To give a sense of the magnitude of the forces, a hub Torque Arm china electric motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm is certainly another piece of metal attached to the axle that may take this axle torque and transfer it further up the frame, thus relieving the dropout itself from acquiring all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is loose, then axle can rotate some quantity and the bolt will slide in the slot. Though it is going to bottom out and stop further rotation, by enough time this happens your dropout may previously be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it may go on correctly snug, or occasionally a tiny amount of filing could be essential for the plate to slide on. In scenarios where the axle flats will be a bit narrower than 10mm and you are feeling play, it is not much of a concern, nevertheless, 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 seated flat against the dropout. If this is actually the case, you should be sure to have a washer that meets inside the lip place. We make custom “spacer ‘C’ washer” for this job, although lock washer that is included with a large number of hub motors is normally about the proper width and diameter.
For the hose-clamp unit, a small amount of heat-shrink tubing over the stainless band can help to make the ultimate installation look even more discrete and protect the paint job from getting scratched. We incorporate 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 durability and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially creating the wheel to fall right out of the bike.

In most electrical 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.