Im using 15 /38 don't forget to reteach the gearing to the bike, check swing arm angle, as there not good out of the box I made new dogbones 136mm to get the 12.7 deg reqd for racing, things change when you go bigger rear sprocket, you get more pro squat! not good, you want antisquat, if you don't understand look up traction dynamics on youtube an youll see for your self