Bottom bracket question
I have a specialized xc fsr pro, im not sure what year it is 2005,2006?
I bought it second hand was told it was a 2006 but cant seem to find the exact spec i got the bike with on the specialized webiste.
Anyway the BB is on its way out so i look on their website and it seems that the size was 68mm with 118mm spindle length from 2005 onwards.
I bought the 118mm spindle but have just pulled out the BB and its a 121mm spindle.
Can i use the 118mm instead of the 121 and what issues will i have?
I also assume this means my frame is a 2004
Cheers for any help
IvanPosted 8 years ago
I'd guess that the chainset might be an LX Octalink one, I think that they needed a 121mm axel whereas the Deore, XT etc. ones needed 113mm to give a 47.5mm chainline or 118mm to give a 50mm one. Frame design might dictate the need for a slightly longer axel for clearance reasons but 121mm seems a bit long.Posted 8 years ago
Ahh thanks for the response guys.
It was an Ocatalink LX chainset with tripple rings.
I have upgraded the the cranks to XT ones and it says on the specialized website that the chainline is 50mm.
So does that mean I am good to go with the 118mm BB with the XT cranks.
IvanPosted 8 years ago
I'm pretty sure that a 118mm axel will give you a 50mm chainline with the XT cranks, they should run on either a 47.5mm or 50mm chainline. I don't know though whether the LX cranks on a 121mm axel would be a 50mm chainline or a 47.5mm one, so you might need to fiddle with your front mech.Posted 8 years ago
So am I correct in thinking that the frame is irrelevant in the whole chain line thing?
And am I correct in thinking that if you have an incorrect BB and crank combo its not the end of the world you will just need to adjust the front mech?
I have done a bit of research(googling) which may be of use to someone else if correct and found that the old cranks Deore FC M470 with a 121mm BB give a chainline of 47.5 and with a 126mm BB.
And the XT cranks M751 with a BB of 118mm give a chain line of 50mm and with a BB of 113mm give a 47.5mm chain linePosted 8 years ago
Well, I'll have a go but I'm very much the amateur mechanic. Generally it's down to the crankset but, certain frame designs may require a certain axle length to give sufficient frame clearance – I've got a standard Inbred with XT M752 cranks on a 113mm axle (47.5mm chainline), the cranks are designed to work on a 47.5mm or 50mm chainline – 47.5mm was the prefered but certain frame designs needed more clearance. My 853 Inbred has very wide chainstays and needs a 50mm chainline to give anything like enough clearance between rings and chainstay, so the M752 XT cranks on that bike are on a 118mm axle.
Frame clearance allowing you should be able to run XT Octalink cranks on either a 113mm or 118mm axle. The point about maybe needing to fiddle with your front mech is this – in Octalink days 47.5mm seems to have been the prefered chainline, the LX cranks you had fitted on a 121mm axle may (I don't know) therefore have given a 47.5mm chainline and your front mech would have been adjusted to work with this. If you then put XT cranks on, fitted to a 118mm axle you'll have a 50mm chainline and the mech may be adjusting.
You can use the XT cranks on a 47.5mm or 50mm chainline because they've been designed to work with either so neither a 113mm or 118mm axle is wrong. If you fit a chainset to a BB with the 'wrong' axle length giving a chainline that it's not designed to work with then you could well have problems, for example you could fit the old LX crankset to an Octalink BB with a 113mm axle (in the sense that it would fit) but since it gives either a 47.5mm or 50mm chainline on a 121mm axle I suspect that you'd have no end of issue.
IFAIK modern external BB cranksets are all 50mm chainlines.Posted 8 years ago
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