Why does a manual transmission make a whirring noise?

When manual transmissions are put in reverse, they produce a loud, whirring noise. What's the culprit? Gears. Marshall Brain explains how helical and spur gears work in manual transmissions in this episode.

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Episode Transcript

Speaker 1 (00:00):
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(00:30):
with today's question, why does a manual transmission make a loud,
worrying noise when it's put into reverse? Manual transmissions use
mostly helico gears, but reverse is a special situation that
requires a different type of gear, a spur gear, and
this is why reverse sounds different than forward in a

(00:51):
normal manual transmission car. The gears that make up the
forward gear ratios are all helical gears. The teeth on
helico gears are cut at an angle to the face
of the gear. When two teeth on a helical gear
system engage, the contact starts at one end of the
tooth and gradually spreads as the gears rotate until the

(01:13):
two teeth are in full engagement. This gradual engagement makes
helical gears operate much more smoothly and quietly than spur
gears do. Also, because of the angle of the gear teeth,
more teeth are in engagement at any one time. This
spreads the load out more and it reduces stresses, which
also reduces noise. The only problem with helical gears is

(01:37):
that it's hard to slide them in and out of
engagement with each other. On a manual transmission, the forward
gears stay engaged with each other at all times, and
callers that are controlled by the stick shift lock the
different gears to the output shaft. You can go read
how manual transmissions work for details on this. In contrast

(01:57):
to that, the gears that are used in reverse are
normal gears like you see in cartoons their circles with
teeth on them. The reverse gear on your manual transmission
uses an idler gear that has to slide into mesh
with two other spur gears at the same time in
order to reverse the direction of rotation. Each time a

(02:18):
gear tooth engages on a spur gear, the teeth collide
instead of gently sliding into contact as they do on
helico gears. This impact makes a lot of noise and
also increases the stresses on the gear teeth. When you
hear a loud, whirring noise from your car and reverse,
what you're hearing is the sound of the spur gear

(02:38):
teeth clacking against one another. For more on this and
thousands of other topics. Because it how staff works dot com,
Streaming TV shows and movies directly to your home is
a breeze with Netflix. As a Netflix member, you can
instantly watch TV and movies on your PC, Mac, mobile device,

(02:58):
or right on your television. Get a free thirty day
trial membership. Go to Netflix dot com slash brain stuff
and sign up today.