To give a sense of the magnitude of these forces, a hub electric motor with a 12mm axle producing 40 N-m of torque will exert a spreading force of slightly below 1000lb on every dropout. A koupl bra is definitely a separate piece of metal mounted on the axle which can take this axle torque and transfer it even more up the frame, thus relieving the dropout itself from choosing all of the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is normally loose, in that case axle can rotate some quantity 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 already be damaged.
The tolerances on electric motor axles can vary from the nominal 10mm. The plate may slide on freely with a little of play, it may go on perfectly snug, or occasionally a small amount of filing may be essential for the plate to slide on. In circumstances where in fact the axle flats happen to be a lttle bit narrower than 10mm and you are feeling play, it is not 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” that come out sideways preventing the torque plate from sitting down toned against the dropout. If this is the case, you will need to be sure to get a washer that fits inside the lip spot. We make custom “spacer ‘C’ washer” for this job, though the lock washer that comes with a large number of hub motors is often about the proper width and diameter.
For the hose-clamp model, a small amount of heat-shrink tubing over the stainless steel band can generate the final installation look more discrete and protect the paint job from getting scratched. We consist of several pieces of shrink tube with each torque arm offer.
However, in high power systems that generate a lot of torque, or in setups with weak dropouts, the forces present can exceed the material durability and pry the dropout open. When that occurs, the axle will spin freely, wrapping and severing off the electric motor cables and potentially triggering the wheel to fall proper from the bike.
In most electrical bicycle hub motors, the axle is machined with flats on either side which key into the dropout slot and provide some measure of support against rotation. Oftentimes this is sufficient.