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 slightly below 1000lb on each dropout. A torque arm is another piece of metal attached to the axle that may have this axle torque and transfer it additional up the frame, hence relieving the dropout itself from bringing all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is certainly loose, after that axle can rotate some amount and the bolt will slide in the slot. Though it is going to bottom out and prevent further rotation, by the time this occurs your dropout may already be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it could go on flawlessly snug, or sometimes a little amount of filing could be essential for the plate to slide on. In circumstances where in fact the axle flats are a bit narrower than 10mm and you are feeling play, it is not much of an issue, nevertheless, you can “preload” the axle plate in a clockwise direction as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways and stop the torque plate from sitting down flat against the dropout. If this is the case, you should be sure to have a washer that fits inside the lip region. We make custom “spacer ‘C’ washer” because of this job, although lock washer that is included with various hub motors is often about the right width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless steel band can produce the ultimate installation look even more discrete and protect the paint job from getting scratched. We involve several bits of shrink tube with each torque arm offer.

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 triggering the wheel to fall proper 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 offer some way of measuring support against rotation. In many cases this is sufficient.