It's not the power, it's the torque. A geared hubmotor has greater torque than a similar DD hubmotor, because of the gear reduction built into it. (just like downshifting on your pedal gears).
The design of your motor's axle won't allow a typical torque arm to be used. They're made to go on an axle that is flatted along it's entire length. Your axle's flatted area is "recessed" within the bearing retainer area of the side cover, so if you try to use a tyipcal torque arm, it would bind on the cover, and if you ground away the lip of that area to prevent that, it would then bind on the bearing itself (at least it's outer race) which you would not want to do any grinding on.
To use a standard torque arm, you'd need to have something else to mount it to, which would require modifying or replacing your axle, neither of which I'd recommend.
It would also be possible to create a torque arm that works like a socket wrench that slips over the end of the axle to reach the flatted area, but with a lever arm that is displaced outboard outside the cover lip--but to do that, the socket portion would have to go "thru" the dropouts, and that would require cutting the dropouts (enlarging them) to accomodate it, or removing the dropouts entirely and creating another way to secure the wheel to the fork...neither of which I would recommend.
So...best I could say is try it out and see. As long as the controller you're using does not not output power greater than the motor and it's mounting system was made for, and it has "soft start", meaning it ramps up the power to the motor when applying throttle or PAS, or you start pedalling and moving first then apply power, it shouldn't apply undue stress to the fork...as long as you keep in mind that none of the bicycles that don't come with motors were designed with resisting motor torque of any kind in mind. (generally even those that do come with motors on the front fork don't have forks any different from non-powered bikes, and are thus not designed for the motor torque either).
ChrisOfBristol said:
I've got a mountain bike with aluminium front forks and a 250W motor. The motor shaft has a 4mm long, 12mm diameter step with 10mm flats for Bafang-style torque washers. However I have read elsewhere that on aluminium forks the washer can force apart the dropouts so I should use a torque arm. I'm sure that would be a good idea with a more powerful motor, but I'm not sure with 250W. What is your view?
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