Bottom Bracket Length for XT M730 cranks on touring bike

[Kevin K]

Does anyone know the optimum bottom bracket length for XT M730 cranks on a touring bike?
Sheldon Brown's table states 122.5mm, but that seems awfully wide.
The bike in question is a Thorn Sherpa with 135mm rear spacing and an XT 11-32 cassette.
Hope someone can help.
Thanks

[cycleruk]

The 122mm length might be down to the design of the crankset.
Some XT chainsets only require 107-113mm.
The difference may be due to how the chainrings align to the square hole center.
You could ask Thorn if they can help?

Another dimension affected by the BB is chainline. If the chainline is too far outwards then you may have trouble with the front derailleur not reaching far enough to move the chain to the big ring.
See:- http://www.sheldonbrown.com/bbsize.html#chainline
If the BB is too short then the small chainring or the crank arm may catch the chainstay.

Sorry I can't be more specific.

[531colin]

Bung the crank on the existing BB. 3 options;-
1 just right--- job done!
2 too long-----now you can see how much shorter it needs to be.
3 too short----unscrew the right BB cup until you can see how much longer it needs to be.

Simples!

[Kevin K]

Two problems with that approach:

1) I don't have an existing bottom bracket;
2) It doesn't help me to sort out the best chainline.

Thanks anyway.

[531colin]

Two problems with that approach:

1) I don't have an existing bottom bracket;
2) It doesn't help me to sort out the best chainline.

Thanks anyway.

I think fitting it to the bike is the best way to sort out the chainline for your particular ring sizes and how you intend to use the gears. Manufacturers have been known to spec. longer than necessary, maybe to allow for big inner rings or un-dimpled chainstays.

[borderglider]

I just had an identical issue rebuilding a Bob Jackson touring frame. I bought a 'Solida' chainset off Ebay and it arrived with a 122.5mm bottom bracket, which - as you say - looked longish. I rang Bob Jackson cycles - who were as always - amazingly helpful. They said that for a 68mm wide bottom bracket shell - 122.5 was not unusual for a classic ten speed using a two-ring chainset. They also said that if you measure from the centreline of the down-tube to the centre-line of the teeth on the chainring nearest the bike, the distance should be 54mm.

When I put it all together the 122.5mm axle was perfect. It is all to do with how far the crankset body slips down the square-tapered axle. This will presumably be different for every brand of chainset - it depends on how they mill or forge the square locating hole.

I still don't understand how any bottom bracket of 113mm could ever fit a classic ten speed, but I am told that with certain chainsets they do

[CREPELLO]

As you don't have an existing BB to experiment with, you'd be best off taking it to a a good LBS for them to work out the right length BB.