I found with mine that a longer one, 100mm? works better because I can rotate the u bolt clamp on the handlebar away and until the arm sits on the clamp at the right angle. Then it doesn't move down any more because it can't. With the shorter arm that I also have, it can move around on the move and I'm often tightening the clamp through the day. This was with the 60CSX and seems to be the same with the Montana. The end position of the GPS is the same, but the way I did makes it more stable with the longer arm. Suppose you want a picture now?