Good question, we get this a lot. If they are alloy rims you are safe to use an acidic wheel cleaner to help break up a bulk of the build up. My suggestion would be for Poorboy's Spray & Rinse Wheel Cleaner. This should break up a majority of the brake dust leaving you to get the real embedded stuff off. From there I'd go with P21s Finish Restorer which will take a little bit of elbow grease but should help restore the wheel to a nice luster once again and rid caked on brake dust. If you want to help prevent this from happening again, Poorboy's Wheel Sealant is a great investment to make in the long run to minimize cleaning time as well as reducing the amount of brake dust build up. A similar product people may have heard of is Wheel Wax, however I feel the Poorboy's Wheel Sealant is more durable for an extra dollar.

Let me know if you have any other questions.

Sincerely,

George @ Detailed Image