Torque Arm

To give a feeling of the magnitude of the forces, a hub electric motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A torque arm can be another piece of metal attached to the axle that may have this axle torque and transfer it further up the frame, as a result relieving the dropout itself from spending all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is certainly loose, then axle can rotate some sum and the bolt will slide in the slot. Though it is going to Torque Arm china bottom out and prevent further rotation, by the time this occurs your dropout may already be damaged.
The tolerances on engine axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it could go on flawlessly snug, or occasionally a small amount of filing could be necessary for the plate to slide on. In conditions where the axle flats are a little narrower than 10mm and you feel play, it is not 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” that come out sideways and stop the torque plate from relaxing toned against the dropout. If this is the case, you will need to be sure to have a washer that fits inside the lip location. We make customized “spacer ‘C’ washer” because of this job, though the lock washer that comes with many hub motors is often about the right width and diameter.
For the hose-clamp version, a small amount of heat-shrink tubing over the stainless steel band can help to make the ultimate installation look even more discrete and protect the paint job from getting scratched. We consist of several pieces of shrink tube with each torque arm package.

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 electrical bicycle hub motors, the axle is machined with flats on either side which key in to the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.