robin-d

Would a long mech with the new box 9 speed wide-cassettes work with a 20" wheel?

(https://www.vitalmtb.com/features/On...rivetrain,2760)

I'm interested in a larger cog for hills on my folder.

I have a single 60t chainring up front

Robin

BromptonINrio

it will work but may be too close to ground. if you give an sharp turn ou going on some light trails it may hit the ground or a rock.

Hinge

I agree. Long cages will work, but when the cage is fully extended they can get pretty close to the road. That derailleur in particular looks longer than most.

However, here is something to think about:

The Box 9 speed cassette you linked to is 11-50T. 50T is HUGE! With 20" wheels, it's YUUUUUUUGE. Total overkill. You could probably ride up a vertical wall with that. Forget about the length of the derailleur cage, the 50T cog is probably bigger than the rear wheel! I'm kidding of course, but that's REALLY big.

For comparison, on a 700c road bike with a compact 50-34 crank and 11-32 cassette, the gear inches is ~27.88 in the 34-32T combination. This is generally considered pretty easy gearing. On 406 wheels (I'm assuming you have 406 and not 451), in the 60-50T combination, the gear inches would be ~21.35. That is significantly less, 25% roughly. You would be spinning so fast your knees will be hitting your chin! You will likely think it's too easy and impractical.

I'm probably completely out to lunch, but that's my story.

tomtomtom123

I have 12-34T cog with Shimano 4700-GS rear derailleur. The bottom of the cage is almost in line with the outer edge of the rim when fully extended, so all that's keeping it from touching the ground is the inflated tire height, which is acceptable for me. I can't remember the measured length of the cage. Different models will probably have slightly different lengths even if they're all GS.

I then have a front derailleur with double chainring with 24/44T. It gives me 519% range. The front derailleur was attached to a braze on adapter that I made myself.

pinholecam

I was using a XT mid cage and 11-36t rear cassette on my Moulton TSR, no issues with it.
It was rather near the the ground though its still above the rim. (just a bit)

tomtomtom123

Also, regarding the comment that 22 gear inches being too low, it's too high for me. I need minimum 18. I have 14 gear inches on the 24t chainring and 34t cog. It lets me go up 15 percent incline at 70 rpm.

bikebikebike

It is so bike dependent.
Look at the Greenspeeds 16". Less than 20 Gear inches you can climb phone poles, which is sometimes useful
On my 20 inch trikes I am often forced to use sidewalks too ovoid high-traffic streets
They sure as hell are not off-road capable with the mech hanging that low
and one has to be exquisitely careful on rough pavement and curbs
If that's an issue you have to go to an IGH or Schlumpf
. Folding bikes and RD are always got to be a compromise, they are just not designed for small wheels and the exposure of folding
Part of the reason the Brompton is so funky is that it optimizes protecting the rear Mech.
Everything is going to be a compromise to optimize