What's that black powder on my tires?

Dear Car Talk

Dear Car Talk | Oct 01, 1989

Dear Tom and Ray:

I have a fairly minor cosmetic problem, but I would like to solve it. My wife's car is a 1988 Volvo 760 Turbo station wagon with 4-wheel disc brakes. On the front wheels only, a black powdery substance from the brakes discolors the wheel covers. It is easy enough to clean off, but I am curious as to why it doesn't happen on the Volvo's rear wheels or on my car, an Acura Legend coupe, which also has 4-wheel discs.

TOM: The black powder is residue from the brake pads. It comes out because the air which cools the brakes sends it out in that direction. There's a plastic shield you can buy that goes behind the wheel and stops the powder from getting out. That would eliminate the dust problem, but it might interfere with brake cooling.

RAY: The reason it doesn't happen on the rear wheels is that the rear brakes are smaller and do far less work than the front brakes. And the reason it happens on your wife's Volvo and not your Acura probably has to do with the design of the brakes, the wheels, and the air flow around the brakes.

TOM: We'd recommend against the brake shields unless they're Volvo approved. If they do interfere with brake cooling, you'll get poor braking at high speeds, your discs can warp, and you'll find this cosmetic problem has suddenly become a financial problem. I'd just wipe the powder off every couple of weeks and apply the money you're saving on brake jobs to that inevitable turbo replacement you're going to need.

Get the Car Talk Newsletter

Got a question about your car?

Ask Someone Who Owns One