To give a feeling of the magnitude of these forces, a hub motor with a 12mm axle producing 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is definitely a separate piece of metal mounted on the axle that may have this axle torque and transfer it additional up the frame, therefore relieving the dropout itself from currently taking all of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is usually loose, after that axle can rotate some amount and the bolt will slide in the slot. Though it will eventually bottom out and stop 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 bit of play, it may go on correctly snug, or occasionally a tiny amount of filing could be necessary for the plate to slide on. In scenarios where the axle flats are a 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 path as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and stop the torque plate from seated flat against the dropout. If this is the case, you will need to be sure to have a washer that fits inside the lip spot. We make customized “spacer ‘C’ washer” for this job, although lock washer that is included with many hub motors can often be about the right width and diameter.
For the hose-clamp version, a small amount of heat-shrink tubing over the stainless steel band can make 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 offer.
However, in high electrical power devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present may 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 leading to the wheel to fall proper out from the 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.