How to measure the Bottom Bracket

[RodWatts]

Just building up an old 1950s racing bike (starting from frame up)
By trial and error I've found that none of the bottom brackets (Italian thread, square taper) in my shed fit, spindles not long enough.
So is there a method to calculate how long the spindle needs to be or is it really down to trial and error?
Surely there must be a way!!
Thanks
Rod

[9494arnold]

Are the cups Italian Thread? If they are and it's a British frame,the cups wont fit. (French ones are different again)
Really need to know what chainset , single/double /triple/cottered/cotterless.
If it's alloy and not low profile (ie Stronglight 49/80/90/100/200/Shimano 600/SR Apex /Campag GS/Record etc Chainset, it's about 118mm for a double , but there is a degree of trial and error too, sometimes the bearing surfaces on the bottom bracket axle are too wide for an older frame. ) I had some help recently from St Johns St Cycles on this very subject .
Good Luck

[Manc33]

Vernier caliper.

[Mick F]

At the risk of being too simplistic ................

The BB shell is of a standard width. There are only "so many" standards.
This is a good table:
http://www.sheldonbrown.com/gloss_bo-z.html#bottom

Also, chainsets have recommended compatible BBs. You should have the correct BB for the chainset you are using bearing in mind the shell width.

Questions:
What is the shell width?
What threading is it?
Which chainset will you be using?

[RodWatts]

It's a pretty standard 68mm Italian thread square taper as I've already said, but I don't have the original to check the spindle length, nor is it common frame manufacturer (Leonino da Bologna/Paletti). All I can work out from old BBs in my spares box is that the spindle length is more than 111mm. Trial and error then? I assume it's not offset....
Will be fitting a Stronglight 49D.


Rod

[Brucey]

It's a pretty standard 68mm Italian thread square taper as I've already said, but I don't have the original to check the spindle length, nor is it common frame manufacturer (Leonino da Bologna/Paletti). All I can work out from old BBs in my spares box is that the spindle length is more than 111mm. Trial and error then? I assume it's not offset....
Will be fitting a Stronglight 49D.

Chainset designs vary enormously; commonplace double chainsets (with the same chainline) could use anything from a 102mm ISO spindle (which has a smaller taper width than JIS) all the way to a JIS (or similar taper) in a length well over 120mm.

Stronglight 49D is an 'old' stronglight design, which uses a taper that is close (but not quite identical) to JIS. Depending on how worn the taper in the crank is and the clearance to the chainstay, a 49D double will often use a spindle that is 118 to 120mm long. A triple will often need a slightly longer spindle than that.

BTW a standard Italian threaded bottom bracket is normally 70mm wide with two RH threads in it, 36mm dia.

If you offer up your cranks to a JIS BB (almost any length), you ought to be able to measure what 'chainline' you obtain, i.e. the distance from the midpoint between the chainrings to the middle of where the BB shell would be. Most road bikes (with a double of about that vintage) use a chainline of about 43mm.

You can buy a BB-UN55 with Italian threading in nominal 118mm width. If this is the correct length then the 'chainline' on a similar 111mm (nominal 110mm) unit will be wrong (too short) by about 4.5mm.

hth

cheers

[recordacefromnew]

With Stronglight 49D, if not already do ensure you have the right puller before you start, and if in any doubt get help.