The ones in front aren't covered and stay pretty clean on their own, though I do rinse them off when I clean the lot. The ones in the rear under a canopy get pretty grungy, probably from getting dragged through tire dressing. I pull them off two at a time and wash them in a SS bay, after which they look pretty much new again. I don't try to clean the insides anymore - I tried once and just created a bigger mess. The trash going through them keeps the insides cleaner than anyone will ever care.