Bigger tyres won’t make your car faster!

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

20 March 2016

Contrary to what you might expect, The Philosophical Magazine is not a publication for philosophers! It is one of the oldest and most pretegious academic journals for physicists. Its name is probably an appreciation of the fact that, even though physics is the study of physical things, the subject can involve itself with serious philosophical discussions.

This came to mind after last week’s article where I stated that a vehicle with large tyres does NOT travel faster than another with smaller ones when the two are doing the same speed. Several readers wrote to me saying that, when they fitted their cars with larger tyres, they moved faster.

Well, the question is: what do we mean when we say that a car is doing 80km/h? Is it that the speedometer pointer is at the number 80 or is it that, if the car continues in that state of motion for one hour, it will cover a distance of 80km? That sounds like a philosophical question!

We must first appreciate that the speedometer is just a maesuring instrument and, like all measuring instruments, its measurements are never correct; it is bedeviled with errors. Nevertheless, it does give fairly good readings for general motoring.

Reader Matu Gachanja notes that, if a car designed with 175-60-14 tyres is fitted with 185-70-14, then, when driven with the speedometer pointing at 100km/h, it will actually be travelling at 103km/h. I agree; but that’s not the point I was making.

The notion I was dispelling was the one that says that a bus moving at 80km/h moves faster than a car at 80km/h because of the bigger tyres. That is totally not true! Still, in Matu’s example, we could ask a further question: what is the speed of the car? Is it 100km/h or 103km/h?

Of course it is 103km/h; therefore, if my car has the design tyres and I drove it at 103km/h (never mind how I will find 103 in the normal speedometer), Matu would not be able to overtake me. Apparently, then, fitting bigger tyres does not make the car go faster; it just adds errors to the speedometer reading!

Worse still, the bigger tyres are more difficult to rotate. So, the engine has to apply more force to move the car. Consequently, acceleration is reduced and so is the ability to go up hills. Fuel consumption also goes up.

To understand what’s happening, think about how we open doors: we normally hold the farthest edge from the hinge line. If you tried pushing the door at a point close to the hinges, you will need much greater force. It is the same effect when a car is fitted with bigger tyres…and you thought the “load-fulcrum-pivot-effort” you were taught in school was a waste of time!

The only justifiable reason you should think of fitting bigger tyres is to increase the height of your car so that it can handle potholes and bumps without scraping the underside.