Have you ever wondered why when you drive in reverse, your car makes that strange, high-pitched whine? Of course you have; you’re a human, or at least something close enough to a human to go on the internet, drive a car, and wonder. There’s actually a simple, satisfying reason why this happens, so let’s kick some ignorance to the curb.

Just in case you’re not familiar with the reverse whine, I found some samples of it online, in these videos of people driving in reverse:

So, we all heard that sound. Why does the car only make that sound in reverse, and not in any of the forward gears? The answer has to do with the gears themselves.

You see, in most transmissions, the forward gears are what’s known as helical gears, which have angled teeth and are in constant mesh with one another. The angles of the teeth keep the teeth in constant contact with one another, which makes the gears quiet.

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For reverse gear, most transmissions use spur gears, also known as straight-cut gears. These are used because they’re much cheaper to produce than helical gears, and since a reverse gear actually uses three gears to reverse the direction of the input shaft, that adds up.

Also, with straight-cut gears, you don’t need any synchronizers for the gears, which saves even more money. The accountants must love straight-cut gears.

These are like the archetypal gear you picture in your head when you think of “gear.” The teeth on these gears are straight, a simple extrusion of the shape of the gear.

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That means when spur gears mesh with one another, each tooth essentially slams into another tooth on the other gear as they mesh, because they don’t have the same buttery-smooth engagement as helical gears. When those straight-cut teeth make contact, they impact and make a little sound. Individually, one of these gear-tooth clicks probably wouldn’t be noticed, but when the gears are in motion, you end up with a series of clicks, and all those clicks together become a whiny tone.

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Just in case you’re not sold on this series-of-clicks-become-a-tone idea, I set up a little computer simulation, using a very, very outdated computer:

So, that’s what’s happening. When you select reverse, the straight-cut gears engage, their teeth make tiny little taps on one another, and you hear that whine. The whine gets higher the faster you go, because you’re increasing the frequency of those little taps.

Now, in race cars, often the transmissions will be all spur gears, because spur gears do not produce something called axial load, or thrust force, which is generated by the sliding, constantly-meshed contact of the teeth of helical gears.

This extra force must be compensated for in stronger transmission housings, which means more weight, which is exactly what you don’t want in your race car. Here’s what a full set of spur gears sounds like there:

That’s a lot of angry whine, right? In that context, it sounds pretty cool, but you can see why your average driver probably wouldn’t want that on their daily highway commute.