I commute with a bike with 20" wheels and I have to be careful the spring on the arm is strong enough to come all the way down to hold the 20" wheel securely. About one in ten aren't so I move the bike to the other position in the rack if it is available. Always give the bike a good tug and jiggle it a bit to make sure the arm is secure. The drivers tell me there have been cases where the bikes feel off and were run over by the bus - not a pretty sight I am sure. I also have to remember to shift into high gear so the derailleur won't bend against the rack. These days I rarely put it on the bus though, just when I'm running late.