To give a feeling of the magnitude of these forces, a hub electric motor with a 12mm axle making 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is certainly a separate piece of metal mounted on the axle that may take this axle torque and transfer it additional up the frame, therefore relieving the dropout itself from taking all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is certainly loose, in that case axle can rotate some quantity and the bolt will slide in the slot. Though it is going to bottom out and prevent further rotation, by enough time this happens your dropout may already be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it may go on flawlessly snug, or sometimes a tiny amount of filing may be essential for the plate to slide on. In circumstances where the axle flats happen to be somewhat narrower than 10mm and you feel play, it is not much of an issue, but you can “preload” the axle plate in a clockwise way as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and prevent the torque plate from seated flat against the dropout. If this is the case, you will need to be sure to get a Torque Arm china washer that meets inside the lip area. We make customized “spacer ‘C’ washer” because of this job, although lock washer that comes with many hub motors is normally about the right width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless band can produce the final installation look more discrete and protect the paint job from getting scratched. We contain several pieces of shrink tube with each torque arm bundle.

However, in high electrical 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 occurs, the axle will spin freely, wrapping and severing off the electric motor cables and potentially leading to the wheel to fall right out of 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. Oftentimes this is sufficient.