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I'm building a tandem at the moment. The frame appears to be threaded for a 1.45" Chater Lea bottom bracket.

I'd like to fit a modern bottom bracket. Any ideas? Here's my ideas so far:

  • Threadless cartridge bottom bracket: Shell ID is probably way too big for these, but maybe make a sleeve to go inside? Will that give enough grip?

  • Machine a counter bore for press fit bearings: Doesn't leave a lot of material left on the BB.

  • Drill the ID of the cups to accept a JIS spindle: No idea how I'd calculate what size balls to use. Also cups are really hard so difficult to machine?

EDIT:

I should clarify, the question in hand is about the rear BB, which has a shell width of 74.5mm. Chain tension is not an issue, since the front BB is eccentric and I'm sure I've got a solution for that.

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  • Just wondering if you found a solution to the 1.45" bottom bracket conversion as I have found my self in the same boat.
    – stuart
    May 17, 2021 at 16:03
  • @stuart In the end, I faced the frame shell down to a normal size (73mm?) and made a part to adapt the shell. To make the adapter I bought a pre-tapped ISO shell from a frame builder and then turned down the outside to have the Chaterlea threads on it. I kept a lip on one side and added flats to it so it had something to butt up against. It's torqued in pretty tight with some thread lock. No problems with it whatsoever.
    – Chris Adams
    May 17, 2021 at 20:39

2 Answers 2

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You can just buy Phil Wood bottom brackets for Chater Lea.

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  • That would be a superior solution, but how does OP deal with slack in the timing chain?
    – Criggie ♦
    Oct 16, 2016 at 5:45
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    They don't say anything about needing help with that aspect, from which I infer that it's got a pulley type tensioner, as many old tandems do, or it's got an eccentric with Chater Lea threads.
    – Nathan Knutson
    Oct 16, 2016 at 7:38
  • I'd thought about this.
    – Chris Adams
    Oct 16, 2016 at 11:41
  • ... There's two problems. One is the expense. Secondly from Phil's site philwood.com/products/bbhome.php he says it's available for a shell up to 73mm, so I'd have to face my frame 1.5mm? Also do you think a 120mm spindle would be long enough? Chain tension is not a problem. The front BB is eccentric and I have a solution for that.
    – Chris Adams
    Oct 16, 2016 at 11:43
  • The listing of shell widths you see there is missing a few obvious ones (such as 70 for Italian) and looks to just be a recital of the standard ISO widths, plus 63 for some reason. The Phil design uses relatively thin cups without shoulders, so unless they weren't able to sink in to the frame sufficiently (not likely), it should be totally fine. The spindle length depends on your target chainline and the cranks you're using. Once upon a time there was a hack setup involving using Phil cups with certain Shimano BB cartridges to save cost, but they haven't been made for a while now.
    – Nathan Knutson
    Oct 16, 2016 at 16:31
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I'd avoid machining anything if you can get away with it ... "standard" sized components would be best, if you can find them.

You need something that can be adjusted to take up the slack on the timing chain, or you would need a chain tensioner and they're ugly.

This is an eccentric bottom bracket, which will allow the use of a standard 68mm English bottom bracket cartridge for the front rider. The whole unit can be rotated inside the bottom bracket housing to take up the slack of the timing chain.

http://www.bushnelltandems.com/graphics/eccentric-retro.gif

So ideally you'd find a 1.45" eccentric, and fit a boringly common modern threaded bottom bracket.

This item will NEVER wear out. The only things that will kill it are corrosion, or a BB cartridge that is unremovable for some reason.

Do try and avoid the press fit BBs - creaking is bad enough when it annoys one rider - do you want to annoy two riders at once ?

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    Thanks for your response. Unfortunately I looked and there's no such thing as a 1.45" OD eccentric BB. Chain tension is not a problem anyway, the front BB is eccentric and I have a solution for that already.
    – Chris Adams
    Oct 16, 2016 at 11:45

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