Tyres determine acceleration of cars; not engine power

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 July 2023

On September 23, 2022, a team from the University of Stuttgart got into the records books for accelerating an electric car from zero to 100km/h in 1.461 seconds. For comparison, the best commercially available, high-performance cars do it in about 2.8 seconds or more. Formula-1 race cars have been clocked at about 2.5s.

Interestingly, the maximum acceleration of a car depends more on the friction between the tyres and the road than on the power of the engine. Think about it, no matter how powerful the engine, if the road is slippery, the wheels will just skid on the surface and the car will not go anywhere.

So, the question then is this: how much friction is there between the road and the tyre? Friction between two surfaces is measure by the coefficient of friction. This is the ratio of the frictional force to the compression force between the surfaces.

On a dry road with new and warmed-up tyres, the coefficient of friction is almost one. That is, the maximum frictional force is approximately equal to the force of gravity acting on the car. Therefore, if the accelerating force is greater than the car’s gravitational force, the tyres will slip and spin as if it was on a slippery surface.

It follows, therefore, that the greatest acceleration attainable by any car on a normal road surface is the acceleration due to gravity. This has constant value of about 9.8m/s per second for all objects – heavy or light.

A better way of conceptualising this figure is to convert it to kilometres per hour. An object traveling at 9.8m/s for one hour (3,600 seconds), covers a total of 35,280m or about 35km. Therefore, the acceleration due to gravity on the earth is about 35km/h per second. In other words, after every one second, the speed increases by 35km/h. from zero to 35km/h, to 70km/h to 105km/h and so on.

So, how much time would it take to reach 100km/h starting from zero accelerating at 35km/h per second? The answer is simply 100km/h divided by 35km/h; this comes to 2.86s. This is the shortest time for a car to get from zero to 100km/h.

How then did the University of Stuttgart team manage 1.461s? The answer is that that they must have used special tyres that have greater grip than usual. That is, their coefficient of friction with the road was more than one. Clearly then, contrary to popular belief, it is the nature of the tyres that determine a car’s maximum acceleration, not the power of the engine!