Double chainring modification, I've moved away from that apart from emergency granny-gear.

The first thing that happens if you literally simply bolt a 28T on the inside is the bolt heads promptly scrape all the paint of rear triangle as you fold. That's after you've realized the chain rubs on the stay. If you then post-bike-damage then put a slightly longer BB on, e.g. with a spacer, then the next issue you find is the shorter distance from the sprockets means the inner-chainring / outer-sprocket combination, the chain rubs on the left side of the inner-chainring. So then you have to use an even longer BB and wider spacers til it all works. You need to push the right chainring out by about 3mm and use 3mm-3.5mm chainring spacers.

For a bike which either won't be used often in hilly areas, or the steeper hills are infrequent, a double chainring makes sense. Since I made up customized front-ends, Brompton came out with the BWR which produces a much wider range and that kills 80% of the reasons for why folks did such stuff pre-2009.

I have a made 30T/54T on my 2-speed Brompton and I have on order a new Brompton with BWR on a 44T "gear reduced" but I'd still be interested in a double chainring just to be able to handle the once-a-year steep hill I might find ahead. For short hills, get off and push but some hills take an hour to climb and then biking is easier and faster than walking.

Bolting a smaller chainring on the OUTSIDE of the larger chainring, true that means not having a longer BB, but

• The inner sprocket is your lowest gear so you're creating a very crossed chainline at your most intense effort.
• you still need a quite wide chainring spacer to make the chain clear.
• If manually shifting, its easy when the hill's gradient is showing its needed, to push the chain to the left with your foot.