Want to know what bottom bracket length you need?

I'm with shinobi how does that help, I admit I don't understand what I'm looking for when mating BB shafts to chain rings to rear cassette. I have either jut copied what was previously installed or taken the parts/advice from the local bike shop

But know I want to learn!

Any advice appreciated

The chain line is the distance between the centre of the bike (normally centre of seat tube is where you can measure it) to the middle ring on a triple set up.
The longer the axle the further out the chainset will be from that centreline.
4mm can make a LOT of difference - I was trying to use a 122.5 on my Rocky Mountain and it all looked fine but I had no end of problems shifting into the granny ring. When I realised the furthest the mech would go (ie up against seat tube) still was no good I changed the BB for a 127mm one and all was fine.
Measure your current bikes to get a feel as to where the measurement is taken.

That's the front. If it's a double, then centre frame line to between the two rings, for a single then ditto to centre of the teeth. With a multi-cog rear, then from centre of frame line (which you can work out from drop out spacing/hub offset) to middle cog.

If you want to see if front "matches" rear, then do what the fixed riders do - place a straight edge across face of the middle ring (if a triple) and see where it encounters the cluster. If it's near enough the middle then that will do.

So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul

No, not entirely! Most c'sets will fit most bikes/frames given the axle length recommended for that chainset.

oldave":3uuzb72z said:

No, not entirely! Most c'sets will fit most bikes/frames given the axle length recommended for that chainset.

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Hi ,sorry what i ment was if you are gathering bits together for a build and you have no old bits to refer to then you could end up with two options for the same chainset depending on chainline , ie you could buy a 110mm then run into shifting problems and need a 113mm ?

cheers paul

shinobi":1jw5i6oh said:

So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul

Click to expand...

No.
Get the length your chainset is specified for.
Retro, aim for 47.5, then move to 50mm if needed (and it's generally got a wide band front mech on the frame)
Modern, then why the hell you buying it

(i.e. 47.5 is the spec, 50mm is an alternate spec.)

If you don't have the spec's, look or ask for them or look at the profile of the crank. It is the profile that determines the length.

FluffyChicken":3p3yjlfr said:

shinobi":3p3yjlfr said:

So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul

Click to expand...

No.
Get the length your chainset is specified for.
Retro, aim for 47.5, then move to 50mm if needed (and it's generally got a wide band front mech on the frame)
Modern, then why the hell you buying it

(i.e. 47.5 is the spec, 50mm is an alternate spec.)

If you don't have the spec's, look or ask for them or look at the profile of the crank. It is the profile that determines the length.

Click to expand...

Ahh i see , right , excuse me being a bit of a dumbass

in retroland the 50mm is (mainly) for fatter seat tubes.

since nowadays almost everything has a fatter seat tube, 50mm has become the standard

My Dave Lloyd needed a 127mm BB for the DX chainset - 50mm chain line. And that had the skinniest tubes I've seen on an MTB!! ;-)