12mm axle in 10mm dropout, does it need to be filed?

[Comrade]

I'm assuming the most typical rear dropout on an MTB is 10mm nominal (mine is 10.4mm actual) and 135mm width. Most hub motors designed for this size have a 12mm axle that is ground down on each side by 1mm.

This is how an actual 12x10 axle will look like in a 10mm dropout (well, 10.4mm in the illustration).

Filing away 0.4mm of material will make it sit flush. But does it really matter?

 

[motomech]

But does it really matter?

Depends on how much power you intend to run.
Use the right-sized round file and work carefully to get the best fit while removing the least amount of metal.
Then use a well-fitting torque arm (I always use two).

 

[Comrade]

Depends on how much power you intend to run.

Not much. Sub 1000w. And I plan to have dual torque arms out of 1/4 steel which would grab the axle tighter than the dropout, which is realistically about 11mm wide without the paint.

 

[Chalo]

If you leave sharp inside corners in the slot, you can predispose it to crack by concentrating stress in the corners. Better to use a round file (like a chainsaw file) to add clearance for the corners of the axle.

 

[Comrade]

What about removing less than 0.5mm of material from the axle itself?

 

[E-HP]

What about removing less than 0.5mm of material from the axle itself?

It depends on your goal. If you want the center axis of the motor axle to line up with the centerline of the bike axle, then you need to file the dropout 1mm deeper first and shape it to match the contour of the motor axle.

In your case, the dropout tolerance is so sloppy, you won't get much from filing from a strength perspective, but if you need the alignment so things like your brake calipers align, maybe it makes sense. That can also be solved with adjusting a washer or two.

 

[Comrade]

It depends on your goal.

Just looking for some opinions. I guess my goal is to have the axle sit solid in the dropout. There is no suspension, so I imagine dynamic loading on some bumps would eventually deform the dropout to take shape of the axle, which should be much harder steel.

 

[Comrade]

With this torque arm, if the distance between the motor axle center and the bolt holding the bracket to the frame is 37mm, and the motor can generate say 50nm peak torque, does that work to be 304lb of shear force on the bolt? The SATs were so long ago. :lol: