Depends on the rear of the bike. Seat post racks are made for that. But a universal rack with do just fine if you have all the eyelits

I've got this on my road bike, 700c and I love it:

http://www.nashbar.com/bikes/Product...7_10000_202601

It's cheap, strong, adjusts like crazy, includes a lot of hardware (even if you don't have eyelits on the frame, but you do need them on the axle mount)
The bottom adjusts to different heights and I think the lowest one is 24", I'm too lazy to open my door to look.