We have all heard the whiny noise that comes from the transmission when a car reverses. Many of us have also judged that the car was being reversed just by hearing the sound. But the real question to ponder over is, why does it occur? Because the same transmission runs smoothly while moving forward. What’s with the reverse gear? Well, that is precisely what this article addresses.
Let us look at the construction of a manual transmission first.
The picture shows that all forward gears are made of specially shaped gears which are unlike our usual orthodox gears, while the reverse one is different.
The gears that encompass the forward gears are known as helical gears while the reverse gear is a spur gear. Let us see what these are:
These are a special type of cylindrical gears with slanted teeth. The angle at which these teeth slant is known as the helix angle. They have a larger contact ratio and are able to transmit large forces. What happens is that when two teeth on helical gears start to engage, the contact starts at one end and gradually increases until all the surface area of one tooth becomes in contact with the other. This gradual meshing of teeth allows for helical gears to operate without noise and makes them ideal to transfer large forces without the development of excessive stresses. Furthermore, as a number of teeth simultaneously start to engage due to the helix angle, the total force that has to be transferred spreads out over a number of teeth and makes it easier for the gear to handle stresses. This is advantageous for forward motion.
These are also cylindrical gears but have teeth that project out radially from the centre. As they don’t have any angle unlike the helical gears, these are also known as straight cut gears. Technically these are not straight but have a special contour which the teeth follow, but generally, they are taken as straight cut gears. It is easier for two straight cut gears to mesh together than the helical gears but because two gear profiles cannot be exactly identical, when two teeth of the corresponding two gears, mesh together, there remains some “unoccupied space” when the gears are idle. This is known as backlash. What happens is that when these are engaged, the backlash region the tooth travels makes it “hit” the other tooth with force producing noise. This collision makes a lot of noise and also increases the stresses on the gear teeth. This is the noise that is heard when the car is reversed.
Why Use Spur Gears for Reverse?
This is because of two reasons:
- The forward gears are constantly and throughout in contact with each other as they are difficult to engage while the reverse gear is used seldom. But as spur gears can engage easily, it makes them better to utilize for reverse gears.
- Reverse gears are spur gears because reverse requires an idler gear to prevent accidentally shifting into reverse while moving. This is what is known as the gears not being synchronized, and this is also why one cannot shift into reverse while moving.
So yes. It is the shape of that spur gear that makes that whine when you reverse. Those cuts/grooves of the cog mesh into each other at speed making that high pitched sound. You might have heard the same noise in various race cars’ videos even if the car is going forward. That is because race cars don’t need to be quiet. The straight cut gears are generally stronger as well. So they work fine in front forwarding configuration as well. It is the noise that makes them unsuitable for normal passenger cars.