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A long time ago, I found a document or a forum post that gave me the information I require. However, I can't find it now so I hope one of you knows the answer.

Take the normal bottom bracket, BB-UN55, LL113. It's a symmetrical 113mm spindle.

The 107 is apparently the same as the 110 on the left side but shorter on the right side.

D-NL, D-EL etc supposedly match an older length sizing system and some of them have a longer drive side than non drive.

The D-EL at 127mm nominally should be the correct length for a replacement I need, but I need more of it on the drive side for a triple. There is no technical document I can find to tell me whether it is symmetrical or not.

Why not suck it and see? This is for a tandem and the timing chain wants to be in line on the LHS. I don't want to "invest" in a bottom bracket that I decide I don't want to use when the specification should be available.

Thanks.

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One of the potentially confusing things about looking at the sources that are out there for this is that the numbers are all referenced to the key dimensions of the pre-cartridge world, where cups and spindles were typically modular components you'd mix and match. JIS spindle codes are basically a way of dividing a cup and cone spindle into three sections: the middle section length between the bearing races, and the race to the tip on either side. The different variables in each code are giving you the length of each of those three sections plus telling you whether the spindle is bolted or nutted, and that's all the system is. When a cartridge BB bears a JIS spindle code, it's telling you what the equivalent looseball spindle would be or what it would replace, but it doesn't have the same physical reference points.

Here is a snippet from Sutherlands 6th that shows its take on how to interpret the codes:

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And here is the physical length data: enter image description here enter image description here enter image description here enter image description here

The JIS is still a living standards body. They appear to publish a current version of the JIS bottom bracket standard, but it costs money. I'm curious what's inside but I've never seen it.

It gets tricky with tandems as you know, since chainline concerns applying to both sides and not just one means fudging things with spacers doesn't apply in the same way that it can when making a symmetrical modern repair BB work with cranks that want asymmetrical.

One solution if the exact right asymmetric spindle is unavailable is to get complete Phil sets front and rear, make the stoker DS chainline be what it should, then make the captain sync chainline match where the stoker sync chainline landed. You can do the same in a cheaper, messier way using 73mm Shimano BBs (presuming the frame is 68mm) and using spacers under the drive side cups to adjust the chainline. The principle if you do it that way is you have some liberty to make the captain DS crank Q be whatever, which it might have to if they're non-matching cranks.

On some tandems, the eccentric overhangs past the shell enough that you could also use it to dial chainline and still have 100% contact with it. Obviously that's not really ideal, but it could also be reasonable if the budget isn't there to get one of the solutions that is ideal (Phil BBs or modern cranks, for example).

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  • Thank you Nathan, that was exactly the document I was after. I should have thought of Sutherland but it's a book that's less and less used now, as components have changed even where basic frame standards have stayed the same.
    – Noise
    Jun 25, 2022 at 15:26
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    I was able to recreate the chainline I wanted for the LHS while getting a workable RHS chainline by taking a 124mm cartridge bottom bracket and spacing the cartridge 3mm to the driveside, giving the same non driveside measurement as a 118mm spindle. This was possible with an RPM bottom bracket but I would have struggled with a genuine Shimano bb because its thread is shorter and the locking cup has a shoulder. Overall, excellent suggestions, thanks Nathan.
    – Noise
    Jun 25, 2022 at 15:32
  • @JoeK Glad it worked out! Jun 26, 2022 at 7:36

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