Chainline / Cranks / BBs

Ok, so how does it work?! Do all Shimano cranks fit the same on a given BB length? What about other brands? Does a BB length go along with just the frame, or does it vary with the chainset to be fitted?

My 856 came with an XTR BB, but the modern deore chainset sticks out way too far on it... Can I get a different chainset to remedy this (was planning to sooner or later anyway), or does it just mean I've got the wrong BB fitted for the frame?

Haylp?!

things like this are difficult to discuss with just writing but I'll try.

You just have to keep in mind the goal: getting the middle chainring lined
up with the middle of your cassette.
There are 2 shimano standards for tapered cranks, in perspective of the offset, but the
'other' standard you normally only see on the cheap bolted together cranks.
As far as BB's being compatible with your frame, there are three standards:
68mm, 73mm and 100mm, but if you have the wrong one you can spot this
easily as the cranks are not centered.

But wait a minute now I read 856, as in 1996? with as standard an XTR
BB, that would mean the splined one? Then your problem is pretty clear:
The early splined XTR, DA and ultegra BB's are NOT compatible with the
later version from XT,LX and Deore. I keep forgetting what way around
but either you had to use a lot of force getting them on either there is
some play right now. But this also explains the offset, compare the pictures:

XTR 950

Deore

Looks like you need either a new BB or a set of old XTR 950's Cranks
but don't keep on riding like this it will destroy your cranks

Thanks! & sorry for not being clear - it's a square taper BB & I've no idea if it was in there from new - I was told it was XTR when I got the frame earlier this year but I never checked as it didn't really bother me!

I'll pull it out to find out more if necessary... I think it's 110mm (measured it to compare to my other spares earlier but I can't remember for sure!).

So is it likely that a modern deore chainset is 'incompatible' with my BB? Or is it more likely that I just need a shorter BB to get any chainset to fit right in my frame?

I had a similar problem with my Fuquay, was running old LX cranks on a 118mm UN53 bottom bracket......changed to Middleburns and I had to switch to 127.5mm.....wiiiiide :shock:

Seems to be trial and error but the other size ones do get used, I bought an incorrect axle width UN73 which has now found a home on my Dave Yates

Stu

this might help?
http://www.bikepro.com/products/bottom_ ... able.shtml

I believe sheldon browne also has a similiar table.

sounds like you've got this BB http://www.bikepro.com/products/bottom_ ... un90.shtml

ok sorry then for my misplaced reaction, I work at a bikeshop
and it does happen that people try to mix up those splined ones.

Square tapered ones are almost all compatible (exc. campa sometimes)
with a deore you would normally take a 107mm so 110mm is not that wide.
just take a ruler and see or your middle chainring is between 45-50mm from
the center of your bike (eg seattube)

Hmmm... Measured it & the middle chainring is indeed about 45mm from the centreline... It just doesn't look right though!



& I'm having issues with shifting onto the top chainring... Just seemed to be the obvious explanation.

:? I'll investigate further... Maybe try a few other chainsets too (I'm sure there'll be some in the bits box!) & see how they work...

What size chain rings you using? The compact mechs will struggle with 46/48 outers. BB looks fine in photo

Sheldon brown is pretty good .

I have same issue with my Yeti . My tnt cranks require such a wide bb , that it is nearly impossible to shift to outer rings .