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?

That's what I did. I simply pulled the legs of the rack out a little and it fit just fine. For my MTB, I have disc brakes so I had to go with a special rack.

Mine spread to fit no problem. It was the rack that came with the Topeak seat for my daughter.

You could also mount it on the inside of the frame dropouts and use some plastic spacers to fill in the space between the rack and the frame.

I stretched mine to fit because fitting inside the dropouts interfered with my RD. No problems one year later...and it was a cheap rack anyway.

Thanks everyone for the good advice. I stretched it out a little and it fits good now!