Single chainring crankset - need advice!

[Fatmat]

Hello. I’ve clearly no idea what I’m doing! I noticed that the axle/spindle was quite long, I offered up a standard MTB BB, I’m left with a lot of space, is this a Boost axle? Will it work on a standard MTB with spacers? Any advice would be greatly appreciated

 

[clubby]

What crank? That looks like a lot of extra axle. Any chance they are fat bike cranks? They were designed for a 100mm bb shell.

Boost generally uses the offset of the chainring to give the correct chain line.

 

[Fatmat]

Hi clubby, thanks for your reply. It’s a race face ride cinch. I think the spec is 137mm but actually measures 135mm (unless I’m measuring wrong). 137 seems to be quite a common size but ‘fat bikes’ did crop up when researching!

 

[Twozaskars]

Just took that measurement from my XTR M980 and it's 115mm.

 

[Fatmat]

Yes, this looks to be about 20mm to wide!

 

[Fatmat]

The best explanation for measuring a spindle that I can find is ‘the length + 5mm’ which makes sense as this is supposed to be a 137mm and I can make it measure 132mm if I ignore the 3 small pre-installed spacers. So I guess my question now is what setup does a 137mm spindled crankset work on?

 

[clubby]

https://res.cloudinary.com/fox-factory/image/upload/v1600385412/images/d9715b92f0f94eadb807eb34529905adfc447228_Crank_Q_factors_and_chainlines___16th_April_2020.pdf

https://res.cloudinary.com/fox-factory/image/upload/v1634842892/images/2de4b81fcb6f594268a1c681fdd4860a310f28ce_B10187_C_Aeffect_Ride_Cinch_Installation_Web.pdf

Have a look at these. 137mm looks the correct size for normal MTB use.
From the second link it looks like there should be a spacer between the drive side crank and bb. That would take up the slack. I had the fat bike version and from what I remember, the preload was determined by different sized spacers and shims to provide the correct preload. It was a bit of a crap system compared to Shimano but once set, actually stayed fairly consistent and trouble free.

 

[ishaw]

It could be a dh spec axle which are longer, iirc 83mm BB shell width over the standard 68/73mm?

On ht2 style cranks, the arms press up against the bearing covers to preload them. I don't think any amount of spacers will make what you have work.

I have an E13 crankset that uses spacers and a wavy washer for preloading. Only 2-3mn and I ended up not using the wavy washer as it caused the cranks to be too tight on the bb and it did not spin freely. 20mm is a large gap.

 

[ishaw]

Can you swap the axle on the cinch cranks?

 

[Fatmat]

Thanks guys, yes, it did come with a spacer, it fits inside a recess inside the drive arm flush with the lock ring / chainring so unfortunately doesn’t offer any padding - the RF data sheet link is really useful & the heads up on the 83mm bb, together with this and some digging on how RF measure their spindles I think I’ve nailed it down to a 156mm (full length of spindle end to end ignoring the non-drive arm surrounding it) - according to Merlin it’s only suitable for the Cannondale Ai Asymmetric System / Cannondale AI Offset. Apparently it works with BSA 83mm / BB104 / B107 & PF30 83mm. It’s not gunna fit the intended frame so I’m interested to find out what type of frame it will work with. As suggested potential a DH frame, even with an 83mm bb I’m guessing it would still need more spacers to take up the 10-15mm left, looking at some setups online people seem to use clamp-on spacers which might help. At this stage I’m just intrigued as to what it would fit so any more thoughts are welcome, thanks