Perplexing chainline question: 145mm tandem spacing, 3x to 2x crankset conversion and Cues components

I have a perplexing chainline question.

I intend to ‘build’ a new 2x11 drivetrain for our tandem based around Shimano Cues components.

Unsurprisingly, Shimano does not produce a Cues tandem crankset. As such, I will use generic to-be-determined 11-speed chainrings mounted to our existing Deore XT M730-T crankset(s). This is a triple crankset.

The chainline of an XT-M730-T crankset is as follows:

122.5mm BB = 47.5mm chainline
127.5mm BB = 50mm chainline

Those figures, of course, assume the middle-chainring as the centre of your chainline.

Converting the crankset to 2x, I will either mount the chainrings to the middle and outer, or middle and inner. This which will result in a different ‘effective’ chainline’.

The chainline for a Cues 2x crankset is 48.8mm.

Making matters more complex, our tandem has a 145mm-spaced rear end. The wheel is built around this hub. The bike does, at least, use a standard 68mm shell.

In summary, what combination of chainring positions and BB spindle lengths will result in a chainline (or effective chainline) closest to 48.8mm?

Further considerations: I intend to spec an 11-45t cassette. If I use the middle chainring position (110mm BCD) for the ‘inner’ I will be limited to a minimum 33t chainring, giving me a 20in low gear.

That’s approx 2in higher than our current 24x36t 9-speed setup. It would be preferable if I were able to use the inner chainring mount so I can spec smaller chainrings. The position of the inner chainring is also determined by removable spacers, providing chainring spacing adjustability.

Apologies if that’s as clear as mud! Happy to elaborate further if required.

Why are you trying to recreate the stock Cues chainline? That'll all be designed around a different hub than you are using.

I am not necessarily trying to recreate the 48.8mm chainline (which I assume is designed around a 142mm disc rear end?). I want to end up with the same 'effective' chainline – or at least one that is as close to optimal for my 150mm rear end using my existing cranks. Essentially 48.8mm + whatever to make it work with the 150mm rear end.

I asked the same question on my Instagram and the most authoritative answer suggested the longer BB with the chainrings mounted on the outer position would be best (edited for clarity):

I’d suggest, because of the wider-than-standard rear hub, you actually
want the chainline to be 53.8mm-ish.

The rear hub is 10mm wider than you’d normally use on Cues, so the cassette
is 5mm more outboard. As such, add 5mm to chainline. The bistance between
chainrings is approx. 8mm, so +/-4mm to convert triple chainset to outer/inner
double.

So the outer positions with the longer BB should work well. As you’ve
used the crankset with the rear hub before I wouldn’t be surprised if
it works in either position though, especially with an even longer BB.

I unfortunately don't have the bike to hand so can't confirm all this but it sounds reasonable to me.

The Instagrammer is correct. On a tandem, I spend most of the time in the big ring, so tend to have the chainline slightly inboard of ideal. I would run the chainline as 50mm or a bit more but that would be difficult if you are using the inner two chainring positions.

A very fair point. We almost never use the middle chainring on our tandem, mostly because we're either going dangerously fast or excruciatingly slow.

Provided the claims re increased lifespan and smoother shifting under load hold true I reckon the Cues/Linkglide stuff could be perfect for tandems. A few colleagues have tested ebikes equipped with Linkglide XT drivetrains and speak highly of it.

Bigger BB, chainrings on your preferred place and try. You can measure as well but if it's visually centered and works trough all gears with no issues or very bad angles at either end it's good enough.

I don't mind experimenting but obsessively researching extremely niche tech questions is almost as fun as riding bikes 😇

Something blindingly obvious I haven't taken into account with all of this – front derailleur compatibility!

The Cues FDs are, of course, designed around a 48.8mm chainline. I'm sure I could push this a little but not to the extremes required for 'optimal' chainline on the outer chainring mounts.

A 127.5mm BB would result in a chainline of approx 44mm-ish if I use the inner and middle chainring mounts (and would enable me to use the smaller chainrings preferred).

Better still, cheap 131mm BBs are available, resulting in a 47.5mm-ish chainline – not great, not terrible. If that works well, I'll eventually plump for a Phil Wood or TA BB.

Looking back at my notes, we're currently running a 122.5mm BB with an 11-36t cassette, and it's been fine (admittedly with friction shifting). I'm sure the 131mm BB will work just fine.

I realise I am massively overthinking all of this but what else is one to do on a Saturday night?

Gave it a think. If the chainline is designed around a 142mm rear hub and you have a 145mm one it's only a 1,5mm each side extra. So you'd want 50,3mm chainline to have it perfect. FD should be able to go 1'5mm extra out without too much drama.

Apologies – I forgot to respond when you first replied. This is tremendously helpful, thanks. The parts will be ordered soon and I'll report back in the new year.