To give a feeling of the magnitude of the forces, a hub electric motor with a 12mm axle creating 40 N-m of torque will exert a spreading force of just under 1000lb on every dropout. A torque arm is another piece of metal attached to the axle that may have this axle torque and transfer it even more up the frame, hence relieving the dropout itself from taking each of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is normally loose, in that case axle can rotate some sum and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by enough time this takes place 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 lttle bit of play, it may go on correctly snug, or in some instances a little amount of filing could be necessary for the plate to slide on. In conditions where the axle flats will be a bit narrower than 10mm and you are feeling play, it isn’t much of a concern, but you can “preload” the axle plate in a clockwise route as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways and prevent the torque plate from sitting down flat against the dropout. If this is actually the case, you will need to be sure to get a washer that suits inside the lip region. We make customized “spacer ‘C’ washer” because of this job, although lock washer that comes with a large number of hub motors can often be about the right width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing over the stainless steel band can generate 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 program.
However, in high ability devices that generate a lot of torque, or in setups with weak dropouts, the forces present can 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 causing the wheel to fall right out of your bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.