Questions about adding a two gear crankset to a bike not prepared for it.

Hello to everybody.

I've recently acquired a folding urban bike, and I'm having some issues with the current gear ratios. It's a 6 speed bike, and there's not much space available in the frame to add 7 or 8 gears in the back, so I'm considering options for adding a new crankset with 2 chainrings. That would be enough for my needs. Current chainring is 48 teeth.

The main issue I'm seeing is the front derailleur. Two problems, one is that the frame's vertical strut has a diameter of 40 mm, and I haven't seen any derailleur that can manage that. I've seen derailleurs with bottom bracket attachment, that are mounted to the crank's cassette, so that would do it.

The second issue is the cable housing, as the frame has no place for it, and I've seen zero front derailleurs with a housing for it, analog to the one in the rear derailleur. I haven't seen any option for solving this particular issue.

I'm seriously considering simply adding the new crankset, with no derailleur, and moving the chain by hand when I need it. Given the expected use for this bike, I can live with having to stop to switch from high to low gears. But then, will the absence of a front derailleur cause the chain to slip out of gear easily? Would the tension provided by the rear derailleur be enough to keep everything stable?

Thanks for reading 🙂