To give a feeling of the magnitude of these forces, a hub motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm can be another piece of metal mounted on the axle that may take this axle torque and transfer it additional up the frame, hence relieving the dropout itself from bringing each of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is definitely loose, in that case axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by enough time this occurs your dropout may already be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it may go on perfectly snug, or occasionally a small amount of filing could be essential for the plate to slide on. In conditions where the axle flats are a little narrower than 10mm and you feel play, it isn’t much of a concern, nevertheless, you can “preload” the axle plate in a clockwise way as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and stop the torque plate from sitting flat against the dropout. If this is actually the case, you should be sure to have a washer that suits inside the lip area. 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 right width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless band can make the final installation look more discrete and protect the paint job from getting scratched. We contain several bits of shrink tube with each torque arm program.
However, in high power devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present can exceed the material power and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially leading to the wheel to fall proper out 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 way of measuring support against rotation. In many cases this is sufficient.