I don't know if the SRAM Force is very different from the rival, but, I am using an 11-28 and 34-50 combination and and have no chain rub at all even at extreme cross-chain. Check the angle and height of your FD in relation to your chainset and make sure they are within tolerance. Check also the chain you are using. A wide/thick chain will also rub. Look for another LBS while you're at it.