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How do you work out what length axle to use in the bottom bracket? Is it a case of "suck it and see"?
I've been using the bottom bracket that came in the bike: I bought the "last ever" Deore geared inbred some years ago and it came with a 118 axle BB and an octalink chainset. Over the years, I've changed the BB once or twice, always just buying the same: I used the same BB when I sawpped the inbred frame for the Cotic Soul, then again when I went for the 853 inbred. At that point I changed the rings from 22-32-44 to a 24-35-bash setup. Brant did advise me to use a shorter axle in the BB, but I didn't change it as I was concerned about chainsuck and clearance: looking back at the chainsuck I did have, maybe I'd have had fewer problems with less clearance as the chain wouldn't have been trapped between rings and frame as it wouldn't have fitted. Anyway...
I've now got the Ragley Blue Pig and the chainring clearance is simply *immense*. I've been in the garage all night, as I was recently given a 110mm bottom bracket: I've swapped out a square taper chainset from another bike, and there's *STILL* tons of clearance. Is it worth going even shorter than 110mm? The donated BB is a raceface one with replaceable cartridges, so using that would be a bonus I expect, but looking at CRC I can get a 107mm for £15 (or a carbon and titanium 102mm for a staggering £90!), so it's worth considering: I've found that shimano square taper is pretty good, they do tend to last me qiute some time in the bikes that I do run them on.
your comments and help will be appreciated, ta
You work out chainline from the hub width and your middle chainring distance from the centre of the BB. Road tends to be shorter than MTB.
Standard Shimano (and most) is 47.5mm to the middle ring.
You need to measure the distance from the middle of your rear hub to the middle gear on your rear cassette, that dictates the chainline, then set up the BB to suit taking into acount your crank setup.
If using a double and bash, set it up to be right for the big chainring.
The crank you are using should specify a chainline with a specific size BB. 110mm would be pretty normal for a double and bash.
What crank are you using?
Thanks for that. Cranks are Shimano Deore, the old style where the sides were parallel before they introduced a bt of flare. Rear is a Hope XC with an Sram PG950 11-32 cassette (I think!). Will check the measurements tomorrow: I bet if I check I will find something online as someone will already have measured it.
Obviously as well a 9spd chain is prety flexible and setup is not as critical as with a single speed. + or - 5mm wont matter much.
AS much as anything the wider the front chainline, the bigger the q factor or distance between your legs, some find wide uncomfortable e.g. external BB's are quite wide and it is these where shimano has a 47.5mm chainline for the middle ring.