To give a sense of the magnitude of the forces, a hub electric motor with a 12mm axle making 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm can be a separate piece of metal attached to the axle that may have this axle torque and transfer it further up the frame, hence relieving the dropout itself from bringing all of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut can be loose, in that case axle can rotate some quantity and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by the time this occurs your dropout may previously be damaged.
The tolerances on motor axles may differ from the nominal 10mm. The plate may slide on freely with a little of play, it could go on perfectly snug, or sometimes a tiny amount of filing could be necessary for the plate to slide on. In conditions where the axle flats happen to be a bit narrower than 10mm and you feel play, it is not much of an issue, nevertheless, you can “preload” the axle plate in a clockwise route as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and stop the torque plate from relaxing toned against the dropout. If this is the case, you will want to be sure to get a washer that fits inside the lip location. We make customized “spacer ‘C’ washer” for this job, although lock washer that is included with various hub Torque Arm china motors can often be about the proper width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless band can help to make the ultimate installation look even more discrete and protect the paint job from getting scratched. We include several bits of shrink tube with each torque arm program.
However, in high electricity systems that generate a lot of torque, or in setups with weak dropouts, the forces present may exceed the material durability and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the motor cables and potentially causing 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 provide some way of measuring support against rotation. In many cases this is sufficient.