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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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.
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