Jockey wheel size in RDs
 

I'm wondering what are the pros & cons with some RD that are long cage but uses smaller jockey wheels while some especially MTB RDs have shorter cage but with larger jockey wheels.
I'm thinking the latter allows for the same chain capacity but at a shorter form factor. But how about shift quality?

Cannot speak to shifting feel , just that yes there are 10, and 11 tooth pulleys , used on many derailleurs.
also a bigger one .. proprietary Shimano , I guess , on their lower end RD's
for the non performance enthusiast cyclist.

a smaller upper jockey wheel can sit in closer to the cassette with less b-tension, which would give better shifting due to the chain having a sharper angle to the cassette.

but, IMO, not a big difference.
you can always read up on the marketing dribble from the makers of after market jockey wheels.

|3iker, I see no shifting benefit that can be attributed only to the jockey and tension wheel's size, except for the smaller jockey wheel/B screw clearance AEO mentioned.

A weak argument could be made that the larger wheels are kinder to chains, I suppose.

Brad

years back Sun Tour made a 3 pulley RD, I suppose to try to make a shorter cage
wrap chain slack up like a longer one..

Were it to work, seems it practical, using on small rear wheels
where the bottom pulley gets within inches of the Road surface,
as is on Bike Fridays and some Tadpole Tricycles with Dual Drive Sram Hubs..
and 34 t Low gear cog cassettes, + a triple crank..

But there are none made like that, any more..
Big wheel MTB stuff just gets adopted ..

Seems to me, larger jockey wheels would take up more chain slack for a given angle of the cage.

certainly increases the distance the chain passes thru, But the side plates of the RD
have to be on either side of the chain, so need to be manufactured around that size pulley..

maybe 10 on top and 11 on the bottom
but 14 or larger , tooth pulleys may be unsuitable , if available.

I can't say what's in the heads of manufacturers but if the same manufacturer uses smaller wheels on long cage RDs and larger wheels on short cage it might be to compensate for using the same body on both versions. The Jockey wheel diameter factors into the cassette clearance, and changing wheels is simpler and cheaper than building two complete models to accommodate various cassette sizes.

The biggest difference would be that larger pulleys run smoother.

You are assuming all are running on the same bearing surface,
usually a plain bushing,
or a metal bushing and the inside of the plastic pulley itself..

Aftermarket Pulleys use a cartridge sealed bearing. in which case smooth
is independent of size
as the race bike RD use an 11t with a ceramic bearing, if you pay for that ..