Torque arm
A torque arm braces a hub motor's axle so it can't spin in the dropouts. It matters most on powerful motors, aluminum dropouts and bikes with regen braking.
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A torque arm is a metal plate and arm fitted to a hub motor's axle. Grin Technologies explains that when the motor drives the wheel, the axle tries to turn the other way, and the torque arm passes that load to the frame so the dropout doesn't carry it alone. Without one, the dropout can spread, the axle can spin out, the motor cables can be damaged and the wheel can come loose (Grin torque arms).
An axle that has turned in its dropout can damage the motor cable and let the wheel come loose. Stop riding and have a shop check the dropout, axle and cable if you see the axle has moved.
What it's used for
Grin treats a torque arm as needed on high-power setups, on aluminum dropouts, which can crack rather than bend, on bikes with regenerative braking, and on weak or shallow dropouts. It says a lower-power motor in steel dropouts with tight axle nuts may not need one (Grin). Refit it whenever you remove the wheel, as in Fix a flat on a rear hub motor wheel.
What to look for
- Axle size. Grin's plates fit 12 mm and 14 mm axles, and axle flats are nominally 10 mm, though real sizes vary (Grin).
- Dropout type. Horizontal or vertical dropouts, and any quick-release tabs, which may need a spacer washer so the plate sits flat (Grin).
- Front or rear. Front forks often have less clearance (Grin).
- Where it fixes to the frame. Fender eyelets, the seat stay or the fork leg decide which arm style you need (Grin).
- A snug fit. Grin says to tighten the plate to arm bolt as snugly as possible and the axle nuts firmly, and to recheck them after any wheel removal (Grin). See Torque and specs and Torque wrench.
Where to buy
- Your bike maker, if it offers one for your model.
- The component maker. Grin Technologies makes a range of torque arms (Grin).
- Independent specialists. See Where to buy e-bike repair parts.
- A local shop, through find a shop.
Guides that use it
Sources
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