Science Puzzle
Why Do Tractors Have Such Wide Tyres?
Farm tractors are heavy machines. A big one can weigh more than ten tonnes, and it spends much of its life in soft, wet fields. Look at its tyres and they're enormous: some are nearly a metre wide, far wider than a car's.
Why does a tractor need such wide tyres?
The Answer
Wide tyres spread the tractor's weight over a bigger patch of ground, so each bit of soil carries less of it and the tractor doesn't sink.
What matters to soft ground isn't the total weight; it's the pressure, which is the force divided by the area it's spread over. Double the area touching the ground and the pressure halves. A narrow tyre puts the tractor's weight onto a thin strip of mud, which gives way, and the wheel digs itself in. A wide tyre puts the same weight onto a much bigger patch, and the mud holds.
There's a second reason farmers care. Heavy pressure squashes soil, closing up the tiny air spaces that roots and rainwater need. Squashed soil can stay damaged for years and grows poorer crops, so many farmers let some air out of their tyres before working in the field. A softer tyre flattens out and touches even more ground.
The same idea is at work in snowshoes and in the caterpillar tracks on a tank. It works the other way round too. A stiletto heel puts a person's weight onto an area smaller than a fingernail, so it can press harder on a floor than an elephant's foot does.
The principle: Pressure is force divided by area. The same weight spread over a bigger area presses less on each part of it, so it sinks less into soft ground.