To give a feeling of the magnitude of the forces, a hub motor with a 12mm axle making 40 N-m of torque will exert a spreading force of just under 1000lb on each dropout. A torque arm is a separate piece of metal mounted on the axle that may take this axle torque and transfer it further up the frame, therefore relieving the dropout itself from bringing all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is usually loose, in that case axle can rotate some volume and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by the time this takes place your dropout may previously be damaged.
The tolerances on electric motor axles may differ from the nominal 10mm. The plate may slide on freely with somewhat of play, it could go on flawlessly snug, or sometimes a small amount of filing could be necessary for the plate to slide on. In conditions where in fact the axle flats happen to be a lttle bit narrower than 10mm and you feel play, it isn’t much of an issue, but you can “preload” the axle plate in a clockwise route as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways preventing the torque plate from resting flat against the dropout. If this is the case, you should be sure to get a washer that matches inside the lip region. We make customized “spacer ‘C’ washer” for this job, although lock washer that is included with a large number of hub motors can often be about the proper width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing over the stainless band can generate the ultimate installation look more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm program.
However, in high electric power devices that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material power and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the motor cables and potentially creating the wheel to fall right from the bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and provide some measure of support against rotation. Oftentimes this is sufficient.