Originally Posted by
Jipe
I fully agree with this: starting at 10t or even 9t would have been better for small wheel bikes and give a wider gear inch range.
I have a Shimano Ultegra RD-R6800GS and RD-R-8000GS medium cage on my two Birdy with 50x355 wheels and never had any ground clearance issue. The RD-R8000 goes till 34t but according to 3T website accept 36t (I use 9-32t and 9-34t cassettes with them).
Capreo seems to be dying quickly.
The derailleurs made for 11T are usually just fine with 9T or 10T.
While this setup may be picked up by folding bike riders, the ad appears to indicate kid's bikes. And, a kid riding 20" wheels likely doesn't need a 9T cassette (although there is a 9T BMX sprocket).
So far my only issue with size and rear derailleurs is a 20x4.25 small FAT wheel that the derailleur overlaps with the rim slightly, and thus risks getting caught on the rim.
My Bike Friday with 451 wheels seems to be fine with older generation long cage Ultegra.
I'm not sure 1x gearing will work for my needs, although 1x works fine with a Dual Drive rear hub (which is also dying out).