To give a sense of the magnitude of the forces, a hub electric motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every single dropout. A torque arm is normally 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 bringing all the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is 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 the time this happens your dropout may already be damaged.
The tolerances on electric motor axles may differ from the nominal 10mm. The plate may slide on freely with a little of play, it may go on properly snug, or in some instances a little amount of filing may be essential for the plate to slide on. In situations where in fact the axle flats are a little 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 speedy release “lawyer lips” which come out sideways and stop the torque plate from resting flat against the dropout. If this is the case, you will need to be sure to possess a washer that meets inside the lip region. We make custom “spacer ‘C’ washer” because of this job, although lock washer that comes with a large number of hub motors can often be about the proper width and diameter.
For the hose-clamp version, a small length of heat-shrink Torque Arm china tubing over the stainless steel band can help to make the final installation look more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm package deal.

However, in high electric power systems that generate a whole lot of torque, or in setups with weak dropouts, the forces present can exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the engine cables and potentially causing the wheel to fall right 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 provide some way of measuring support against rotation. Oftentimes this is sufficient.