Question

why the rubber tyre of the car sinks in the mud

Answer 1

It's the grip of the tyre on the road and is caused by the friction between the rubberand asphalt. The rougher the two surfaces, the higher the friction and the better the traction. ... The same principle applies to driving in mud and snow: the grooves in the tread expel the snow or mud from under the tyre

Answer 2

The key to a good tyre is traction. It’s the grip of the tyre on the road and is caused by the friction between the rubber and asphalt. The rougher the two surfaces, the higher the friction and the better the traction. Rubber and asphalt have a high coefficient of friction, so a smooth rubber tyre could get you along a perfectly dry road without disaster. The problem, as those of us who have face-planted outside the Singaporean Ministry of Education in our rubber flip-flops understand, is when it rains. When the roads are wet, the tyres are sitting on a layer of water and the friction between rubber and water is low (face-plantingly low).

This is where tyre treads come in. The grooves (or “voids”) on tyres are there to provide a channel for the water. Allowing the water to escape from underneath the tyre keeps the rubber in contact with the asphalt, maintaining good traction and avoiding slippage. The same principle applies to driving in mud and snow: the grooves in the tread expel the snow or mud from under the tyre. Typically, the deeper the grooves, the better the traction in wet conditions.