actually i didn't put any spacers for the caliper as the rotor is too near the frame. Instead, i put a spacer between the hub and the dropout...anyway,i placed another spacer at the other side so this is not the cause of the problem. The problem is the brake pad touches the rotor only at certain position ( i mean the wheel ) If what Elliot said is correct [the pads often rub against the disc a bit, and according to the manual], then i am not so worried...