Why do cars (transmissions I assume) sound different when they are backing up than they do when they are going forward?

Question

In reverse they tend to make a whining noise. Isn’t it the same parts doing basically the same thing either way?

In: Engineering

Answer 1

Most transmission gears are straight cut. The interactive elements are parallel to the spin axis. This is less durable, provides more power loss, but is nearly silent while operating. In transmissions with a distinct reverse gear (almost certainly all of them for the last 30 years or so) the gear is a helically cut gear. The interactive elements are cut on a diagonal to the spin axis. These are stronger for their size, and substantially less prone to torque loss.

Helically-cut transmission gears have a distinct whine. This increased strength allows them to be smaller than the other components, and the time reverse is typically used makes the effect less annoying.