Status-Q

I have a theory, which I’d like to expound, and my clever readers can tell me whether it’s right or wrong.

It came to me in the mid-1980s, this theory, when I got my first car: a Hillman Imp, which I purchased for £90. It had no manuals, and, actually, no ignition key: you had to put an old screwdriver into the slot where the ignition switch had once been, and twist it to start…

Anyway, because I had no documentation, I just guessed at the appropriate pressures for the tyres, and pumped them up to 30 PSI all round. Several months later, in mid-winter, I finally found out that the front of the Imp was so light that the front ones were only supposed to be at 16 PSI. This, I imagine, accounted for the fact that when it was snowing, turning the steering wheel didn’t do very much on its own, and I had to make handbrake turns to get around some of the tighter corners on my daily commute.

And this came back to me, a quarter of a century later, when I got my first electric car: a BMW i3, which had large and very narrow tyres — almost like motorbike wheels — to improve the airflow..

Some of my friends and neighbours said they’d prefer bigger, fatter tyres so there would be more rubber in contact with the road.

But I pointed out that, to a first approximation, this shape didn’t make any difference. And here’s my reasoning:

The thing keeping your car off the ground is the air pressure in the tyres pressing on the patch of tyre that’s in contact with the road. That downward force must equal the pressure in the tyre times the area of contact. Or, to put it another way, if your tyres are at 40psi and your car weighs 2000 lbs, then you must have roughly 50 sq. ins. in contact with the road – 12.5 sq ins per tyre – whatever your shape of tyre.

So, for the same pressure and weight of vehicle, if your tyres are wide, you’ll get a wide, short patch touching the ground. If they’re narrow, you’ll get a longer, thinner patch, but they should be about the same size in either case. Double the pressure, and you’ll halve the area in contact with the ground. Halve it, and you’ll double the area. But buying wider tyres will only make you look more macho.

Now, this is an approximation, partly because tyres aren’t perfect spherical balloons and the area doesn’t change smoothly with the pressure, partly because the forces go towards stretching the rubber as well as supporting the car, and partly because the pressure is providing rigidity to the structure of the tyre, so you get some support from the vertical bits of rubber as well, but the basic principle holds: it’s primarily the pressure, not the shape of the tyre, that’s important in determining how large an area touches the road.

Now, in practice, I came to rather like the long, thin tyres of the i3: they cut through water and slush very well, and were less likely to aquaplane when you encountered a puddle. (A counter-argument, I guess, would be that if there’s a rut on the road, you’re likely to have a higher proportion of your tyre on it for longer if your contact area is long and thin.)

So, yes, the best way to get a better grip is generally to lower the pressure, if you can face the resulting fuel costs. However, even this isn’t as simple as it may seem, because you may remember from your school physics lessons that it’s the perpendicular force times the coefficient of friction that counts; Guillaume Amontons showed in the 17th century that if you’re sliding two surfaces over each other, the area of contact isn’t important – it’s the force with which they’re being pressed together.

If we had perfectly flat roads, increasing the area in contact with the ground would make little difference. But sadly, the roads are getting ever less flat around here, so dropping the pressure a bit will not only get you a better grip as winter approaches… it’ll make the potholes more comfortable too.