eli5 why does the cars R gear sound so different to the 1st gear? you can always hear the different when a car is backing up.
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Straight from Google:
Reverse gears are straight cut while forward gears are helical (a helix or spiral shape) because it creates a bigger contact area for the gears which creates higher torque.
You don’t need a lot of torque when reversing so the gear is cut straight creating less contact area which makes a different sound then the forward gears.
ElI5: Smooth gear makes different sound than a gear with grooves.
Straight cut gears just make that noise due to the way they mesh. The helical gears are quieter and stronger because they stay meshed continuously. Helical gears are more expensive to produce, and since You don’t typically go far with reverse and strength in reverse is less important manufacturers choose to use the cheaper parts to save money.
Most of the gears in your car are [helical gears](https://upload.wikimedia.org/wikipedia/commons/e/ed/Anim_engrenages_helicoidaux.gif) where the teeth are tilted. This means that only a tiny part of the tooth it making/breaking contact at any given time and generally multiple teeth are partially engaged at all times. Helical gears run smoother and quieter making them good for things where you want speed and lots of power
The reverse gear in your car is a [straight cut spur gear](https://upload.wikimedia.org/wikipedia/commons/1/14/Gears_animation.gif) and because the teeth are straight a whole tooth makes contact across its entire length at once and later releases across its entire length at once. They’re rougher and noisier because of this since the teeth are never *perfectly* spaced so there’s always going to be some chatter. The straight cut teeth are also cheaper so they’re good for the reverse gear where you don’t need speed, power, or quiet operation because its only occasional so it gives a minor cost savings.
Short version is cheaper and simpler straight-cut gears instead of helical. Gear cutting gets pretty complex actually. The simplest version is just a triangle spike shape around a disk. You could cut some gears like that and match them together and they’ll kind of work, but you’ll see a lot of issues the more power you try and put through them. The meshing will be more of an impact as the teeth switch, which is noisy and puts a lot of stress on the gears. The contact point will slide around, which generates more friction and stress, and also changes the exact gear ratio a little. Excessively sharp tips might also tend to chip and flake, and any chips will tend to bounce around the gearbox and cause rapid wear.
So if you want to make gears that can transmit a lot of power for a long time with low noise and vibration, you pay more attention to and spend more money on the teeth profiles. You carefully contour the teeth to minimize sliding and keep the contact point as close to the same radius as possible during the whole engagement, minimize sharp corners prone to chipping and cracking, and angle the teeth so that there’s always at least one tooth in engagement to get rid of impacts. This also tends to generate twisting and axial forces on the gear, which you’ll need better bearings and stronger housings to handle.
So Reverse often has cheaper and simpler gears since you don’t go far or accelerate hard in it. Making it better and stronger is more expensive and can make the whole transmission bigger and heavier, which hurts overall costs and fuel economy.
Also worth noting that traditional air raid sirens are nothing more than a pair of straight-cut gears driven by a big engine. That’s how loud putting a lot of power through poorly-cut gears can be.
I drive Subaru with CVT and the only difference between forward/reverse gear is small gear that doesn’t make recognisable sound. I’ve driven in reverse pretty fast, don’t ask me why.
Forward gears – angled tooth design, smoother and quieter meshing, more expensive to manufacture.
Reverse gear – straight cut, good for low speed applications, not as smooth meshing of gears which produces destinctive whine when in operation.
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