The upshot of all of these discussions seems to be that if you stick with readily available components, you can swap out most of them after experimenting.

• The front chainrings are dirt cheap, available in LitePro brand for example, in different sizes for about $20 online.
• Standard Shimano cassettes and gears are also easy to alter - especially up to 36 teeth which I have on one bike. I swap individual gears quite often as my bike moves with me to different locations.
• Moving to 20 inch wheels is also easy [there are way more tyre choices at 20" 406 than 18" but they cannot be too fat or you cannot fold the bike].
• I have four birdys and I prefer to buy a cheap one secondhand, then source secondhand or sometimes new components. it is more satisfying and the equivalent of 'car restoration' I suppose.
• If buying new, I guess getting the best model is important, but sometimes you can retrofit a really expensive drivetrain that you got cheap somewhere else, and save money.
• I have a carbon seatpost and they save a lot of weight, and flex a tiny amount front to rear, but be careful - they are probably not as strong [the $70 versions from Aliexpress], although I have not broken one.
• The orange color bike you can get from Pacific cycles is really light, under 8kg without lights and so-on - it has a carbon chainring and wheels, with disc brakes. https://www.facebook.com/photo.php?f...type=3&theater [join Birdy Global Community]
• I have one 20 inch birdy at 8.5kilos with secondhand ultegra, carbon seatpost and bars, secondhand rear end and suspension, some sort of light shwalbe 1.25 tyres, and could go much lighter with a chainring and hollow axle bottom bracket. Litpro brake calipers rather than the 20 " conversion kit. That I keep in Australia these days and the terrain is pretty flat - bike is really fast. I also have a heavy 24 speed with a rack, in England, with 18x1.5 Marathon tyres, for shopping and touring over in Europe. The standard mk2 8 speed I have, 8 years old, actually has a 36 tooth low gear and 10 tooth top- this is occasionally useful.
• By the way the Marathon 1.5s roll well and are comfy - the lighter 1.35 punctureproofs seem to have more resistance, do not know why.