BB axle length experts please

[Simon Masterson]

Hi all, just wondering if anyone around here is good with chainlines and axle lengths etc? I'm doing a '90s TT build with Campag pista cranks, and the 111mm Veloce cartridge I bought isn't long enough. I was hoping that 115.5 might work, but I gave it to my LBS, and they say that the current production Campag one in that length that they tried didn't work.

Long term plan is to get a nice Royce BB, and they do a good range of lengths, but any idea what length I might be after? I was really hoping to be able to do it on the cheap for now...

 

[mdvineng]

You could put a 2mm shim ring behind the lock ring on the drive side, that move the chainring out enough, then check the none drive side crank arm for clearance.

 

[torqueless]

Re:

What follows is theoretical rather than practical, but it is a good starting point-

To get an accurate chainline you need to work from the 'centre-line' of the bike:

1.Find the point on the rear hub that is precisely central between the dropouts.
2.Measure the distance in mm from that point to the 'centre' of the sprockets- i.e. for an 8 speed cluster the point between sprockets four and five.
3. Find the centre point of the BB shell.
4. Measure the distance in mm from that point to the current chainring line.
5. If your chainline was accurate, the measurements from steps '2' and '4' would be identical. The discrepancy between the two figures tells you how far your current axle-length deviates from your ideal axle-length.

Some of that measuring is much easier in theory than in practice!

Personally I would be OK with that sort of clearance between non-drive side crank and chainstay. Does the drive side currently have similar clearance, or significantly less clearance?

 

[964Cup]

Re:

You may also be running into issues with different tapers here. Campag used what is effectively a JIS taper until (I think) 1993, then switched to ISO. If you're using pre-93 cranks on a current-model BB, the cranks will travel further down the spindle, reducing your effective axle length. Phil Wood and Velo Orange both produce JIS-taper BBs in a wide variety of spindle lengths. You may need an asymmetric spindle (Super/Nuovo Record BBs of the period were asymmetric) which is longer on the driveside than the non-driveside.