How are bottom brackets measured?

swee'pea99

Legendary Member
On one like this, where it's described as 68mm x 113mm, what's 68mm? And I presume 113 is the width of the BB, but is that precisely from one external 'face' to t'other? Or what? How many different kinds are there? (I'm looking for something to fit an old British frame - is there a 'standard' it's likely to be?)
 

4F

Active member of Helmets Are Sh*t Lobby
Location
Suffolk.
68 mm is the distance across the bottom of the frame, 113 is the axle length so this depends on what chainset you want to use. As long as your frame is not Italian then you will want a 68.

When I replaced the bb on my fixed I used a shimano UN54 68 x 110
 

gwhite

Über Member
As has been said, 68 mm is the width of the BB shell (the brazed on tube holding the BB) And the BB is measured from the end of the tapered shaft to the other end.The exact length will depend upon the type of chain-set you want to fit in that a triple tends to need a longer BB than a double and the actual width of the chain-set has to be taken into account (Q factor). Again some BBs tend to be asymmetric which also complicates things. As you may have gathered it's not altogether straightforward but the goal re length is to achieve an acceptable chain-line. Your LBS may be able to assist.
 

PpPete

Guru
Location
Chandler's Ford
As ever Sheldon has all the answers. Here.
The two tables at the bottom of the page are the most useful for the first measurement.

Measure the width of the BB shell on the frame. It will be 68 mm on 99.9% of old british frames.

Further up that page are the tables which should enable you to tie down the second measure which is the spindle length.... which is determined by which chainset you choose.

Oh yes and I used a UN54 68 x 110 on my fixed too!
 
OP
swee'pea99

swee'pea99

Legendary Member
Thanks all (and sorry this thread seems to have appeared twice - don't know how that happened)...so, for a fixie on an old British bike, I almost certainly need a 68 x 110, yes?
 

4F

Active member of Helmets Are Sh*t Lobby
Location
Suffolk.
swee said:
Yes, however you could probably also use a 68 x 108 as the difference of 002 between the two is on the non drive side.

My conversion was of an old 1980's Dawes and the chainline was spot on first time, well chuffed :biggrin:
 
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