Antifriction

Reddleman has been probing the limits of 1xN gear range on folders, starting with a Microshift Advent 9-speed 11-42 on a Tern, & then fitting another Tern with an Advent 11-46. The limiting factor is ground clearance, and while it's not for everyone, the clearance he gets, with a medium-cage derailleur, works for him. I've also pushed my old 7-speed Dahon in this direction, with an 11-34 cluster and a medium-cage Tourney TX derailleur (cage 77mm centre to centre, clearance 70mm).

The 10-speed Advent X groupset also uses a medium-cage derailleur, which by measurement of photos is no longer than the 9-speed version (although the pulleys are larger, 14t, which increases pulley radius by 4mm over that of a 12t). So here's the question: am I right in assuming that equal cage length means equal ground clearance, less that 4mm for the pulley? If so, I'll probably try the upgrade once parts are again available.

mirfi

Lets say it all fits. What size is the chainring, or what gear inches are you expecting

Antifriction

I'll take a 48t, giving 20-87" - which puts the 436% range lower than most would want, but then I'm 65, in a hilly city, and hoping to ride the folder up ski-hill switchbacks I used to climb on a 3-ring mountain bike.

Reddleman

Exhibit A is the 42T block, exhibit B is the 46T block. Both Terns are running Schwalbe Big Apples on 406 wheels. Factor in the extra 4mm of the bigger cog on the Advent X rear mech, the extra distance on the B tension screw for a 48T block, and how lucky you feel.

With a 48T chainring your minimum will be 21.3”, 19.5” or 18.7” with a 42T, 46T or 48T max cassette anyway, so the only considerable difference between Advent X or Advent at the low end is going to be the gaps between shifts and how much these might improve with an extra cog. The main benefit of the 46T over the 42T in my experience is the ability to keep the same low gear of 20” but use a bigger chainring for a higher top gear of 88” vs 75”.

Fentuz

I was talking with microshift about clutch derailleur for my smooth hound (which would be required on a 11-42 and above) and they don t do anything for 20” wheels.
they advise small cage only.

if you need range, the best option is something like Sora 2x with a 11-34 cassette on a short cage derailleur and then you select both chainring depending of the range required an you maximise the range ensuring there is a 16T gap between gears

cost wise, 39-53 (14T) is the cheapest option but you could find 56-42 and if you go for 11-30 cassette, 39-56 (17T) might work.

seat_boy

You need to look at the clearance in the largest cog. I mocked up a Shimano 10 speed (46t) on my 20" wheeled Bike Friday, and the cage barely cleared the ground in the largest cog. Too close for me.

Antifriction

Thanks for the feedback. It's that B-screw adjustment I was wondering about; sounds like it will push the tensioner arm down a few mm that really can't be spared.

pinholecam

For ground clearance, it will depends on users appetite for how low it is and where they cycle.
I have run mid cages on 20" wheels without issues for many years.
I have also done mid-short cage Shimano Zee 1x10 on 18" and done gravel riding.

Generally, road riding will be lower risk the low cage gets struck by anything,

The main drawback to me is dirt accumulation which I find is very fast with the cage so low on the ground.