To give a sense of the magnitude of these forces, a hub motor with a 12mm axle making 40 N-m of torque will exert a spreading force of just under 1000lb on each dropout. A torque arm can be a separate piece of metal attached to the axle which can take this axle torque and transfer it further up the frame, thus relieving the Torque Arm china dropout itself from currently taking all of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is certainly loose, then 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 currently be damaged.
The tolerances on motor axles can vary from the nominal 10mm. The plate may slide on freely with a bit of play, it may go on properly snug, or sometimes a tiny amount of filing may be essential for the plate to slide on. In circumstances where in fact the axle flats are 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 way as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways preventing the torque plate from relaxing toned against the dropout. If this is the case, you will need to be sure to possess a washer that meets inside the lip place. We make customized “spacer ‘C’ washer” for this job, though the lock washer that is included with various hub motors is normally about the proper width and diameter.
For the hose-clamp model, a small length 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 contain several bits of shrink tube with each torque arm bundle.
However, in high power systems that generate a whole 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 motor cables and potentially creating the wheel to fall right out from the bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some way of measuring support against rotation. In many cases this is sufficient.