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 dropout. A torque arm is normally another piece of metal mounted on the axle which can take this axle torque and transfer it even more up the frame, as a result relieving the dropout itself from choosing each of the stresses.
Tighten the 1/4″ bolt between your axle plate and the arm as snug as possible. If this nut is loose, in that case axle can rotate some sum and the bolt will slide in the slot. Though it is going to bottom out preventing further rotation, by enough time this takes place your dropout may already be damaged.
The tolerances on engine axles may differ from the nominal 10mm. The plate may slide on Torque Arm china freely with a bit of play, it may go on flawlessly snug, or sometimes a tiny amount of filing could be essential for the plate to slide on. In situations where the axle flats are somewhat narrower than 10mm and you are feeling play, it isn’t much of an issue, nevertheless, you can “preload” the axle plate in a clockwise course as you tighten everything up.
Many dropouts have speedy release “lawyer lips” which come out sideways and stop the torque plate from resting flat against the dropout. If this is the case, you should be sure to get a washer that fits inside the lip place. We make custom “spacer ‘C’ washer” because of this job, though the lock washer that is included with many hub motors is normally about the proper width and diameter.
For the hose-clamp style, a small amount of heat-shrink tubing over the stainless band can make the ultimate installation look even more discrete and protect the paint job from getting scratched. We include several pieces of shrink tube with each torque arm bundle.
However, in high electrical power systems that generate a whole lot of torque, or in setups with weak dropouts, the forces present can exceed the material power and pry the dropout open. When that happens, the axle will spin freely, wrapping and severing off the engine cables and potentially triggering the wheel to fall right out of your bike.
In most electric bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and offer some measure of support against rotation. In many cases this is sufficient.