Torque Arm

To give a sense of the magnitude of these forces, a hub motor with a 12mm axle generating 40 N-m of torque will exert a spreading force of slightly below 1000lb on each dropout. A torque arm is certainly a separate piece of metal attached to the axle which can consider this axle torque and transfer it further up the frame, thus relieving the dropout itself from choosing all the stresses.
Tighten the 1/4″ bolt between the axle plate and the arm as snug as possible. If this nut is usually loose, then axle can rotate some volume and the bolt will slide in the slot. Though it will eventually bottom out preventing further rotation, by enough time this happens 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 bit of play, it could go on flawlessly snug, or in some instances a small amount of filing may be necessary 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 an issue, but 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 prevent the torque plate from sitting smooth against the dropout. If this is actually the case, you will want to be sure to get a washer that fits inside the lip spot. We make customized “spacer ‘C’ washer” for this job, although lock washer that is included with a large number of hub motors is normally about the right width and diameter.
For the hose-clamp model, a small length of heat-shrink tubing over the stainless band can make the final installation look more discrete and protect the paint job from getting scratched. We contain several bits of shrink tube with each torque arm package deal.

However, in high vitality systems that generate a lot of torque, or in setups with weak dropouts, the forces present may exceed the material strength 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 into the dropout slot and offer some way of measuring support against rotation. Oftentimes this is sufficient.

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