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PostPosted: Tue Mar 12, 2013 7:46 am 
Old School Hero
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Location: Barrrkshire
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


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PostPosted: Tue Mar 12, 2013 8:02 am 
Windmilling for a Scotch Egg
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Location: Huddersfield, West Yorkshire
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.


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PostPosted: Tue Mar 12, 2013 10:54 am 
Gold Trader
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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.


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PostPosted: Tue Mar 12, 2013 12:20 pm 
Retro Guru
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So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul


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PostPosted: Tue Mar 12, 2013 12:41 pm 
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No, not entirely! Most c'sets will fit most bikes/frames given the axle length recommended for that chainset.


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PostPosted: Tue Mar 12, 2013 12:53 pm 
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oldave wrote:
No, not entirely! Most c'sets will fit most bikes/frames given the axle length recommended for that chainset.


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


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PostPosted: Tue Mar 12, 2013 9:57 pm 
retrobike rider
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shinobi wrote:
So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul


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.


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PostPosted: Wed Mar 13, 2013 9:42 am 
Retro Guru
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Location: worthing
FluffyChicken wrote:
shinobi wrote:
So if you have just a frame and no previous components its still hit and miss which length to use ?

Cheers Paul


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.


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


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PostPosted: Wed Mar 13, 2013 9:44 am 
retrobike rider / Gold Trader
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Location: Bristle
in retroland the 50mm is (mainly) for fatter seat tubes.

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


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PostPosted: Wed Mar 13, 2013 9:52 am 
Windmilling for a Scotch Egg
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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!! ;-)


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