To give a feeling of the magnitude of the forces, a hub engine with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm is certainly a separate piece of metal attached to the axle which can take this axle torque and transfer it additional up the frame, as a result relieving the dropout itself from acquiring all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is certainly loose, after that axle can rotate some amount 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 currently be damaged.
The tolerances on motor axles may differ from the nominal 10mm. The plate may slide on freely with a bit of play, it may go on perfectly snug, or in some cases a little amount of filing could be essential for the plate to slide on. In circumstances where in fact the axle flats happen to be somewhat narrower than 10mm and you feel play, it is not much of a concern, but you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways preventing the torque plate from relaxing smooth against the dropout. If this is actually the case, you will want to be sure to have a washer that fits inside the lip spot. We make custom “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 unit, a small length of heat-shrink tubing over the stainless band can generate the final installation look even more discrete and protect the paint job from getting scratched. We include several bits of shrink tube with each torque arm bundle.

However, in high electrical power devices 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 happens, the axle will spin freely, wrapping and severing off the motor cables and potentially creating the wheel to fall proper out of 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 way of measuring support against rotation. Oftentimes this is sufficient.