To give a feeling of the magnitude of these 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 every dropout. A torque arm is certainly 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 acquiring all of 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 sum and the bolt will slide in the slot. Though it will eventually bottom out and stop further rotation, by the time this happens your dropout may previously be damaged.
The tolerances on engine axles may differ from the nominal 10mm. The plate may slide on freely with a little of play, it could go on properly snug, or in some cases a little amount of filing may be necessary for the plate to slide on. In situations where the axle flats will be a lttle bit narrower than 10mm and you feel play, it is not much of an issue, but you can “preload” the axle plate in a clockwise path as you tighten everything up.
Many dropouts have speedy release “lawyer lips” that come out sideways and stop the torque plate from relaxing flat against the dropout. If this is the case, you will want to be sure to possess a washer that meets inside the lip spot. We make customized “spacer ‘C’ washer” because of this job, although lock washer that comes with a large number of hub motors is often about the right width and diameter.
For the hose-clamp unit, a small amount of heat-shrink tubing over the stainless band can generate the ultimate installation look more discrete and protect the paint job from getting scratched. We contain several pieces of shrink tube with each torque arm package.
However, in high electrical power 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 happens, the axle will spin freely, wrapping and severing off the engine cables and potentially creating the wheel to fall proper out from the bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and provide some way of measuring support against rotation. In many cases this is sufficient.