The wheel squeaked when driving. I saw that he didn't get caught on the handrails - he slacked off and fell sideways. I loosened the screw holding the wheel mount and saw that the wheel axle needed to be raised a little higher so it would fit snugly against the rail. Used a drop of epoxy on the bottom of the "U" and the problem was solved. Others must have the same problem as it is caused by the cast "U" being the wrong size.