Mixing road and MTB components

[The French Tandem]

This^^  is a very good point. I tend to see gear overlap in a triple chainring drivetrain as a sign of poor design, but you make me realize that I may be completely wrong! I never calculated exactly, but we probably ride on similar gear ratios with both the 39t and the 52t rings, by using different sprockets.

Chain tension is probably not an issue in our case. The optimal single chainring would be a 44t, which falls in between our current 39t and 52t rings.

[Ningishzidda]

This^^  is a very good point. I tend to see gear overlap in a triple chainring drivetrain as a sign of poor design, but you make me realize that I may be completely wrong! I never calculated exactly, but we probably ride on similar gear ratios with both the 39t and the 52t rings, by using different sprockets.

Chain tension is probably not an issue in our case. The optimal single chainring would be a 44t, which falls in between our current 39t and 52t rings.

Which is what I have, on a double with a 22 for steep hill climbing 
Very seldom need the 22.

The 44 ring gives me 50 to 100 inches using the 24 to 12 sprockets on a nine cassette.

[The French Tandem]

So, If you had bigger sprockets than your 24, you would probably never use your 22 chainring! That was my reasoning at first. What is likely to put me off this path is the silly price of 10-42 cassettes. The Ultegra cassettes that we use now are €40 if I buy them in Germany. Sram's cheapest 10-42 is €220.

[zigzag]

any gearing setup is a compromise in one way or another, you just choose the one that suits you best. i rode pbp with 42t and 11-34 cassette, never felt disadvantaged but in england on very steep hills it was hard work. my previous commuter bike had 36t ring and 11-34 cassette, did a fair few audaxes on it (8hr 200s on m+ tyres) including two easter arrows, simple, reliable and hasn't given me a single problem re gears.
i now ride triple, but i reckon ideal gears (if not racing) are 44/30 with 11-32 cassette; most modern rear mechs can handle 32t sprockets. i rode 45/30 with 11-28 in the alps and scotland last year and it worked great, probably the best setup i've had.

[mzjo]

When I built up my tandem for the first time (in 1981) I had a single 44T ring and a 16-28 4sp freewheel (which turned out to be a 5sp without the 14 sprocket) which was terrible because the 16 was the wrong way round so the chain wouldn't sit on it (effectively a 18-28 3sp). Soon afterwards I fitted a 14-34 5sp freewheel (and a Huret Duopar mech) and this was sufficient for our needs, including with a heavily laden trailer for camping - but we were young! (The tandem weighed about 25kgs being an anonymous pre-war frame).   

The last gearing on this beast was a 6sp 13-34 Mega-range freewheel and a 40-30 double chainring (in-line drive). Sufficient for some quite reasonable Limousin hills with an underfit blind stoker (who preferred my tandem over one of the club Look's because it was much more comfortable!)

I think there is a law of diminishing returns with sprockets bigger than 32-34; the complication of fitting and running the set-up outweighs the gearing reduction achieved. I would go for a 9sp 11-34 and keep the compatibility issues at bay (in fact I would prefer 8sp on a tandem, I have not been impressed with the reliability of Shimano's 9sp offering). But then I am not really bothered about indexing.

[Polar Bear]

Reading this thread has made me appreciate all the more my stable of 3x9 speed Shimano machines.   Perhaps this is the perfect setup.