4iiii Road Power meter on MTB

Ultegra 4iiii

I’m looking at a powermeter for my XC bike. I was considering the Garmin Rally pedals, but this seems really cheap. Yes, it’s a road crank and won’t match, but that won’t bother me. Fundamentally, is there any difference in the actual powermeter part of the crank?

Serious question, how do you expect to actually fit the Ultegra crank to your XC bike?

IIRC you can’t use an Ultegra crank arm on XT cranks and even if you could, the q-factor is different between the arms.

AfIK, I do think the spline pattern is the same on all the Shimano cranks, so you can fit them on any combo.

The issue as mentioned is the Q-factor difference between road and MTB. I know they are different, but am not sure if the delta comes in the crank arm (pedal face to bb face distance) from just the crank spindle width (assuming the distance above on arms is equal)… or some combo of spindle and crank arm changes.

I don’t have time to Google it now, but I am certain the answer is out there.

I have a spare Shimano crankset so I’m able to trial fit, so that’s not the question. I’ll be able to figure out where the difference in Q-factor comes from. If the road cranks don’t work, then that’ll be OK.

The question is do we know if the actual powermeter unit is the same across the range of road and MTB powermeters?

Thats a question best directed to 4iiii. I would guess they are the same, but have nothing to back that guess.

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Sorry don’t have the answer about the modules (not sure anyone outside 4iiii will have that info). Even if they look identical they may be programed / calibrated differently. I suppose it might depend if there’s a measurable torsion difference in the longer MTB axle which requires to be offset differently to a road axle, but that’s really just speculation.

I have a 4iiii power meter (105) for my road / CX bike. It won’t fit on my MTB because Shimano road / MTB crank arms are a different shape as far as I can tell - the MTB ones flare out more.

I run a SRAM MTB crank so I’ve not tried to fully fit it, but from holding it up against the BB there would be a clash between crank arm / pm module and chainstay if I fit it to my Orbea Oiz.

Try and fit your MTB drive side and non PM road non drive side to your MTB and see what the clearance is like.

@AndyGajda I don’t think Ultegra is a good fit for MTB from a reliability perspective. I’ve seen so many of these cranks break during gravel rides. They tend to break along the seam on the spider and also the non-drive side crank tends to fall off (although I suspect the latter is most often user error). If you are going to go this way, stay close to home…don’t get over extended to the point where you’re in trouble if you can’t ride your bike out.

I have orbea oiz and will prob buy 4iiii xt left crank pm…do i understand that you tried and there is a clash on the chainstay with pm module on crank?

No, I tried with a 105 (road) crank arm and it didn’t fit. XT is more likely to fit since it’s a MTB crank, but I can’t confirm personally if it does or not.

If you have the original XT crank arm I’m sure there’s a check you can do to see if the PM will fit. I suggest checking the 4iiii website for instructions.

Yeah my Ultegra chainset disintegrated a couple of years ago in this way, it was the drive side that went on mine, and it wasn’t a catastrophic fail, it just developed a creak and a wobble which when inspected found it was split at the join. I ‘upgraded’ to 105 after that!

Ok thx, i will try and fit my friends xt before buying then, currently i have sram cranks

I think some early R8000 chainsets had a problem with the bonding agent used between spider and spindle.

This would have been 6800 but maybe the issue carried over to 8000.

It wasn’t between spider and spindle, rather between the two halves, outer and inner of the crank arms.


Yup, we’ve seen that issue thru both generations of those cranks. Typically drive side from what I have seen, but wouldn’t shock me if there were non-drive side issues too.

I didn’t pay close attention to the detail, just wanted to make sure it worked. But I put a Stages/XTR crank arm on my gravel bike with a GRX group. But I guess that is a bit of the opposite situation.