To give a feeling 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 just under 1000lb on every single dropout. A torque arm can be another piece of metal mounted on the axle that may consider this axle torque and transfer it further 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 certainly loose, then axle can rotate some volume and the bolt will slide in the slot. Though it is going to bottom out preventing further rotation, by the time this takes place 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 bit of play, it may go on properly snug, or in some cases a little amount of filing may be essential for the plate to slide on. In conditions where the axle flats happen to be somewhat narrower than 10mm and you are feeling play, it is not much of a concern, but you can “preload” the axle plate in a clockwise direction as you tighten everything up.
Many dropouts have quick release “lawyer lips” that come out sideways and stop the torque plate from sitting down toned against the dropout. If this is actually the case, you will need to be sure to possess a washer that matches inside the lip area. We make customized “spacer ‘C’ washer” for this job, although lock washer that comes with many hub motors is normally about the proper width and diameter.
For the hose-clamp style, a small length of heat-shrink tubing over the stainless steel band can make the final installation look more discrete and protect the paint job from getting scratched. We involve several bits of shrink tube with each torque arm program.

However, in high electric power systems that generate a whole lot of torque, or in setups with weak dropouts, the forces present may exceed the material durability and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the engine Torque Arm china cables and potentially triggering the wheel to fall right 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.