Originally Posted by
noglider
It has always seemed to me that when you combine derailleurs with an internally-geared hub (IGH), you get the worst of both worlds. Derailleurs are vulnerable to bending and breaking and need more frequent maintenance. IGHs are heavy and expensive. You may have a plan to mitigate the cost. Rigging up these two cogs might be a technical challenge, but that can be fun and rewarding. If you want it to be durable, I suggest you get one of those ugly derailleur guards that prevent the bending-in of the derailleur/hanger. They work, and you want a heavy duty bike, right?
Another consideration is that IGHs are not rated for the high torque that you make when you gear them very low. You can probably get away with it, though.
(bold above) My feelings exactly. Which is why I don't like the mix on a Brompton, but then, it needs that long-arm derailleur to take up chain slack when folding.
To the OP, what is your goal? Full chainguard (that a front derailleur would preclude)? Does the frame have horizontal dropouts for chain/belt tensioning? Going full IGH would allow full chainguard, plus belt drive if you desire, a big plus for an all-weather commuter. If you want 2 sprockets with an IGH, find a Brompton hub (BWR, Brompton Wide Range) so equipped, but be aware it's intended to be geared for 16"/349 wheels, but you may remedy that with chainring choice, possibly sprockets, more limited I think. Or just go front and rear derailleur, or these days, wide 1X cassette, which is super easy to do on large wheels; It's small wheels where there becomes more limitation on really large cogs, though you can still get into the low 40s on 20"/406 wheels. With larger wheels, you can easily go 11-50+ teeth on the cassette, and with the appropriate chainring, gets you plenty of range for a commuter, I think that is both your easiest and cheapest solution, and quite a common setup. My townie is 21 gear inches lowest gear, enough for all but heavy touring with long and steep hills. My high is 85, don't need higher, so about ~400% range, I think minimum unless you live in flatland. (You've currently got just under 300%.) More than that is gravy, which you can easily get with wide 1X cassette, and rear derailleur "slope" designed for it.