Why Are Wheels So Good?

Step 1 of 5

Dragging means constant grinding.

Slide a crate across the floor and every square centimetre of its base scrapes against the ground for the entire journey.

Step 2 of 5

A rolling wheel does not slide at all.

The bit of the wheel touching the ground is momentarily stationary. It sets down, pivots, and lifts away — it never scrapes.

Step 3 of 5

So it takes far less force to move.

Rolling friction is typically 10 to 100 times smaller than sliding friction. A crate you cannot budge becomes a cart you can push one-handed.

Step 4 of 5

The axle is the real invention.

A rolling log works but you must keep moving it round to the front. Fixing a wheel onto an axle keeps the rolling under you permanently.

Step 5 of 5

But you still need grip.

Wheels only work if they do not skid where they touch. That is why tyres are rubbery: low friction in the rolling direction, high friction sideways.

Trains use steel on steel because it is so low-friction — which is also why they need a mile to stop.