If you mount a hub motor that is over 500 watts on any bike frame you’re going to want to use a Torque arm to prevent your dropouts from opening up under load and in some cases catastrophically failing. There are a couple good options from Grin Tech that involve hoseclamps (the v3 and 4v torque arms) but no Torque Arm I’ve seen yet is quite as sexy as Dr Bass’s Torque Arms from Endless-Sphere.
The advantages of Dr Bass’s torque arms are that they are much more stealthy than other brands that rely on hose-clamps. The disadvantage is that once you epoxy it on, it isn’t ever going to come off again. If you want to mount your old non-motorized wheelset again with a QR you’ll have to find one with a slightly longer axle QR than what you were using before you put on the TA.
I would advise you grind off the paint on the side of the dropout that you want to mount the TA on. I did a test fit with the hub motor and rim then wrapped the axle on the side of the hub motor on the TA side with a ziplock baggy. You can put the nut over the end of the axle with the bag on it and tighten it down to about 1/5 the torque you would usually mount the wheel with. Tighten the opposite side nut as well and give it 24 hours before you take the motor off again. Keep the temps in the 70+ degree range or the epoxy won’t set right. Mix the epoxy till you get bored doing it, then mix it for a few more minutes. Wear nitrile gloves when using the epoxy to keep it off your skin.
Dr Bass advises using grease instead of a baggy, but that ends up being pretty messy and the grease can accidentally mix with the epoxy and keep it from hardening right. A ziplock plastic baggy seems to control the chaos and still has pretty good results.
If there is room on the inside of the dropouts, you can mount it there to hide it a little better, but you might have to really fudge to get the disk brake to line up. If I have to move the disk brake I split the difference and put 1/2 the washers on the rotor and the other half on the brake mounting bracket. You’ll need to use longer bolts for both which I usually get from Mcmaster Carr. Longer disk mounting bolts (16mm instead of the standard 10mm) are here and longer 25mm disk brake mounting bolts are here. You have to get creative when mounting non-standard hub sizes on fatbike non-standard width dropouts. Be aware that if you try to mount the TA on the inside of a dropout you will not be able to mount the old non-motorized hub back into the dropout as it will be far too narrow to fit.
I’ve fit 145mm motors on 170mm dropouts by using a lot of washers and pushing the chainstays together a bit. Aluminum is harder to do this with than steel is as aluminum tends to break rather than gracefully bend when under a lot of tension. You can usually get a few mm of play with aluminum but don’t push your luck.
For through-hole axles you are out of luck mounting a hub motor unless you want to cut openings at the bottom of the through hole to slide the motor in and then use Dr Bass Torque arms glued on either side. You should be able to mount a 135mm Mac on a 150mm Bluto front fork with the TA glued to the inside of the fork, but I have yet to try it. Someone else destroy their $500 fork and send me pictures and I’ll give you your 15 minutes of fame.
If your front fork is steel you probably don’t need a TA until you hit over 250 watts. For anything over 500 watts on Front or Rear or any front mounted hub motor on a non-steel fork you should really use a TA. If you don’t know if your fork is steel use a magnet, if it sticks then it is steel or ChMo. That being said on beefy rear dropouts I’ve run up to 1000 watt Direct Drive motors on aluminum frames without TA (for testing purposes only) but I am insane and care very little about my life, so don’t do what I do. Do what I say.
I say if you must have a Torque Arm then I think these are the ones to use.