Crank Compatibility

[Richard G]

You lot are pretty knowledgeable about this sort of thing...

Anyone know what the crank compatibility is like for the old XT M785? I want to keep my eye out for a used power meter to throw on my old trainer bike (my ancient trainer is only providing power readings intermittently now) and I'm not sure what will work.

I know with 12 speed they've changed to some completely overengineered bollocks ( https://www.youtube.com/watch?v=_T8e2Uv9OJQ&t=207s ) but I think most of the 10 and 11 speed cranks (road and MTB) shared the same system (two bolts and axle end pattern). So in theory I could grab an older Ultegra crank arm or similar?

[whitestone]

Road and MTB cranks have different Q-factors (168mm vs 146mm) so you won't be able to stick an Ultegra crank arm based power meter onto an XT spindle. The arm will fit the splines on the spindle but will foul the chainstays.

POI: The "Q" stands for "Quack"!

[Richard G]

I see. I assumed the Q-Factor was controlled by the length of the spindle, but you're saying the crank arm would be shaped differently too?

Tedious. Means virtually no chance of finding one (obviously lots of roadies use them, very few MTBers).

[Lazarus]

Being unaware of the different q factor i have just compared
sram descent mtb
Sram rival 2x (road)
Uktegra 2 x ( road)

Seems longer spindle to me as all non drive crank arms were ending up in the same/ are the same shape
The rivals have a 10 mm shorter spindle 100 mm thebother twonarw 110mm( rough measures )

My guess is it might fit andn younl would need to eyeball one next to your crank

[schmiken]

Get a Sigeyi Axo PM - I have two and they have been 100% rock solid.

[Richard G]

Get a Sigeyi Axo PM - I have two and they have been 100% rock solid.

I'm sure they're great, but I'm looking to spend as little as possible, hence used (not least as I really can't be bothered with all this training bullshit any more).

Seems longer spindle to me as all non drive crank arms were ending up in the same/ are the same shape
The rivals have a 10 mm shorter spindle 100 mm thebother twonarw 110mm( rough measures )

My guess is it might fit andn younl would need to eyeball one next to your crank

I have a road bike here that I never use, I guess it wouldn't harm to pull the (105) crank arm off and see what I get.

[whitestone]

I see. I assumed the Q-Factor was controlled by the length of the spindle, but you're saying the crank arm would be shaped differently too?

Tedious. Means virtually no chance of finding one (obviously lots of roadies use them, very few MTBers).

The Q-factor is the distance between the pedal attachment points. Quite how the crank gets from the bottom bracket to there is presumably dependent on whether it's road or MTB

[jameso]

FC-M785 is 72.2mm from centreline to inside face of the arm at the pedal end, Ultegra 11s FC-6800 is 57.7mm to the same point. If you account for the 2.5mm each side that is the difference between road 68mm BB and MTB 73mm (the crank arms themselves are the same width across at the end), there's 12mm of difference - that's the amount of additional bend in the MTB crank.

[Richard G]

Thanks James. I'll venture into my garage with a ruler and see how much space I have!

[Dyffers]

I had a 5800 105 left 4iiii crank on my 785 right crank on my 73mm BB shell for a few weeks in 2021 when I was working abroad.

It fits on the splines but the crank arm was narrower than the left 785 so I remember some kind of bodge I had to do where I added a BB spacer or two inboard of the left side HT2 BB cup to take up the gap. I wasn't very happy with it and removed it when I got home.

So it will work but not perfectly.

[Richard G]

The bike hasn't been off the trainer for five years, so I'm definitely not concerned about it being pretty!

[Richard G]

Alas, I have basically exactly 12mm of clearance. Definitely not worth the risk!

Apparently the gravel ones are slightly wider, so I could keep an eye out for those.