My bike is a folder with 16 inch wheels, and I have the opposite problem - the arm on the rack is too big. But a friendly bus driver figured out that I could put the arm over the frame rather than the wheel and it holds the bike very securely. Of course, I could (and have) just fold and carry the bike on board, but it is nice to get off the bike and put it in the rack, and to take it off the rack and get on and ride away without stopping to fold or unfold.