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Why are the front wheels of my car always much dirtier than the rear ones?

Guy Cox
Sydney, Australia

I suppose Rob Janes [who posed this question] normally drives forwards, so the front wheels of his car encounter all the muck and slush on the road first and either accumulate it or swish it out of the way. We could plan a scientific experiment here: would he be willing to drive only in reverse for a month to see what happens to his wheels?

David Muir
Edinburgh, UK

The vast majority of particulate pollution from modern cars comes from friction between the tyres and the road surface, as well as from the brakes within the wheels. The amount of polluting particles produced depends on vehicle weight, driving style and the material composition of the brakes, tyres and roads.

When drivers brake, their weight is thrown forwards, hence seatbelts. The weight of a braking vehicle is also projected forwards, so the job of slowing the vehicle lands mainly on the front brakes. This causes greater wear on these than on the back brakes, and thus more particle pollution from within the front wheels than the back, which makes the front wheels dirtier.

Harald Mandl
via email

The front wheels of cars usually have disc brakes (where a calliper squeezes pads against a disc to create friction). The abrasion of these disc brakes is deposited on the rims. Rear wheels typically have drum brakes (where a cylindrical drum that surrounds the brake shoes is attached to the wheel). Much less abrasion can escape from drum brakes because they are closed, while disc brakes are open systems.

If disc brakes are also mounted on the rear wheels, then there is much less abrasion here than there is in the front, because the mass of the vehicle shifts to the front when braking and therefore the front brakes are much more heavily loaded. That is why disc brakes at the rear are much smaller than those at the front.

Paul Gibbons
Banstead, Surrey, UK

This is because the front brakes work between two and 10 times harder than the rear ones.

The rear braking limit is set to the point where the rear wheels lock up. This happens at a much lower level than the front wheels due to forward-shifting weight while braking. So the front brakes grind away five times more brake pad material and this gets deposited on the front wheels.

Martin van Raay
Culemborg, the Netherlands

There are special products to clean brake dust off aluminium alloy wheels. If this isn’t removed regularly, it will damage the surface of alloy wheels as the brake dust etches and stains the surface. If this damage has occurred, then only a more expensive treatment by a specialist will restore the wheels to their original splendour.

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