To give a sense of the magnitude of the forces, a hub motor with a 12mm axle producing 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm is usually another piece of metal attached to the axle which can consider this axle torque and transfer it even more up the frame, as a result relieving the dropout itself from choosing 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, 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 takes place your dropout may currently 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 could go on correctly snug, or occasionally a small amount of filing could be essential for the plate to slide on. In conditions where in fact the axle flats will be a little narrower than 10mm and you are feeling play, it isn’t much of a concern, nevertheless, you can “preload” the axle plate in a clockwise direction as you tighten everything up.
Many dropouts have quick release “lawyer lips” which come out sideways and stop the torque plate from sitting down smooth against the dropout. If this is the case, you will need to be sure to have a washer that suits inside the lip location. We make customized “spacer ‘C’ washer” for this job, though the lock washer that is included with a large number of hub motors is normally about the right width and diameter.
For the hose-clamp version, 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 involve several bits of shrink tube with each torque arm package deal.
However, in high power devices that generate a whole lot of torque, or in setups with weak dropouts, the forces present may exceed the material power 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 right out of 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.