The rear rack I just purchased is supposed to fit both road and mtn bikes. I bought this for my mountain bike, but the rack doesn't seem to fit. I just realized that I have 135mm rear end on the mtn bike, and the rack is probably skinny enough to fit a 120mm rear end on a road bike.

Basically, the rack is too skinny to fit the back end of my mtn bike. Can I just install it and let the rack stretch into place? What can I do?