The absolute best way to clean plastics is take them off bike and spray them with vinyl siding power wash cleaner. I use Krud Kutter. It makes my plastics clean as new no matter how many scratches are filled with dirt and grime. You can do it on the bike, but it is strong stuff and can react with some metals. I follow the directions on the back and make it slightly stronger. I mix it up and put it in a bug sprayer, spray then power wash. It literally makes my whole bike frame engine and all look like the bikes in the pits at supercross. NEW.