To give a sense of the magnitude of these forces, a hub motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm is a separate piece of metal mounted on the axle that may take this axle torque and transfer it further up the frame, as a result relieving the dropout itself from spending all the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is usually loose, in that case axle can rotate some volume and the bolt will slide in the slot. Though it is going to bottom out preventing further rotation, by enough time this occurs your dropout may previously be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a lttle bit of play, it could 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 somewhat narrower than 10mm and you are feeling play, it is not much of a concern, nevertheless, you can “preload” the axle plate in a clockwise direction as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways preventing the torque plate from resting smooth against the dropout. If this is the case, you will want to be sure to possess a washer that fits inside the lip location. We make custom “spacer ‘C’ washer” because of this job, although lock washer that comes with various hub motors can often be about the proper width and diameter.
For the hose-clamp model, a small length 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 consist of several bits of shrink tube with each torque arm program.
However, in high electricity devices that generate a lot of torque, or in setups with weak dropouts, the forces present may exceed the material strength and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the electric motor cables and potentially leading to the wheel to fall right out of 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 way of measuring support against rotation. In many cases this is sufficient.