XTR M950 / M952 Cranks, chainline, BB and chainrings

[gtRTSdh]

So I've been tinkering this afternoon and I want to try to collate all the knowledge in one post.

Sorry for the massive park plug, but without the right tools it's easy to do damage. I knwo other tools are available but let's face it Park is going to continue to be around and the tool part numbers will remain the same.

Bottom Brackets

The cranks use octalink V1 hollow BB system, as far as I know this is shimano's first splined BB system. The original bottom brackets came in 112mm & 116mm lengths. The bottom bracket from the 5500 Shimano 105 road group is still available and fits but does not offer the ideal chainline, it's available in 109mm & 118mm lengths. For me, with the 109mm length, the front mech rubbed on my 2.15"/55mm (actual measurement across the widest part) wide tyres, not great.

It is important to remove the crank ring caps before mounting the cranks as it's easy to damage the splines, use park tool SPA-2, to fit / remove the removal caps.

http://www.parktool.com/product/pin-spanner-red-spa-2

The M950 BB uses BBT-7 & BBT-18 Park tools. The M952 BB uses Tools BBT-22 or BBT-32, or if the left cup has 8 notches (as the 105 raod one does), then BBT-18 can be used.

http://www.parktool.com/blog/repair-hel ... -selection

Chainrings

For the 4 bolt chainring version, Shimano picked unique BCD's, 112mm (middle ring), 68mm (granny ring) which makes finding chainrings, especially the most used & worn out middle ring virtually impossible now.

The spiders are held on with a lockring locked with a spring clip, threaded M30 x 1mm pitch, there are only 1-2 threads on the cranks, these are easily stripped so go easy, the lockring is removed with the M950 BB tool (park BBT-1.

Middleburn made a triple spider, and there is a triple 5 bolt spider from shimano, but these are quite rare & expensive unless you get lucky. There is another option which uses the spider from an XT M750 square taper crank, to give 4-bolt compact triple chainring options. The M750 octalink crank lockring is pressed on and as far as is known is not useful for the M950's. I have measured the spider thickness', the M95* is 4mm, and the M750 is 3.7mm and comes with a 0.3mm washer, the washer goes next to the lockring. I have tried fitting the XT M750 spider and have found that it moves the chainrings 0.6mm towards the seat tube, fairly minor, unless like me you're already too close.

Other shimano cranks share the same crank/spider interface, in partiular STX-RC & LX of the XT M750 era, LX & XT of the previous era (the ones where the big ring is the spider for the other rings) but the spiders are thicker than 4mm and won't allow the lockring and spring clip to fit. It is of course possible to modify the spider's, but the end result is still not as good a solution as the options above.

Here's some pictures comparing the spider options (cranks just placed on top of chainrings /spiders) & to show the relative length of the octalink V1 BB

 

[nwuboy]

Re:

this is enormously helpful, thank you! Did you ever discover a solution for the m952 chainring bolts? is there a modern substitute? several of the bolts available for current shimano ringsets looks nearly identical to the old m952’s.

 

[Woz]

Re:

Good post for us tinkerers.

 

[Woz]

Re:

If you have the M750 square taper version, any chance you could measure the Q-factor for me?

I think they spec'd a 107 or 113 BB axle depending on what chain line you wanted.

 

[nwuboy]

i have an m750 - how do i measure q factor?

 

[Woz]

Re:

Its the overal distance between where the pedals go - ie the outside edges of the cranks.

If the cranks are mounted, measure the left crank upto the seat-tube , turn cranks 180 degrees, measure the right crank from the other side of the seat-tube add them together along with the seat-tube diameter.

You will also need to know the square taper BB length and crank length for the measurement to make any sense to
me. Many thanks for the help before I potentially pull the trigger on one for a CX build.

PS: Think St Sheldon explains Q-factor with diagrams

 

[Woz]

Had these bits close hand to better explain. This is how to measure Q-factor if cranks are unmounted.

This is a LX M563 175mm crank with a UN71 107mm. Q-factor is 165mm.

Probably won't clear the chainstays of a modern AL fat tyre frame, but nice for an old school steel rigid

 

[boxxer]

You forgot to mention that there were AC Cranks which used the same interface and used the same lock ring set up and the 5 bolt spider is interchangeable with M95X

 

[nwuboy]

so q factor is measured outside edge to outside edge or inside/inside edge? your picture makes it seem like ts outside edge but its about chainstay clearance, no?

 

[Woz]

so q factor is measured outside edge to outside edge or inside/inside edge? your picture makes it seem like ts outside edge but its about chainstay clearance, no?

Q-factor is outside edge . Primarily, it's about biodynamics and comfort.
Update: This is a decent read https://www.cyclingweekly.com/news/late ... nce-187403

As you say, inside edge is all about chainstay clearance. You can deduct the clearance from the Q-factor
as crank widths at the tip are around 1cm or so.

I found this list, but it is missing a few. Sadly crank manufacturers rarely spec this measurement. You will
see a significant variation; road will be the lowest. In general, later and low end MTB cranks have a wider Q-factor.

http://konstantin.shemyak.com/wiki/imag ... himano.pdf