Not sure if this is an SS mtb or road bike. Track ends or vertical dropouts with a singulator? If track ends, wheel could be slipping in the drops. May be using a quick release too. Change out the QR to axle and nuts and add a chain tensioner and you're golden. If vert dropouts, slippage could be chainstay flexing.
Last edited by progre-ss; 06-19-06 at 03:27 PM.