What Car? Q&A - Why are front wheels dirtier than rears?

Q: I've noticed that my car's front wheels become much blacker than the rear ones. Why is this?
Kaia Hodnebo

A: It is likely that your car has a braking system featuring discs at the front and drums at the rear. Also, when you apply the brake pedal, the force is predominantly transmitted to the front wheels.

When you brake, the calipers clamp the discs to slow the car, and it is this process that generates particles of brake pad material. Because the discs are open to the air, these particles are deposited on to your wheels.

The rear wheels remain free of this dust because the braking system is enclosed inside the brake drum, sealing off the brake shoes and the dust they generate.

To ensure the efficiency of the rear brakes, the drums are removed when you have your car serviced, and the dust is blown out using compressed air.

If left for a long period of time, accumulated brake dust can cause irreparable damage to alloy wheels and plastic wheel trims.

It is important also to limit the use of alloy wheel cleaners containing acid, because this could leave your wheels vulnerable to corrosion. Using car shampoo little and often is the key to well maintained wheels.

Very careful application of car polish can be a chore, but should have a restorative effect. Remember also to test the brakes by applying them after cleaning or polishing your car, as water can render them dangerously ineffective when they are first applied.