doesn't have anti
lock brakes, you can't steer while you're breaking. Steering is
locked with a b S, you can still steer while
you're breaking, and sometimes that I can actually lead to
drivers making bad decisions and steering off the road. There's
a lot more that we could say about break systems
(42:03):
and newer innovations in the space. Uh there are some
high tech things that we can cover, but I think
I'm going to save that for another episode. We'll we'll
talk more in detail about some cutting edge materials and
techniques and breaking sometime down the line, but for now,
let's put us stop to this. If you guys have
suggestions for future topics for tech stuff, why not write
(42:25):
me and let me know The email addresses tech stuff
at how stuff works dot com or pop on by
our website. The u r L for that is text
stuff podcast dot com. You'll find links there to our
social media as well as to the merchandise store. Remember
every purchase you make goes to help the show, and
we greatly appreciate it. And I'll talk to you again
(42:48):
really soon for more on this and bathans of other topics.
Because it how staff works dot com really really wonder
Get in touch with technology with tech Stuff from how
stuff works dot com. Hey there, and welcome to tech Stuff.
I'm your host, Jonathan Strickland. I'm an executive producer with
How Stuff Works, and I heart radio and I love
all things tech. And you know, we've been going pretty
fast with tech stuff over the past I don't know, decade,
(00:27):
So I felt maybe it was time to apply the
brakes a little bit. I don't mean to slow down
the show or stop it, but rather to take time
to talk about how breaks work and the science surrounding breaks,
because puns are sort of a thing I do, but seriously,
I thought this would give us a chance to talk
(00:47):
about mechanical and hydraulic systems as well as the science
that's behind breaking, the physics that are involved. So first,
let's talk about what makes breaks no necessary in the
first place, which is pretty obvious stuff, but it leads
into a discussion about physical forces that guided the whole
evolution of breaks. So, a body in motion has momentum.
(01:14):
Momentum is a quantity of movement. It depends upon two things,
the mass of the object that's in motion and how
quickly that mass is moving. So really comes down to
how much of the stuff is there and how fast
is it going. A simple equation would be momentum equals
mass times velocity. So if you have something with a
(01:38):
low mass, like a bullet, but it's traveling at a
high velocity, it has a pretty good amount of momentum.
If you have something that's really really huge, like a glacier,
but it's moving incredibly slowly, it still has a lot
of momentum, because momentum depends upon both the mass and
the velocity, not just one or the other. Now, if
(01:59):
you have something that's of a pretty decent size, like
a vehicle, and it's moving at a pretty good speed,
then that has got a lot of momentum too. And
that's a problem, right If you've got a mass that's
moving at a pretty good speed and you need to
stop that mass, you have to figure out how do
you offset that momentum, How do you convert this movement
(02:22):
energy into something else. So momentum is also a vector
quantity vectors and physics have not just a magnitude but
also a direction. Acceleration is the same. It has a
magnitude and a direction. So to fully describe momentum, we
need not just how many kilograms of mass are traveling
(02:43):
at a speed typically in meters per second, but also
the direction of travel. Now, a body and motion has
kinetic energy as well. So to reduce momentum and stop
a body in motion, you need to convert that kinetic energy,
that energy of movement in to something else. Because we
have to remember the laws of conservation, the laws of
(03:06):
the universe. We we energy cannot be created, it cannot
be destroyed. We cannot just make energy, we cannot destroy energy.
We can convert it from one form into another. So
breaks do this by converting the kinetic energy of an
object in motion into heat through friction. And you've probably
(03:26):
heard the term waste heat. Now what we mean by
that is that we've got some sort of system. It
can be pretty much any given mechanical system in particular,
but really other systems as well, biological systems. Uh So,
it could be pretty much anything. And in this system,
some of the energy that we're depending on is going
(03:47):
not to whatever it is we're trying to do, but
rather into generating heat. So with a bicycle, some of
the energy we're putting forth by peddling is not going
directly to making us move we're losing it in the
form of friction, and thus heat um really the heat
that's generated from friction. That's the best way of putting it.
(04:08):
The heat represents energy that we could not otherwise exploit
for whatever it is we're trying to do. So here's
another example, with an electrical generator. We might say that
the friction of the moving parts inside that electrical generator
means that some of the kinetic energy we would otherwise
use to create more electricity is instead converting to heat,
(04:28):
and that heat isn't being captured in any meaningful way.
So the work we did two direct this kinetic energy
to the generator was wasted. Some of that work we
weren't able to get efficiency. So no system is perfect.
Any system with moving parts is going to have friction
to deal with. And there are a lot of super
(04:50):
smart materials scientists out there who have worked really hard
to develop stuff that generates very little friction, and that
is in an effort to increase efficiency and minim eyes
waist heat. But when it comes to breaks, we want
that friction. That's what is stopping an object. So we
need something that is really good at creating this friction.
(05:11):
We want to convert that kinetic energy into heat and
thus decrease the momentum of a fast moving massive object,
perhaps even bringing that object to a complete stop, not
just slowing it down, but stopping it. So in the
very early days of break systems, even before there were cars,
you were looking at a pretty simple device. And this
(05:33):
would be in the horse drawn carriage era. You've got
a carriage that's being pulled by horses. Even when the
horses start to slow down, you know you still have
the momentum of the actual carriage itself. You need to
slow down the carriage uh to to come to a stop.
So the break was typically a lever, and the long
end of the lever was extended up towards the driver.
(05:55):
That was the handle side, So the longside of the
lever is the handle. On the short end of the
ever was a wooden block. The wooden block was when
you pull, the lever, would make contact, typically with the
driver's side front wheel of the carriage to create that
source of friction and convert the kinetic energy of the
turning wheel into heat and thus slow and eventually stop
(06:18):
the carriage. A lever is one of the classic simple
machines it's a bar that rotates around the fulcrum, and
a fulcrum is just a fixed point. So the length
of the lever on either side of the fulcrum determines
the amount of effort or force exerted or acquired per side.
In a classic class one lever, lever makes certain types
(06:40):
of work, such as lifting, easier by reducing the workload
that is needed to affect a change to to create
that lift. There are three classes of levers, and I
just mentioned class one lever. That's the one where you've
got a force that's applied on one end of the lever,
you have a load the thing you're trying to move
(07:00):
or activate on the other end of the lever, and
the fulcrum is somewhere in between those two. If it's
in the center of the bar, then the fulcrum ends
up balancing loads on either side. If you put equal
loads on either side, it should just balance out, kind
of like you know, just a scale if you think
of it that way. So a see saw or a
(07:21):
teeter totter would be an example of a class one lever.
If two people weigh the same on a seesaw, it
balances out. But if you put a heavier person on
one end and a lighter person on the other. The
heavier person sinks down the lighter person goes up into
the air. But if the heavier person were to scoot
forward on the seesaw to decrease the space between the
(07:42):
heavier person and the fulcrumb, then you would eventually balance out.
And if the heavier person kept scooting forward to get
closer to the fulcrum, the lighter person would eventually sink
to the bottom and the heavier person would be up
in the air. So by decreasing the disc dins between
the load and the fulcrum, you can increase the the
(08:06):
lifting power essentially, or the really you're decreasing the amount
of force needed to lift that load, if you're really
thinking about it that way, you're creating a mechanical advantage.
But there are two other classes of levers. I should
probably cover those because some of them actually factor into
breaks as well later down the line. Class two levers
put the fulcrum on one end of a bar, so
(08:29):
instead of it being in the middle, somewhere the fulcrum
the fixed point is actually on one end of the bar.
The load is somewhere in the middle of the bar
and the force is at the far into the bar.
So a wheelbarrow is this type of lever, because these
can be kind of difficult to imagine otherwise. But if
you think about the wheel on a wheelbarrow is a fulcrum,
(08:49):
and you put a load of stuff into the wheelbarrow itself,
and then you lift the other side of the wheelbarrow
by the handles. So the closer the load is to
the fulcrum, the more the mechanical advantage you will have
uh and the less you will the less effort you'll
need to lift the handles for a given load. Or
you can think of it another way, the heavier the
(09:10):
load you will be able to manage as long as
it's closer to the fulcrum. A class three lever has
the fulcrum on one end, the load at the other end,
and the lifting force is in the middle. Now, these
leavers don't provide mechanical advantage, but they can increase the
speed at which a force moves a load. A baseball
bat would be an example of this, or even your
(09:32):
your own forearm would be an example of this. But
we're gonna leave the off from here because class three
leavers don't really factor into the discussion for breaking, So
the wooden block break is a Class one lever. On
one end is the wooden block, its position near the
carriage wheel. A little bit further up from the wooden
block is the fulcrum, the fixed point closer to the
(09:53):
wooden block than to the handle, and the other end
of the lever, the long end has the handle, so
the handle must travel a radar distance than the wooden block.
When you pull back on the brake um, you are
pulling the breakback much further than the wooden block has
to travel to make contact with the the carriage wheel.
But by increasing that distance, you reduce the amount of
(10:15):
work you need to do on the force end to
get a result on the load end. Uh. This is
important because if you're going pretty fast and you need
to slow down that wheel, you need to exert a
good deal of force to create enough friction and convert
that kinetic energy into heat. You couldn't easily do that
if you were, let's say, just holding a wooden block
(10:37):
and just trying to push the wooden block directly against
the carriage wheel. You probably wouldn't be able to do
that with enough force. To make a difference in a
sufficient amount of time. By putting it on this lever,
you can increase the amount of force you're exerting on
the wheel itself on the carriage wheel through pressure with
this wooden block, then you would if you were to
(10:58):
do it directly. So the lever makes the job easier.
Very important part of physics in general. The brake system
made the transition from horse drawn carriages to some of
the earliest automobiles in the nineteenth century, and it was
the same sort of thing. They were using wooden blocks.
The wheels on these early automobiles were often metal. There
(11:20):
was no rubber tires yet, and they worked reasonably well
under certain conditions, namely that the vehicles were traveling at
slower speeds were talking below twenty miles per hour or
below thirty two kilometers per hour, and traffic was pretty
light in those days, so there weren't a lot of
incidents where you would need to break quickly due to
(11:42):
an increased amount of traffic on the streets. But you
also weren't necessarily uh, it wasn't necessarily a good idea
to go, like driving a car up a steep hill,
for example, because your breaking system was pretty primitive. The
only way you would prevent yourself from rolling backwards was
either by applying more acceleration to overcome the force of
(12:05):
gravity that's pulling you down, or to hold a break
real strong against that wheel so that you're not slipping backward. Uh.
This also was probably a good thing. Like that, It's
probably a good thing you weren't going very fast, not
just because the brakes were primitive, but because those wheels
were metal. You would feel it when you would run
(12:27):
over bumps in the road, of which there were many.
It was very similar to if you remember my episodes
about the history of the bicycle, the bone shaker, that
it was a nickname for a type of early bicycle
that was really uncomfortable to ride. It was a very uh,
stiff ride, you might say. But it was clear the
(12:48):
lever approach wasn't going to remain sufficient as cars were
getting faster and heavier and traffic was increasing. You could
stop a faster car with a lever breake, but it
would take longer as you converted that kinetic energy into
heat through friction, so you didn't come to a stop
in as reasonable amount of space. You know, and an
(13:11):
increase in traffic, you had a decreased the email of
time you had in order to slow down to avoid collisions.
So something had to change. A couple of people came
up with some very clever alternatives. One was a proposal
that was made. It was actually not just proposed, it
was developed. It was the creation of Elmer Ambrose Sperry
(13:33):
of Cleveland, Ohio. Sperry's solution involved using disc brakes. Now
I'm going to cover modern disc brakes a little later
in this episode, but it's good to just go ahead
and then explain what the general idea was because it
remained the same even later on. So let's say you've
got a wheel, and you've got that hub of the wheel.
This is part that turns on the axle. Attached to that,
(13:57):
you have a disk that is really just part of
the hub of the wheel, but it's separate from the tire.
So you've got this free standing disc. Positioned over this
disc is a set of calipers, so that either side
of the caliper are on either side of the disk,
and it can pinch down on the disk, just like
(14:18):
your fingers would pinch down if you had let's say
a spinning frisbee between two fingers and you rut uh.
You know, your friend is holding their two fingers together,
and the frisbee is spinning around on the axle of
those two fingers. If you brought your fingers like calipers,
right on the gadge and then pinched, you could stop
the frisbee and its spin. That's the same idea as
(14:41):
this set of disc brakes the the In a sense,
it's working very similar to the way the wooden block
that would press directly against the wheel itself works, except
instead of of hitting the wheel, you're hitting this disc
that's attached to the wheel. Um. Pretty clever. And Sperry
was working on an early electric vehicle. You may remember
(15:02):
in some of my previous episodes, I've talked about how
electric cars actually pre date internal combustion engine cars. The
earliest automobiles were electric vehicles, not not gasoline powered vehicles.
So he's working on this electric car, and he was
actually using electro magnetism to close those calipers, to shut them,
(15:23):
to attract the pincher to the disk and have it
clamped tight enough to start to break the wheel. Uh.
Springs that were attached to the calipers would create the
force that would allow the calipers to open up again
once the electro magnetic field went away. So if you
were to step on a brake in Sperry's design, you
(15:45):
would cause a current to flow through the braking system,
thus generating the electromagnetic field and forcing the calipers closed
and breaking the disk, breaking b R A K I,
n G. The disk. Letting the foot pedal brake would
interrupt the current, so the field would dissipate, the springs
(16:05):
on the caliper would pull the caliper open again. It
was a very ingenious creation, but Sperry's invention didn't catch
on in the States. A similar idea would take off
in Europe, however, and I'll talk about a different breaking
system that would see early success in the United States,
and that early success would extend all the way into
the modern day. But I'll do that in just a second. First,
(16:29):
let's take a quick break. So right around the same
time Sperry was working on the disc brake solution, Gottlieb
Daimler was developing a different approach called the drum brake.
(16:49):
Daimler's idea was to attach a drum mounted on an
axle and to wrap a cable around that drum and
pulling the brake would create tension on the cable, tightening
it around the drum and creating the friction needed to
slow and then stop a vehicle. So if you can
think of maybe a spinning axle and then wrapping a
(17:10):
belt around it and then pulling the belt, really taught
so that that creates that friction and then slowing the
axle down. It's similar to that. So you have this
dedicated drum for this purpose. UH. This was an idea
that will Helm Maybach used in nineteen o one in
some early Mercedes designs, but it was Louis Renault who
(17:31):
defined the drum break as a standard in vehicles. It
was Renault's UH design that really took off. That being said,
there are also a ton of different mechanics and engineers
working on this problem, and so the idea may have
been developing in parallel around the world. Renault gets the
credit for making this the first really practical drum break,
(17:54):
but lots of people were working on this because the
automobile was a rising technology at the time. Now, the
drum brake is a pretty clever invention, and a modernized
version is still used in many vehicles today, though typically
only for the rear wheels for most passenger vehicles. It
was also the dominant form of breaking systems in the
(18:15):
United States until the nineteen seventies, though in Europe things
were a little different. The original drum brakes weren't great
on flat surfaces. These drum brakes that had a a
a belt or a strip of metal or something wrapped
around a drum that would then tighten to slow things down,
(18:36):
those worked fine on flat surfaces. A break race in
nineteen o two pitted a horse drawn coach that had
a lever style break against a Victoria horseless carriage that
had an internal drum brake, and a custom vehicle that
was created by a guy named Ransom E. Olds And
he called it the Oldsmobile. Yep, that's where that comes from. So,
(19:01):
like I said, the coach had a tire break, the
horseless carriage had an internal drum break, which I'll talk
about in a second, and the Oldsmobile had a different
drum brake design that used a band of stainless steel
wrapped around the drum. So pressing on the brake pedal
rather than pulling a lever would contract this band. So
that would grip the drum more tightly and thus create
(19:23):
the friction. The oldsmobile proved it could stop in less
time and thus travel the least distance while breaking than
the other two vehicles. So this meant that for a while,
external breaks, in which the breaking mechanism is on the
outside of the drum, were more popular, but both external
and internal drum breaking systems existed at the same time.
(19:45):
So what was an internal drum break system. Well, in
that case, you have the drum assembly that's mounted on
a wheel or an axle, So just think of it's
almost like a pot, right, It's just mounted on there
and and everything is inside this pot. So you have
these extendable parts inside the drum. They're called shoes. And
(20:08):
these extendable shoes are anchored with respect to the car,
so they are not rotating. They are stationary with respect
to the chassis of the vehicle. So the drum rotates
around these shoes as as the cars in motion. The
shoes have breaking material, so a material meant to generate
(20:30):
friction coding the surface of the shoe itself the stopping surface.
So when you engage the brake, when you step on
the brake, pedal A system extends these shoes on the
inside of the drum, so that that breaking surface is
rubbing up against the inside edge of that rotating drum.
(20:52):
So it's the same effect that you were getting before,
this idea of pressing a surface against a moving object
in order to generate friction and convert kinetic energy into heat.
It's just in this case it's happening on the inside
of a rotating surface, not on the outside of the
rotating surface. Uh. The this is really interesting stuff. It's
(21:16):
a little difficult to envision just from my audio, I imagine,
but if you really want to look into this and
see some diagrams and some animations and things like that,
How Stuff Works has several articles about breaks, including how
drum brakes work, and that's incredibly useful. So if you
want to check in on a visual aid, I highly
(21:37):
recommend that I don't write for How Stuff Works anymore,
but I still respect the heck out all the articles
that are on that site. They are incredibly useful, especially
for stuff like this, to get an understanding of these
mechanical systems. So in addition, uh, these brakes would typically
have some sort of cable or belt wrapped around the drum.
So you would have a lot of cars that would
(21:59):
have kind of a high red system where they have
both external and internal breaking UH systems incorporated into the
same overall brake system and create more friction more more efficiently,
so that you could convert that kinetic energy into heat
and stop faster. Now, early on, the external drum brake
(22:19):
systems were believed to be superior. They could stop a
car faster and less time less distance than internal ones,
but they did have some big drawbacks. One of those
was that the brakes would tend to unwind if you
try to stop halfway up a hill. So you you're driving,
you got a hill. Let's say you're in San Francisco.
That's a great example. You're in San Francisco, you hit
(22:42):
a hill, you're driving up the hill, and then you're
coming up to an intersection where you have to stop. Well,
that was an issue because the brakes would unwind. At
that point. Gravity would create a backward pull on the
car to pull it back down the hill, and that
would cause the bands wrapped around the drum to unwind
(23:03):
in the car would actually start to roll backwards. For
that reason, early motorists would sometimes carry wedged blocks called chalks,
just like you would have for an airplane, and you
would actually there's footage of people who were driving cars
around that time who would jump out of their car
with these wooden blocks, run behind the cars it's starting
(23:24):
to roll backward, and try to wedge those blocks behind
the wheels so that the car wasn't rolling backward anymore.
The external brakes also had no protection against dirt and
other debris, so they would get dirty and they would
wear down relatively quickly, which would necessitate frequent maintenance or
replacement of those brakes. The internal drum braking systems were
(23:45):
protected from debris and dirt right because they're inside the drum,
so that stuff wasn't getting to them. Although as you
use the drum brakes over and over over again, they
do develop dust inside the drum itself because it's actually
wearing away both the brake pads and the inside of
(24:07):
the drum. You know that friction is slowly grinding away
some of that material. But also because of the design,
the shoes inside a drum could maintain pressure for as
long as the brake was engaged, which meant there was
no need to worry about rolling backward down the hill.
If you had the brake pressed, then you know it
wasn't unwinding. If it was an internal drum brake, the
(24:29):
brakes would last longer than external breakes, but they weren't
as effective at stopping the vehicle as quickly, so for
that reason, some car manufacturers chose to employ both types
of brakes on the rear wheels of vehicles, in particular,
so that both an external and internal drum brake system
could work together from the same press of a brake pedal.
In those early systems, everything was purely mechanical. We haven't
(24:53):
gotten to the hydraulic sections yet, so that meant that
it was using stuff like cables and rods and levers
to do mechanical work and to move these different elements
to where they needed to be. Eventually that gave way
to hydraulic systems, but will have a different type of
break to talk about first before we get into hydraulics.
(25:14):
So that means we need to get back to Sperry's
disc brakes. His implementation didn't get much traction, and yes
that's a pun, but others began to work on similar designs,
possibly with complete independence and without knowledge of Sperry's work, because,
like I said, a lot of people were working on
this at the same time. Uh. One of those was
(25:36):
an inventor named F. W. Lanchester in the UK who
received a patent in nineteen o two for a mechanical
approach to the disc brake design, so instead of the
electromagnetic approach, the calipers would be controlled by a cable.
The cable, when pulled taught, would force the calipers closed.
But lanchester solution wasn't ideal. The disc mounted to the
(26:00):
wheel hub was made of metal, which makes sense, but
the brake pads that were actually on the caliper were
made out of copper, So when you were applying a break,
that meant that you were applying to copper pads on
either side of a rapidly spinning metal disc. And as
you might imagine, this created no small amount of noise.
(26:20):
From what I understand, it was the type of high pitched,
screeching noise that was not that different from what you
might get with fingernails dragged down a chalkboard, so not
a pleasant sound. In addition, the brake pads would wear
down very quickly and had the same issues with dart
and debris that the external drum systems had, so lanchester
solution was, like Sperry's, largely unimplemented. A dude named Fruit
(26:46):
made the next big contribution. His name is Herbert Fruit.
He was an English engineer who took the brake pads
and lined them with a substance that would cut back
on that noise. It was a long lasting material that
could withstand a lot of abuse, and it would become
a common brake pad material for both disc brakes and
drum brakes, and still is used in a lot of
(27:08):
breaks today. That material was asbestos. Yikes, So asbestos was
long considered a truly remarkable substance with a ton of
practical applications. It's actually a group of silicate minerals, it's
not just one, and this group all have similar traits
(27:28):
their fibrous which means you can actually draw the stuff
out and create a material that's similar in consistency to
cotton balls in a way. Its heat resistant, it's an
electrical insulator, and it holds up against a lot of
otherwise corrosive chemicals, so it can make other stuff stronger
when you mix the substances together, and it was a
common additive for everything from cement to paper. But what
(27:52):
was not known for a very long time was that
asbestos is actually incredibly toxic. Now, as I said, it's fibrous,
and so they are these tiny asbestos fibers in the
mineral and these can easily be swallowed or inhaled without
your knowledge. Their microscopic in size, so you're not able
to spot them. And due to those qualities I mentioned earlier,
(28:15):
the fact that the fibers are so resistant to so
many different things, it also means that they can last
indefinitely inside a person's body and there's no real way
to flush them out. They don't dissolve, and these trapped
fibers can cause inflammation and even much worse problems genetic
damage to cells. Uh they can lead to development of cancer.
(28:38):
The illnesses take a really long time to develop, too,
like between twenty to fifty years, which is one of
the reasons asbestos remained in popular use for so long.
It took ages to figure out that it was hazardous
in the first place because the effects took so long
to manifest. Now, nearly all the auto manufacturers in the
(28:58):
United States stopped making asbestos brake pads in the ninety nineties,
but there are a lot of aftermarket companies that continue
to do so, largely manufacturing in places like China and
India and Mexico. So to this day, there are aftermarket
brake pads that have potentially dangerous amounts of asbestos in them. Now,
(29:19):
that probably doesn't pose a huge health hazard to the
average driver, but it is a concern for mechanics. For
people who work on break systems regularly, that's something that
they should actively be concerned about and protect themselves against.
Because there's no telling if someone's getting cheap break pads
(29:40):
from overseas, because you know there they are much less
expensive than buying them here Domestically, there's a chance that
one of the materials in there is asbestos and it
could be in a concentration high enough for it to
be a danger. So just to be aware if you
happen to be someone who works on such things, just
you know, you need to just be care full. We're
(30:01):
masks and stuff. Now. I've got a lot more to
say about breaks, including the introduction of hydraulics and modern
breaking systems, but first let's take another quick break. Now,
it didn't take long for engineers to look into hydraulics
(30:21):
to work with car brakes. Cars were getting heavier and faster,
which meant the brake systems needed to be up to
the task of bringing these increasingly speedy hunks of metal
to a stop. In nineteen eighteen, Malcolm Lockheed actually law
feed at the time, began to experiment with hydraulics. So
what exactly is a hydraulic system and why is it important? Well,
(30:44):
hydraulics is a branch of science that is really about
the practical applications of fluids, typically liquids. It's largely, but
not exclusively, about how those fluids moved through systems like pipes, channels,
and tanks. Blaze Pascal and Daniel Bernoulli first worked out
(31:04):
the basic principles of fluid dynamics and hydraulic power. But
people had been making practical use of fluids for some
time already, So this was one of those things where
people had figured out that they could could use fluids
to do work, but no one had quite worked out
the science behind it yet until Pascal and Bernoulli came along.
Pascal figured out that a pressure in an incompressible liquid
(31:27):
transmits equally in all directions, and this law ends up
being incredibly useful if you want to leverage fluids to
do work. Bernoulli's last data that energy and a fluid
remains constant, but that changing things like the diameter of
a pipe will change the pressure in a system. The
energy remains the same, but the flow slows down as
it encounters a larger diameter, and the surface area of
(31:48):
the fluid presses against is increased. And effectively, what that
means is you can create mechanical advantage through hydraulic systems.
So if we have a closed system with income rescible fluid,
so you if you push against the fluid, you can't
compress it to a smaller form. It's going to push
against all areas equally. We can actually transfer force from
(32:11):
one side of a system to another, with the liquid
acting as the carrier for that force. And by changing
the sizes of cylinders and pistons, you can also amplify
force by trading force for distance, kind of similar to
what we were doing when we were talking about levers.
So let's say we've got two pistons connected in a
(32:33):
hydraulic system. It's much easier to understand if we take
a concrete example, or at least a hypothetical example. So
we've got piston one. Piston one is two inches in
diameter or one inch radius. That's a five point eight
centimeters in diameter. Piston two is six inches in diameter
(32:53):
or fifteen point to four centimeters in diameter. So then
we have to figure out the area of these two
p stance and area of a circle is pie times
the radius squared, So piston one has the radius of
one inch. One inch squared is one one times pie
is three point one four, etcetera, etcetera. So we just
(33:15):
will simplify to say three point one four is the
area of our first piston. Our second piston has an
area of twenty eight point two six because it's much larger.
So that means piston two is nine times the size
of piston one in area. If we apply a force
(33:36):
to piston one in this closed system where we have
liquid acting as the uh the transmission force between one
piston and the other. So we push a piston one down,
we're gonna get nine times that force on piston two.
So if we push down on piston one with one
pounds of force, it makes piston two go up. Nine
(34:00):
hundred pounds of force because piston two is nine times
the size of piston one. However, there is a tradeoff.
That tradeoff is in the distance traveled by the each piston.
Piston two will travel one ninth the distance of piston one.
So in order to make piston two rise up one inch,
you would have to push piston one down nine inches
(34:23):
push down piston one nine inches into a cylinder at
one dred pounds of pressure. Piston two will lift up
one inch with nine hundred pounds of pressure, so you
amplify the force you decrease the distance. Lackeed's method, in
which hydraulic pressure would create the force that would push
a brake shoe against the brake drums, wasn't embraced immediately.
(34:46):
According to Popular Mechanics, the first passenger car to have
four wheel hydraulic brakes was the Model A Dusenberg in
nineteen twenty one. A decade later, a handful of car
manufacturers were using hydraulics and the brake systems, but the
rest were still relying on cable brakes. Ford would be
the last of the major manufacturers to make the switch
(35:07):
to hydraulics, and that happened in nineteen thirty nine. Going
back just a bit in n another advance helped make
hydraulic disc brakes practical. It was the power assist technology,
and that would reduce the physical effort a driver would
have to exert to apply the brakes. So if you
didn't have power assist, you would find that you have
(35:28):
to push that brake pedal really hard in order to stop.
So instead of having to really stomp on the brake pedal,
the driver just uses a little effort and the car
itself would help to do the rest. The N Pierce
arrow used vacuum that was generated by the inlet manifold
of the engine to offset the physical force required by
the driver. Diesel powered cars, by the way, actually require
(35:51):
a secondary vacuum pump to generate the vacuum necessary for
the power assiste because their engines don't work the same
way anyway. Describing how all this works is tri key
without using visual aids. It involves a diaphragm that initially
has a partial vacuum on either side of the diaphragm,
but when you press the brake pedal, it opens up
a valve on the vacuum booster side. Of the diaphragm
(36:13):
and increases the pressure on that side and thus gives
the boost to the driver who's pushing down on the brake.
To understand this, I really recommend looking at the article
how power brakes work on the House Stuff Works site,
because like I said, it's really hard to describe just
in audio alone and haven't make any sense. But the
point is this was one of those necessary features to
(36:36):
make hydraulic brakes practical to remove some of that effort
that was required in order to push down on the brake.
Oh and and hey, remember when I talked about the
lever at the top of the episode and how the
distances between a force of fulcrum and a load can
affect how much force you apply on the system. The
same is true with brake pedals. Brake pedals are actually levers,
their class two levers, so the distance the pedal has
(36:59):
to travel tends to be much greater than the distance
from the pedal cylinder to the pivot, so the force
of the pedal will be multiplied at the point of
the cylinder. So these are all both These are all
like mechanical ways to make it easier to actually apply
the brakes physically easier. Typically, a hydraulic brake system has
(37:20):
one master cylinder, so this is the one that's actually
controlled by your brake pedal. The pedal cylinder with a
piston that connects via a rod to the brake pedal,
so it's like a direct line from the brake pedal
through a rod to the the actual piston for the
master cylinder. There's usually some more complicated stuff in there now,
(37:43):
especially for modern vehicles, but this is the basic idea.
Each wheel on a car tends to have its own
secondary cylinder, sometimes called a slave cylinder, the master cylinder
and the slave cylinders. The master cylinder tends to be
smaller than the slave cylinders, so we get that force
amplification effect I just talked about. This is what allows
(38:04):
the brakes to apply enough force to slow down a
massive vehicle traveling at high speeds, just from a human
stepping down on a brake pedal. Alright, so we've covered
drum brakes, which you can still find on many car
models on the rear wheels, and we've covered disc brakes,
which were adopted quickly in Europe and later in America
and tend to be the brake system used for front wheels.
(38:26):
Both systems now rely on hydraulics to transmit force from
the brake pedal to the brakes attached to the respective
wheels to those those UH brake shoes or the brake calipers.
There's also the emergency brake, which may have a physical
cable attached to a brake shoe UH that in turn
is attached to one or more wheels. And there's one
(38:49):
other advance I feel like I should cover, and that's
anti lock braking systems or a B s. What are
they and how do they work well? A skidding wheel
has less traction and a non skidding wheel, and a
skidding wheel is one in which the patch of tire
in contact with the road is sliding relative to that
road and the wheel the tire itself is not rotating anymore.
(39:11):
It's locked. So if you can break a car without
locking the tire, like without locking the wheels down entirely,
but rather applying pressure so that the wheels are slowed
to a stop, you're in better shape. You're not gonna
skid out and have a terrible accident, or at least
you won't have a terrible accident in that way. This
is particularly important on slippery road conditions, and that's what
(39:32):
a b S or anti lock brake systems do so.
And a b S has four main additional components on
top of the regular brake system. You have speed sensors
that's what's monitoring the speed of rotation of the wheels.
You have a set of valves, you have pump, and
you have a controller. The sensors are pretty self explanatory,
(39:54):
Like I said, they monitor the wheels. They live for
signs that the wheel is about to lock into position,
which means the wheel would actually stop spinning entirely. The
sensors may be located at each wheel, or it might
be located at the differential. The valves are meant to
control break pressure from the master cylinder, so the master
cylinders providing the pressure for the overall brake system. A
(40:17):
valve can be open and thus send pressure onto the
brake system as per normal. So in other words, it's
it's almost as if the valve is not even there.
It could be closed and block the hydraulics from going
to the break from the master cylinder. This would be
for each individual wheel would have its own so it's
not like one for all four wheels. It's one for
(40:39):
each wheel, so in that case, the hydraulic fluid would
essentially bypass that wheels brake system, and then the valve
has a third position to release some of the pressure
from the brake system. Now, because there's a pressure release system,
the a B S needs a pump to build pressure
up again, and the controller is essentially the brains, it's
in charge of the whole thing. The controller sends signals
(41:01):
to decrease or increase break pressure to individual wheels to
avoid lock up, while still allowing for deceleration. The hydraulic
system begins to pulse a bit as this happens, which
can feel a little weird if you if you're not
used to it, and those that pulse can be fast.
It can be like fifteen times per second with some vehicles.
(41:21):
A BS doesn't magically make cars safer in all conditions,
but they do come in really handy, as I said,
in those slippery road conditions, so they do have a
real benefit. But that does not mean that a BS
is magically going to make every driver safer. There are
possible problems that you can encounter. So, for one example,
(41:43):
when you're breaking with a system that doesn't have anti
lock brakes, you can't steer while you're breaking. Steering is
locked with a b S, you can still steer while
you're breaking, and sometimes that I can actually lead to
drivers making bad decisions and steering off the road. There's
a lot more that we could say about break systems
(42:03):
and newer innovations in the space. Uh there are some
high tech things that we can cover, but I think
I'm going to save that for another episode. We'll we'll
talk more in detail about some cutting edge materials and
techniques and breaking sometime down the line, but for now,
let's put us stop to this. If you guys have
suggestions for future topics for tech stuff, why not write
(42:25):
me and let me know The email addresses tech stuff
at how stuff works dot com or pop on by
our website. The u r L for that is text
stuff podcast dot com. You'll find links there to our
social media as well as to the merchandise store. Remember
every purchase you make goes to help the show, and
we greatly appreciate it. And I'll talk to you again
(42:48):
really soon for more on this and bathans of other topics.
Because it how staff works dot com really really wonder
TechStuff News
Host
Jonathan Strickland
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