Why do cars have four wheels?
03 Nov 2020
Why do (most non-solar) cars have four wheels?
Suppose your task is to design a vehicle that can transport a single person from Darwin to Adelaide as efficiently as possible, powered by solar energy. How many wheels should it have?
One wheel is not enough. It is inherently unstable, and keeps falling over. Stopping it from falling over requires machinery such as gyroscopes or reaction wheels, which add mass and complexity and require energy to run.
Two wheels are not enough. It is stable in the fore and aft directions, but still falls over sideways. It is difficult to fit a solar collector on a motorcycle.
Three wheels works. Each tyre touches the ground, even if the ground is uneven. The tyres contact the ground at the three corners of a triangle. If the wheels are arranged so that an imaginary pendulum hanging from the centre of mass is always inside this triangle then the car will not tip over. Achieving this is straightforward—don’t make the triangle too narrow, keep most of the weight down low, and put the driver in the middle.
Four wheels works, but when the car is on uneven ground you are relying on suspension movements to keep each of the tyres in contact with the ground. Uneven ground will try to twist the chassis, so you need to build a stiffer car that can resist these twisting forces. Stiffening the chassis adds mass. The extra wheel and suspension add mass. Extra mass requires extra energy to transport from Darwin to Adelaide.
The advantage of four wheels over three is that, for a given lane width and car length, there is more area within the rectangle formed by the wheels of a four-wheeled car than there is within the triangular area of a trike. If you want to carry a passenger, or cargo, four wheels may be better.
A common misconception is that a four-wheeled vehicle is inherently more stable than a three-wheeled vehicle. Determining stability is not as simple as counting the wheels. You must also consider the shape formed by the tyre contact patches, where the centre of mass is in relation to the tyre contact patches, the suspension, and the forces that the suspension puts on the car.
Five wheels works too; indeed the BWSC allows any number of additional wheels. However, there is a penalty of weight and complexity when going beyond four wheels, with no obvious advantage to be gained.
In 2021 we are giving Challenger teams the opportunity to explore the advantages and disadvantages of four wheels versus three wheels. We have also added regulations that require teams to ensure the stability of their cars, independent of the number of wheels. The Challenger class results will show whether four wheels are better than three. Will there be a clear winner in 2021, or could the optimum design take two or more events to emerge?
Associate Professor Peter Pudney
Emeritus Professor John Storey