Bottom bracket length XTR M900

[kaiser]

Sheldon lists the M900 as 107 -113 for the M900, what do you have fitted? How much difference is this going to make?

 

[FluffyChicken]

as an easy guide

107 for 28.6
110 for 31.8
113 for 34.9

(actual front mech band size, not shimed down size)

Though any and all will work.


I have (had) fitted BB-UN70 107/68 on my RM Altitude 1991 with M900 cranks on it, 24 to 48 rings, now both moved across this year to RM Fusion 1992. Using M900/1 28.6 front mech. Works wonderfully.

 

[kaiser]

Nice one Fluffy, you truly are the man with the ken

 

[Xesh]

as an easy guide

107 for 28.6
110 for 31.8
113 for 34.9

(actual front mech band size, not shimed down size)........

Interesting....

In all the years I've been working on bikes this is the first I've heard of this sizing scheme. I've always used 107 on M739 cranks (I've never been able to afford M900), what ever size BB shell or seattube the frame had, and I never encountered any problems with setup of the f/mech. :?

 

[Stanton]

I've just received some Middleburn cranks that came with a 113 BB, the BB currently on my bike is 110 (31.8 FR mech). Would I need to swap the BBs over and run the 113 on my bike in place of the 110?

 

[FluffyChicken]

as an easy guide

107 for 28.6
110 for 31.8
113 for 34.9

(actual front mech band size, not shimed down size)........

Interesting....

In all the years I've been working on bikes this is the first I've heard of this sizing scheme. I've always used 107 on M739 cranks (I've never been able to afford M900), what ever size BB shell or seattube the frame had, and I never encountered any problems with setup of the f/mech. :?

It not a hard and fast rule at all. Just a guide to a length rule I use for best guestimate.
107 is the original length for the bottom bracket giving the 47.5 chainline.

The longer lengths are there if the stays are wide or fat, i.e 113 was there for fat tube bikes like Alu or Suspension that had wide stays. These also used the wide bands being Alu and you end up with a 50mm chainline at this longer length.
110 came later on iirc as in in fill for in betweens.
It's the same with all these bottom brackets ranges, that why there is a range from 47.5 to 50.

Of course the modern mechs run at a fixed length bottom bracket, use a fixed 34.9 banded (and are shimmed down to fit the frame) and all run on 50mm chainline. Hence the link to running the wide bands to longer bottom brakcets on the widest chainline.

But all mechs should work fine without a problem with the bottom bracket at 47.5 to 50mm chainline, from BiTD


As for M739, they had gone compact drive so rings were smaller and hitting the stays will have been less of a problem. But it's still a nice rule to centre the mechs stroke over the centre chainline...
and I have no proof of the concept

 

[FluffyChicken]

I've just received some Middleburn cranks that came with a 113 BB, the BB currently on my bike is 110 (31.8 FR mech). Would I need to swap the BBs over and run the 113 on my bike in place of the 110?

Only is the 1.5 mm loss in width casues you problems or the new one is a better bottom bracket or of a different crank fixing. Middleburn state 113 for all there bottom bracket, I believe based around ISIS standard making it easy for them to spec, build and not worry too much about it.

I've happily ran Suntour XC Comp MD cranks on a 122.5 with a 28.6 band front mech (113/115/117 should be the bottom bracket length) It all worked fine and dandy for what it was used for.

Of course it took me 16 years to notice I was running a 122.5 bottom bracket though I did use to mention my stance on the bike was wider than my XTR setup, now I know why.

 

[Stanton]

Cheers for the info Mr Chicken.

To be honest I've never changed bottom brackets on any bike I've owned (until now) so I was unsure as to the difference in the spindle sizes (as you say 1.5mm really doesn't seem a lot!).

They're both square taper UN52s and rather well (ab)used ones at that so I think I'll be replacing the BB at some point soon as it is anyway.

Actually, if anyone happens to be flogging decent a 68mm BB send me a PM.

 

[TreaderSteve]

Re:

Ancient but pertinent (for me) thread dredge-up!

Bike: M800 BotE. I'm going to run single chainring (around 49 or 50T) with single speed freewheel (17T or so) on a Surly rear hub. So, the rear cog will be in the 'outboard' position. Chainring will be on the outside in the usual large chainring position.

I've had random luck with BB lengths before, or swapped-out like for like, but for the M900 we have choices. And, I don't really want to buy more than 1 new BB!

Which do you think will be the 'best' choice for my M900 cranks running single speed to get the straightest chainline do you think? 107, 110 or 113?

Thanks for your input!

 

[FluffyChicken]

107 gets you the 47.5mm chainline and it *the* bottom bracket to use with M900
But that's a chainline form middle ring and 8speed read setup.

110 gives the identical chainline but pushes the non-drive side out 2.5mm, some bikes later on where designed with wider stays I guess.

113 gives that 50mm chainline more common with 9speed onwards as being the default, but perfectly fine back then...

The problem you have is your rear is going to need to be setup for an outer ring chainline, that's not 47.5 but another ring out (+7mm, i forget)

You probably cannot go narrow and use road bottom bracket, 103 etc, since, for me at least on all mine. the arms are only a few mm from the chainstay on a normal early 90's steel frame.

BoE may get away with it, fatter tubes.
So in my opinion, it's buy 107mm, see if it works for you (does the hub allow you to push the ring out to the big ring position.

Or work the other way, see what chainline the Surly hub has and work backwards.