I never had a problem with the various seat bags on my mountain bike. However, I found out the hard way that my inner thigh rubbed on the seat bag when road biking: wore a hole in my shorts! I think my shorts contacted the velcro loop part of the strap, and that rubbed through the shorts on a long ride. Now I just use the bags on the mountain bike and the commuter. Shorts are too expensive to take a chance!