What will be a good sprocket sizes to for racing MX ? I'm running 18's on the rear now with a 15t front and a 40t rear . I figured i would ask because this coming spring i'm going to be running some district 6 races and some local mx racing also . I'm going to be doing a complete overhaul to the drivetrain ( front sprocket, rear sprocket ,chain slider, chain rollers and rear chain guide .So any information would be helpful .