Issue with crank offset across bike..

[wafter]

Bit difficult to explain, but I'll do my best as I'd like some reassurance before throwing another day down the pan..


My Fuji Touring is fitted with a 175mm Shimano Deore M591 triple crankset. This is apparently correct; it seemingly fits photos of the bike and literature either states simply "Deore" or "T591".

I'd prefer 170mm cranks for toe overlap reasons and recently scored a Deore M590 crankset; which I though was exactly the same barring the lack of a chain protector.

Today I tried to fit the M590 crankset and found it to be different to the M591 - having an axle that's 6mm shorter. The M591 however has a deeper web at the base of the crank arm and uses a 6mm spacer between the crank and BB.. the important take away being that with the M591 fitted with the 6mm spacer it gives the same lateral chainring locations / chainline as the M590, and the same amount of axle poking out of the NDS to fit the crank arm to.

Here are the two together in an effort to illustrate the above. Mounting points for the chainrings are approximately in the same place, as are the NDS ends of the axles..

The practical difference between the two is that in this configuration the DS crank arm of the M590 sits 6mm inboard compared to the M591; and of course we can't be having cranks at different distances from the bike's centreline.

After a day of cock-ups I checked the positions of various bits on the Fuji with the original M591 now refitted, with surprising results..

Measuring pedal position across the bike from the outside face of the crank arm where the pedal screws in to the OD of the downtube on the opposite side (easiest distance to measure with a caliper) suggested 111.9mm on the DS and 101.6mm on the NDS - suggesting that the NDS pedal is over 10mm closer to the bike's centreline than that on the DS!

So, while I was cursing the difference in lateral pedal position between the original M591 and new M590, it appears that given the current discrepency with the M591, the M590 would actually work in my favour - bringing the DS offset figure down to around 106.9mm; now only around 5mm more than the NDS with the chainline remaining the same.

Further, the BB appears to have 2x2.5mm spacers between the screw-in bearing housing and BB shell on the the DS, and only one on the NDS. So.. swapping one of these from the DS to the NDS should bring the offset figures down to around 104.4mm on the DS and 104.2mm on the NDS.. which would be pretty close to perfect.

Of course the latter would affect the chainline; bringing it 2.5mm closer to the centreline of the bike. Measuring as best I can between the centreline of the middle ring and the bike's centreline suggests an offset of around 51.4mm; which sounds ballpark correct as IIRC Shimano state 50mm for this setup. So, assuming this is correct, swapping the spacer to the NDS would drop the chainline to around 48.9mm.. a little more inboard but arguably closer (measurement error permitting).


While this is clearly an odd situation does my thinking seem sound? The bike's an old steel tourer so there's no odd intentional frame asymmetry going on.

Of course it begs the question of how this situation occurred in the first place - intentional compromise for something I've missed, design cock-up, lack of availability of the correct parts, wrongly-retro-fitted replacement..?

Interested to hear peoples' thoughts as this has been a bloody pain today and I'd like some closure; especially if it means I can use the bits that I'd otherwise assumed were incorrect / would need to be sold on..

 

[Ming the Merciless]

The two 2.5mm spacers on DS and one 2.5mm on NDS is indeed what the manual suggests for fitting cranks designed for a mtb 73mm width to the 68mm BB width of a road bike. By changing the spacers you may find your front shifter has issues shifting onto the granny ring, with it being further inbound. But you won’t know if it’s an issue with the derailleur you have fitted till you try. I’m surprised about the Q factor differences you mention between DS and NDS,

 

[wafter]

The two 2.5mm spacers on DS and one 2.5mm on NDS is indeed what the manual suggests for fitting cranks designed for a mtb 73mm width to the 68mm BB width of a road bike. By changing the spacers you may find your front shifter has issues shifting onto the granny ring, with it being further inbound. But you won’t know if it’s an issue with the derailleur you have fitted till you try. I’m surprised about the Q factor differences you mention between DS and NDS,

Thanks - that was my finding too, after I'd deciphered all the various references in the manual.

The spacer setup for 73mm cranks with 68mm BB shells seems a little odd since it must introduce some 2.5mm / one spacer's width more offset to the drive side. Plus the 7.5mm total of spacers is 2.5mm more than the increase in BB shell width; so the third spacer is both introducing an alignment error (assuming BB shells are symmetrical / on the centrline of the bike) and making the total effective witdh of the BB shell assy (shell and spacers) wider. Why would this be desirable?

Further to that, the fact that this setup works with the shorter, 120mm M590 axle suggests that this must be intended for 73mm BB width (which would figure as it's an MTB groupset after all). This begs the question of what the 6.5mm longer axle of the M591 is supposed to fit.. that would imply a 79-80mm BB width and my knowledge of the horrors on BB standards is (thankfully?) too scant to know if that's a legit standard.

I can't currently see any issues to at least swapping the cranks over (if leaving the spacers in place for now) other than potential heel strike issues on the DS due to my wonky feet.. the bike runs 135mm axle spacing (so again the same as a contemporary MTB) and I don't see any asymmetry in the frame that would suggest more crank clearance would be necessary on the DS.

Besides, surely from a biomechanical perspective any lack of symmetry across the bike has to be an obvious no-no..

 

[Ming the Merciless]

On a 73mm shell you use one 2.5mm spacer on drive side. Thus you’re just adding 5mm of spacers to cope with difference in BB width.

You’re getting into the realms of overthinking it. It definitely all works, go ride.

 

[T4tomo]

as Ming says, as long as your FD has sufficient adjustment to shift across all 3 rings, plus check the differing position of the crank arms don't cause them foul on the frame, and the middle ring is somewhere near-ish the centre of the cassette chain line wise, then all is good.

 

[wafter]

Thanks both!

On a 73mm shell you use one 2.5mm spacer on drive side. Thus you’re just adding 5mm of spacers to cope with difference in BB width.

You’re getting into the realms of overthinking it. It definitely all works, go ride.

Cheers and yes, a bit of digging through manuals for the BB came up with the same approach - 73mm shell plus one 2.5mm spacer for 75.5mm total width, or 68mm shell plus three 2.5mm spacers for the same total value..

I just can't get my head around why there's such a difference in crank position side-to-side, why the 6.5mm longer-axled M591 even exists and why the manufacturer would choose to fit this over the apparently better-suited and far more common M590.

Of course I'm over-thinking this (that's what I do) however sometimes even attempting to consider every variable isn't enough to avoid cocking something up so it's always good to get a second / third / fourth opinion.

Will re-fit the new M590 today, repeat the measurements and take it out for a test ride

 

[wafter]

Thanks again both; trial-fitted the new crankset earlier; all was good so it's now on there permanently.

You can ensure you never hear any of the gory details by avoiding my Fuji thread like the plague

 

[Drago]

Different cranks with different Q factors for different frames.

 

[wafter]

Different cranks with different Q factors for different frames.

Indeed.. just seems a bit weird that this would apparently come from the factory with the pedal offset across the bike being decidedly on the wonk from one side to the other.

Plus, without thinking about it too hard I can only assume that the M591 is intended for a bike with an offset BB shell, if that's a thing..