This video explains why cars make a spooling sound when they run in reverse.
Never in my many years of driving cars have I encountered a vehicle that makes no sound when it is in reverse. Even the Honda CRV, one of the most smooth transmissions, makes a sound when I press the R button. Every car I’ve had the opportunity to drive, manual or automatic, makes a spooling sound. It sounds like a machine about to explode.
It turns out that there are both good and pragmatic reasons for this. The resident engineer geek explains why in this episode of Mike’s Mechanics, which is available on DriveTribe’s Youtube channel. Reverse gears are different from the ones used to move forward, as shown in the video. This is why you must be in total standstill before changing to reverse.
Reverse gears have straight-cut teeth and look similar to a standard sprocket. Normal gears, on the other hand, have a helical shape where the teeth are angled in an arc formation. This is done to make the operation smoother.
Noise is caused by the large amount of surfaces that come in contact with one another. The large surface area of the straight-cut gears makes the noise. Nearly everything touches it, which is why there are so many. It transfers torque easily, or, as Mike said, “it’s almost like ripping off a plaster at the same time.”
Straight-cut gears are simple and cheap to make but they can’t be used in other gears due to the noise they make – unless it’s a racecar. Straight-cut gears don’t cause an axial load so race cars prefer this type of gear.