You pop your car into reverse, you start backing up and you can hear a whining noise, increasing in pitch the faster in reverse you go. We all know the familiar sound of a reversing car, whether it be your own, in busy car parks, or your neighbour parking uncomfortably close to your vehicle, and yet the reason we hear this is commonly unknown.
The automobile is an interesting piece of technology because it’s something we see, drive or are a passenger in every day. Yet there are so many behaviours, noises or features that we don’t know the origin of, or seemingly have no clear purpose (Check our article on the mysterious black dotted border around your windscreen for answers to more of these intriguing features).
So, why do cars whine in reverse? As is so common with these questions, the answer is actually surprisingly simple.
It is to do with the shape of the gears used in the gearbox. There are two types of gears in a gearbox; helical and spur. Helical gears are used in most road car’s forward gears because they offer the smoothest, most reliable and quietest tooth engagement. This is because the teeth are positioned at an angle on the gear, not parallel to the axis of rotation.
When two rotating helical gears engage, the tooth contact point starts small, and increases in a smooth, curved motion until the teeth are fully engaged momentarily before receding away from each other in reverse of how they make contact. This way, the teeth faces gradually make and break contact, allowing for a smoother and quieter action. Helical gears also enjoy the benefit of strength, as the angled teeth contain a greater volume of material than spur gears. Reliability and comfort is a high priority for road going cars designed to last for years, making Helical gears ideal for this application.
However it’s not all great with helical gears. Because of the angle of the teeth, the meshed gears create whats called ‘axial thrust’. This is a force that acts on a helical gear along the axis of rotation, trying to push the gear sideways (imagine a force trying to slide the gears apart). To solve this, a ‘thrust bearing’ must be used to resist this force acting on the gear and additional strengthening to the gearbox housing is required, increasing costs. Helical gears are also less efficient, as the increased friction of the meshing teeth, combined with the sideways pushing forces on gears, mean some energy is lost as heat. These issues with helical gears limit the amount of power that can be put through the gearbox.
Spur gears are the more classic type. The teeth are set parallel to the axis of rotation, straight across the gear. As two spur gears mesh, the straight-cut teeth make contact along their entire width all at once, effectively knocking into each other.
Because of this the engagement is much more violent, with the teeth surfaces hitting together rather than progressively making contact like a helical gear does. The impact of the teeth contacting make a quiet tapping sound. Individually this may seem insignificant, but when rotating at high speeds the quiet taps distort into a loud whine, increasing in pitch the faster the gears rotate as the tapping frequency also increases. And so, it is this rapid clinking of the straight teeth that create the distinctive whine as you reverse your car.
So while they make a lot of noise when in use and have a shorter life, this is an area where manufacturers can reduce costs for both them and us, as the reverse gear is arguably the least used one. This is because of their much simpler shape, making them easier to manufacture. Combined with this, as the teeth are straight, the gears aren’t subject to any axial load, so no special thrust bearings or gearbox strengthening is required, again reducing weight and costs.
Interestingly, racing gearboxes often only use spur gears, where durability, smoothness and operation volume are of less importance. Spur-only gearboxes for racing are an engineer’s choice because as mentioned, they do not produce any axial thrust, so they can handle a much greater amount of power, saving weight without the added gearbox reinforcement, and don’t lose vital energy via heat. The resulting gearbox is lighter, more efficient, and can handle more power.