To give a feeling of the magnitude of the forces, a hub engine with 
a 12mm axle making 40 N-m of torque will exert a spreading force of just under 1000lb on every single dropout. A torque arm is certainly a separate piece of metal attached to the axle that may take this axle torque and transfer it additional up the frame, hence relieving the dropout itself from acquiring all the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is definitely loose, in that case axle can rotate some sum and the bolt will slide in the slot. Though it is going to bottom out and stop further rotation, by enough time this takes place your dropout may already be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it may go on flawlessly snug, or in some instances a tiny amount of filing may be necessary for the plate to slide on. In circumstances where in fact the axle flats will be a bit narrower than 10mm and you feel play, it isn’t much of an issue, but you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways preventing the torque plate from relaxing toned against the dropout. If this is the case, you should be sure to possess a washer that matches inside the lip region. We make customized “spacer ‘C’ washer” because of this job, although lock washer that is included with various 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 ultimate installation look more discrete and protect the paint job from getting scratched. We incorporate several bits of shrink tube with each torque arm package.
However, in high power systems 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 happens, the axle will spin freely, wrapping and severing off the engine cables and potentially leading to the wheel to fall correct out from the 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 measure of support against rotation. Oftentimes this is sufficient.
