Torque Arm

To give a sense 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 slightly below 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 further up the frame, therefore relieving the dropout itself from spending all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is definitely loose, after that axle can rotate some sum and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by the time this happens your dropout may previously be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with somewhat 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 will be a bit narrower than 10mm and you feel play, it isn’t much of an issue, nevertheless, you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways and stop the torque plate from sitting down smooth against the dropout. If this is actually the case, you should be sure to have a washer that matches inside the lip area. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that comes with various hub motors is often about the proper width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless steel band can generate the final installation look even more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm bundle.

However, in high power systems 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 engine cables and potentially triggering the wheel to fall right out from the bike.

In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some measure of support against rotation. In many cases this is sufficient.