Lesson 2
Tyres, Pressure and Grip
See how four small patches of rubber carry the whole car, and read pressure, wear and tread depth from them.
Four postcard-sized patches
Everything your car does, from pulling away to steering to braking, reaches the road through four patches of rubber, each roughly the size of a postcard. That is all the grip there is. The best brakes in the world can only use as much grip as those patches can hold.
The air inside sets the size of each patch. A tyre flattens against the road until the pressure acting over the patch carries that wheel’s share of the car’s weight, so the patch area is roughly the load divided by the pressure. For a 1,500 kg car at 2.3 bar, that works out at about 160 cm² per tyre. The tyre’s own stiffness changes the real figure, but the rule holds: change the pressure and you change the patch.
You can check almost everything in this lesson standing beside the car. Don’t crawl underneath to look at the inner side of a tyre; if you can’t see it, a tyre fitter can.