EBike Torque Arms
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Torque arms or other torque controll devices are generally required on any hubmotor that has more than a few hundred watts of power. The motor design typicaly has an axle that is larger than the standard 10mm round axle on a bike. The axle then has two flat sides machined into it creating a flat 10mm wide axle.
The flat sides keep the axle from rotating within the bike frame dropouts. At some point, the motor will have enough power to bend steel bike frames dropouts, and crack aluminum frames. Then the wheel may leave the bike, possibly while moving fast.
Torque arms are used to add more strength to the bike frame, so the motor axle will not rotate damaging the frame. Often they are designed primarily for front hubmotors, but some designs fit both front and back, or may be easily adapted. Made of thick metal,they fit tightly on the flat sides of the axle, and have a long "arm" portion that gives it a wrench like leverage. The arm is attached to the frame with bolts or clamps preventing unwanted rotation of the motor axle.
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Opinion based on observations: Make sure the torque arm is as thick as possible (5mm or more if you have the axle length to spare) and that it fits *exactly*, to the point it's kinda not easy to put onto the axle, and that the axle hole is not longer than the axle's diameter.
If it is able to rock back and forth on the axle, then just like a wrong-size wrench on a nut, it'll just slip around it and strip the corners off the axle itself if it is harder than the axle, or gouge out the torque arm if the axle is harder.
If there is enough (any, really) extra length on the rounded axle end parts of the hole in teh torque arm, then the thinner part over the flats can be levered outward by the axle as it turns, and the same thing can happen as above.
If the torque arm is thin but hard, it will cut into the axle; if thin and soft the axle will cut into it. The thicker it is, the more surface area in contact with the flats of the axle, and the less likely it is to come loose.
You're better off with a torque arm just a hair softer than the axle so that if something has to give, it's the part you can replace. ;)