Acceptable chainstay clearance for XTR FC-M985 crank

Discuss light weight issues concerning mountain bikes & parts.

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hjb1000
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Joined: Wed Sep 16, 2009 10:25 am
Location: Australia

by hjb1000

So I installed my new lovely new FC-M985 cranks on my Cannondale Flash F3 Alloy, and noticed the clearance between the driveside crankarm and the chainstay is quite tight (about 3mm), but not critical.

I just wanted to confirm the tight 3mm clearance is acceptable for this crank? And I wonder what is the best case scenario for this crank since the q-factor is 3mm less on each side than the M980- ie most frames would have this same issue?

The bottom bracket is 73mm so I have one 2.5mm spacer on the driveside cup. I gather I can't cheat and add another spacer since the non-drive side wouldn't have enough spindle?

Other than that, amazing crank, light and very well finished!

Many thanks

by Weenie


thisisatest
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by thisisatest

3mm is fine. cranks dont flex much in that direction when in the rearmost position.

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hjb1000
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Location: Australia

by hjb1000

Many thanks for the reply thisisatest, pretty much what I was thinking/hoping.

I knew the M985 crank had a small q-factor, but I was surprised to see how small the clearance was! I can see these cranks wouldn't be compatible with a lot of frames eh?

Gchan
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Joined: Tue Jan 31, 2012 9:35 pm

by Gchan

just bought the same cranks, and my clearance is even less. 2mm max. don't know if it functionally matters, but just looking at it is a bit disconcerting. do you notice that your non-drive side has more clearance? the difference is noticeable on mine.

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hjb1000
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Location: Australia

by hjb1000

Gchan wrote:just bought the same cranks, and my clearance is even less. 2mm max. don't know if it functionally matters, but just looking at it is a bit disconcerting. do you notice that your non-drive side has more clearance? the difference is noticeable on mine.


Yes, you're right the clearance on the non-drive side is about 4-5mm. Sounds like I'm not the only one then! I'm guessing this is the price you pay for a low q-factor.

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hjb1000
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Joined: Wed Sep 16, 2009 10:25 am
Location: Australia

by hjb1000

As a follow-up to this I have noticed the drive-side crank arm on my M985 crank has a slight rub mark, as well as on the chainstay. Only very minor, but it is definitely making contact at some point.
So it seems the crank/frame is flexing and under certain conditions perhaps with additional mud there is a rub.

Not good, but do I need to worry about it? I'll post a pic.

by Weenie


Weasel
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Joined: Thu Jul 05, 2007 10:18 am

by Weasel

I tried fitting one (44/30T) to a Trek 9.9 (2007/8?), and the large chainring fouls the chainstay before the crank seats against the BB bearing!
It might fit if I remove the chainsuck plate, but I'm not gonna risk that!

I've now fitted it to my SC Blur XC - it's very close on that - around 2-3mm max, but does fit.

Anyone else had the same issue with other frames?

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