Folding bicycle small enough to fit in hand luggage
I've spent a long time with folding bikes over the years. My advice: bite the bullet and buy a Brompton[1].Yes, Bromptons are expensive.  But they ride like a bicycle, have gears, a stiff enough frame, luggage racks etc.  There's very little compromise.  The fastest I've done on mine is 45 kph and I've ridden 70km in a day. I carry it on and off buses and trains multiple times a day without issue.In the past I had an A-bike[2].  It's a fun and cute little design but the wheels are absolutely tiny and has no gears.  You feel like you're riding a folding contraption rather than a bicycle. The build quality was rather plasticky too.  I like the idea of an A-shape with telescoping tubes. It's quite an ingenious fold.I've also ridden (but not owned) a Pacific CarryMe[3].  The ride experience again suffers for the short wheels and no gears, but it could be useful for some.  The fold still leaves you with quite a long package.I'd love to try the Kwiggle bike.  I don't think the strange seat will cause as many problems as people think, but I suspect the overall ride experience to be a bit bouncy and flexy. Maybe it's worth it for the small fold.For a more out-there design I think the Halbrad/Half-Bike[4] could be quite nice but it never really got past the prototype stage.  Under-seat steering is a bit of an adjustment too.[1] https://www.brompton.com/[2] https://en.wikipedia.org/wiki/A-bike[3] https://pacificcarryme.com/[4] https://www.youtube.com/watch?v=TQIC138vmlE