CrawlingWhisper wrote: ↑
Feb 19, 2018 3:41 pm
I run a Mac 10T hub motor on 52V 30Amp (1560W) and use one GrinTech Rev4 torque Arm:
As mentioned above, a pic of the mount would be good. With a solid mount for the TA it should be okay - but you need to realize that any failure in that single TA is going to have catastrophic frame consequences. Two TAs may not always be strictly required, but 'Sunny Day' design is inherently more risky.
Regardless of remarks above, the Mac has a standard 14mm shaft with flats for a 10mm dropout - the flats are no smaller nor does the shaft make the motor more risky than other conventional motors, DD or geared.
The matter of filing the dropouts is related to your question, but carries with it some dangers if done badly. The first problem is simply one of not filing the dropouts to equal depth which will cock the wheel to one side. To sidestep this issue, the overall fit can be improved but not brought entirely to the ideal by simply rounding out the corners of the bottom of the dropout so the original 10mm radius matches the 14mm radius of the axle. This will get the axle seated with a 2mm displacement without actually filing it deeper - so - a much lower risk of cocking the wheel if you are not handy with tools.
The second and more important issue is to avoid filing sharp corners into the dropout at the bottom - of particular importance with aluminum frames. This causes something called stress risers to appear at these corners when the dropout is loaded making the aluminum more prone to failure at those points. By filing the corners with a rounded contour, the stress is not focused at a single point. It's not necessary that the bottom of the dropout be 100% accurate fit to the shaft as long as there is broad contact at the bottom to bear the weight of the bike. This allows you to slightly over-file the corner a bit to get the desired rounded shape. I find that chainsaw files are a good choice for this task - unlike most rat-tail files, they have a small and constant diameter the whole length. Available at Home Depot, hardware stores, etc.
Here's the improved seating just from reshaping the dropout without actually making it deeper:
Another consideration with aluminum frames is the size of the axle shoulders where the flats end. On Macs, this is quite small. You really want to put a washer or axle spacer inside the dropout so that tightening the axle nut doesn't sink the axle shoulder into the aluminum. This will both loosen the nut and spread the dropout. Dropouts often have a cast circular recess on the inside of the dropout. Make sure any washer you place there fits fully within that recess and does not bridge it or sit cockeyed. Commercially available axle spacers will fit nicely. SAE washers from the hardware store with a small outer diameter can be had by buying one size too small and enlarging the opening a bit.