To give a feeling of the magnitude of these forces, a hub engine with a 12mm axle producing 40 N-m of torque will exert a spreading force of just under 1000lb on every single dropout. A torque arm can be another piece of metal attached to the axle that may have this axle torque and transfer it further up the frame, hence relieving the dropout itself from taking 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 and prevent further rotation, by the time this occurs your dropout may currently be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it may go on flawlessly snug, or in some instances a small amount of filing could be essential for the plate to slide on. In conditions where in fact the axle flats happen to be a lttle bit narrower than 10mm and you are feeling play, it isn’t much of an issue, nevertheless, you can “preload” the axle plate in a clockwise way as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways preventing the torque plate from resting flat against the dropout. If this is actually the case, you will need to be sure to have a washer that meets inside the lip place. We make custom “spacer ‘C’ washer” because of this job, although lock washer that comes with many hub motors can often be about the proper width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing over the stainless band can help to make 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 deal.

However, in high ability 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 motor cables and potentially leading to the wheel to fall right out of your bike.

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