Just One Tube
 

I ride the new Africa Twin and prefer to travel as light as possible. A little obsessive in fact.

I carry a patch kit/spare glue/etc but from experience I would also like to take a spare tube. But just one lightweight (1mm?) one instead of the two heavier ones I typically carry.

It's only for an emergency situation when I really can't patch so it would only be required to allow me to trundle somewhere better.

Point being, which size tube as an emergency single tube for back/front would be best ?

AT tyre sizes : 90/90-R21 / 150/70-R18

I don't think you will get away with just one as the size difference is too much, my G/S has similar sized tyres and I carry one of each when travelling abroad. I also have thicker tubes with slime in them which for me is precaution enough at home.

I certainly wouldn't want to disagree with Mark and as he says having two tubes is the way to go but if you're looking for that last ditch it's better than walking so I might as well try it option I'd suggest taking the front tube.

Obviously it'll fit in the front and you can stuff it in the rear where it'll (probably) stretch to inflate the tyre. The other way round you'll have all sorts of fun trying to stretch the rear to fit in the front. You can do it but it's easier (for a short term fix) to stuff extra bits in than have to try and pull a smaller tube over the rim. All IMHO of course.

All this assumes that puncture repairs are something that holds no fear for you ...

My very first travel bike (below) was adapted to run 18/19'' wheels for one tube.
Nice idea but tubes are always a pain on a long ride and as said, 18 and 21 are too far apart in size.

I would consider converting your AT to tubeless.
I bet other AT owners talk of it.
There are various DIY sealing methods, or the BARTubless system.
But, depending on your OE rims, you may not be able to run them too low psi.
On a heavy AT, that is probably academic, anyway.