The reason why the replica rims of you car turn to be black is that when braking the friction between the brake and the car produces some toner. They originally only lightly cover the surface of the replica wheels, but not always clear. Because the car is driving for a long time, replica wheels will be hot, and the heat causes the surface of the toner in the alloy wheels of coke to become the hard surface layer of dark brown, it is indeed beyond the water wash away.
Why only the front replica rims have and rear replica rims not have? That is because the brake, the vehicle center of gravity forward, all the weight to the front focus, resulting in a heavier burden on the front brake, the brakes are grinding down of toner will be a lot more reasons than the rear replica rims.
To avoid this phenomenon, when the car will also be diligent in washing the new wheels, a day to drive a vehicle at least once a week to wash replica wheels and replica rims, wet washed with water after the first, and then cleaning with a sponge brush, and then use plenty of water.
If the car has been running for some time, the replica rims and wheels are usually not diligent in cleaning and maintenance. The toner will be formed on the replica rims of the hard layer of dark brown coke, then use the more powerful wheel cleaner. Coke cleared to be patient, spray cleaning agents, you must wait a while, let the solvent to soften after coke, then coarse sponge or brush, then rinse with water the same.