I'm in the process of stripping a SWB Claud Butler tandem so I can get it repainted. There are a couple of things I need help with....

a) Can you fit a front mech to the bike, and if so, are there any "gotchas" to avoid, or limits as to chainring size ? I've currently got a 9-speed cassette with long cage mech on the back, and we'd like the bike to be 18-speed if possible....

b) What size is the eccentric cam for front chainset ? Current one is looking like it's seen some (ab)use over the years, so may need replacing. Is there a standard size for these ?

BTW, I'm trying to get it nice and tidy for racing... well it IS what it was designed for...


  • peterverdone.com/wiki/… describes some common eccentric BB sizes, and how to shim one that isn't quite right. Not a proper answer, but might help if you don't get one.
    – armb
    Commented Jan 29, 2013 at 13:52
  • Does these photos more or less match the configuration of your bike?
    – dlu
    Commented Jul 24, 2015 at 12:55

1 Answer 1


You may have a few troubles with the curved stoker seat tube, front derailleurs can be remarkably tricky to set up correctly. You can find some more useful experiences here. In particular, you may have to experiment with some low cost parts before committing to something pricier as your choice of chainring sizes will determine where along the curve the derailleur must sit and so what effective angle it will be mounted at. The maximum allowable chainring size will be determined by the length of your bottom bracket spindle and the clearance of the chainstays. Either crack out some geometry or try it out. It's always tricky to work out chainlines on older parts!

In answer to your second question, your best bet is to have a look here and compare the measurements they have to your frame with a pair of callipers. SJS sell the Thorn tandems so always have a good range, you may have some luck with Tandems East if SJS can't help. Although they don't list the sizes they do offer custom EBB inserts (from the US, at a high cost I expect).


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