Bottom bracket spindle length

[Butler]

Hi there,

I'm in the process of rebuilding up a 1992 claud butler cape wrath and I need to buy a bottom bracket. I have a shimano XT FC M751 cranks I'm putting on and I was wondering what length BB spindle would work?

I know the BB shell is 73mm but not sure on the spindle length.

It would be an octalink BB.

Any recommendation would be great!

Cheers

 

[Cloverleaf]

Shimano used to publish tables in the back of all their catalogues. Sadly I've thrown all mine out a long time ago but a google might throw up something.

 

[Butler]

Shimano used to publish tables in the back of all their catalogues. Sadly I've thrown all mine out a long time ago but a google might throw up something.

Thank you, I appreciate the quick reply! I shall check that out

 

[Butler]

Everything I have read so far suggests either 113mm or 118mm. I'll be running a 9 speed cassette so would either length make that much of a difference?

 

[bikeworkshop]

You'd think if it's a 135mm hub width then the chainline is fixed - but sometimes a bike designed for a 2.4 tyre perhaps also with thick alloy stays pushed the chainrings out a little further. Larger chainrings often needed a longer bb too.
Id assume there was a right length for it, but if you're not finding it online then it might be acceptable to use either length if the stays aren't too wide.
Shops and builders would have both to hand so if the 113 was too short, pop it back in the box and go 118 -
Usually a shorter bb makes better pedaling, front shifting, less chainsuck.
but sometimes the back of the cage catches on a fat tyre

 

[Cloverleaf]

From memory (and it's a while since I actually had to pay attention to front mechs and such), M739 XT was always 110.5mm for 'standard' chainline on our bikes, whatever that actually was. What I do remember is that my dad bought a very nice Hope Ti BB for his Cannondale when it still had the original Sugino cranks which were then shortly upgraded to the aforementioned M739 XT. To his disgust/disappointment the front shifting was never quite right with the 113mm BB and really needed the 107mm to work slickly so a correct UN72 was employed. From memory the equivalent LX cranks did use the 113mm spindle for the correct chainline. I think the answer is f*^%ing Shimano, as it usually was back then.

There's more to chainline than BB as the above anecdote suggests because to get a particular chainline, different cranks need different spindles, and then the whole setup moves in and out. This might sound a little rough and ready but unless the frame is asymmetrical I generally go with the gap between each crank and the chain stay. If it's even then it should all be roughly where it should be, in theory. For your stuff with the Octalink I think there are fewer variations in axle length so at least you know it will be one of two(?) I should think.

Doing some digging as I was curious of how accurate my memory was, Sheldon brown's site suggests 113-118mm for a 47.5-50mm chainline. So identifying what chainline you need and that'll answer your question: https://www.sheldonbrown.com/bbsize.html

This link from Shimano (search 751) suggests that an M751 FD supports both a 47.5mm and a 50mm chainline. Looking at some of the other data for different mechs suggests a 47.5mm chainline for 68mm shells, and a 50mm chainline for 73mm shells (as you have). Ergo my thoughts would be a 73mm x 118mm BB, but 118mm sounds really long for an axle which makes me question it.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjj3-vCkcqEAxVfVkEAHfW5B7YQFnoECA0QAQ&url=https%3A%2F%2Fproductinfo.shimano.com%2Fdownload%3Fpath%3Dpdfs%2Farchive%2F2008_Compatibility_en.pdf&usg=AOvVaw1-8n9vSYoHAJ6dxppa0xA2&opi=89978449

Pretty sure all of that has been no help whatsoever but oh well, I meant well with it

 

[Butler]

Actually everything that's been mentioned above has helped me a great deal. I tend to hyper fixate on things like this and end up going around in circles so all I can say is thank you! I happen to have a 73mm 118mm bb on its way but I've questioned it since ordering lol