To give a sense 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 slightly below 1000lb on each dropout. A torque arm is usually another piece of metal mounted on the axle that may consider this axle torque and transfer it even more up the frame, hence relieving the dropout itself from spending all the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is normally loose, then axle can rotate some volume and the bolt will slide in the slot. Though it is going to bottom out and stop 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 bit of play, it could go on flawlessly snug, or in some cases a little amount of filing may be necessary for the plate to slide on. In circumstances where in fact the axle flats will be somewhat 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 quick release “lawyer lips” which come out sideways preventing the torque plate from sitting down smooth against the dropout. If this is the case, you should be sure to possess a washer that meets inside the lip area. We make custom “spacer ‘C’ washer” for this job, though the lock washer that comes with many hub motors can often be about the proper width and diameter.
For the hose-clamp unit, a small Torque Arm china length of heat-shrink tubing over the stainless band can produce the final installation look even more discrete and protect the paint job from getting scratched. We involve several pieces of shrink tube with each torque arm package.
However, in high ability systems that generate a whole 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 motor cables and potentially triggering the wheel to fall proper out from the bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.