To give a sense of the magnitude of the forces, a hub engine 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 certainly another piece of metal mounted on the axle that may have this axle torque and transfer it additional up the frame, therefore relieving the dropout itself from spending 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, then axle can rotate some volume and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by the 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 may go on correctly snug, or in some cases a small amount of filing may be necessary for the plate to slide on. In scenarios where in fact the axle flats are a bit narrower than 10mm and you feel play, it isn’t much of a concern, but you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and stop the torque plate from sitting down flat against the dropout. If this is actually the case, you should be sure to get a washer that meets inside the lip spot. We make customized “spacer ‘C’ washer” because of this job, although lock washer that comes with various hub motors can often be about the right width and diameter.
For the hose-clamp version, a small amount of heat-shrink tubing over the stainless band can make the ultimate installation look more discrete and protect the paint job from getting scratched. We consist of several bits of shrink tube with each torque arm offer.
However, in high electric power systems 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 happens, the axle will spin freely, wrapping and severing off the motor cables and potentially triggering the wheel to fall right out of 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 offer some measure of support against rotation. In many cases this is sufficient.