To give a sense of the magnitude of these forces, a hub motor with a 12mm axle creating 40 N-m of Torque Arm china torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is usually another piece of metal mounted on the axle which can consider this axle torque and transfer it even more up the frame, thus 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 can be 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 already be damaged.
The tolerances on engine axles may differ from the nominal 10mm. The plate may slide on freely with a bit of play, it may go on correctly snug, or sometimes a tiny amount of filing could be essential for the plate to slide on. In conditions where the axle flats will be a lttle bit narrower than 10mm and you feel play, it is not much of an issue, nevertheless, you can “preload” the axle plate in a clockwise course 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 actually the case, you will need to be sure to possess a washer that fits inside the lip location. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that is included with various hub motors is normally about the right width and diameter.
For the hose-clamp version, a small length of heat-shrink tubing over the stainless steel band can produce the final installation look even more discrete and protect the paint job from getting scratched. We contain several bits of shrink tube with each torque arm package.
However, in high electrical power systems 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 engine cables and potentially causing the wheel to fall correct out of your bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.