Sjtaylor

I知 recommissioning a long cage RD to use a 12-36 where a 12-27 was. I致e got a couple RDs to choose from and I知 noticing that the idler sprocket closest to the cassette has a beefier hub than the opposing idler sprocket. I guess this makes sense since the closer idler sprocket is subject to more side forces when shifting. I知 also noticing that the closer sprocket has more side to side play. I知 guessing that side to side play is from wear.

can I safely swap the idler sprockets? Will the side to side play in the bearing affect the shifting. Also I have a Deore RD and an XTR RD both with a long cage. The XTR has larger idler sprockets than the Deore effectively allowing the XTR RD to take up more chain. I wonder what affect that will have.

FYI I知 lower the gearing on a several year old Serotta road bike. It has Ultegra nine speed.

Thanks.

UKFan4Sure

First, make darned sure your derailleur can handle the new chain wrap. You are increasing your wrap by 9, a significant amount. Whether you can swap idlers or not depends on the RD. I've seen yeah and no.

dedhed

The side to side play on the upper pulley is by design to allow for imprecision in the shifting adjustment.

"In general", on shimano derailleurs, the upper and lower pulleys are not interchangeable

Yes, the side to side play is there to assist shifting.

Andrew R Stewart 

A few points-

Terms first- The top pulley is usually called the guide pulley (as it guides the chain onto adjacent cogs) or the upper pulley as it's above the... tension pulley (as it provides the tension on the chain) or lower pulley. Using the same name for both pulleys might be confusing to some.

Shimano (and others since) have found the tooth profile (both in side and section views) of a best working guide pulley is different then that of a tension one. Different jobs and thus different shaping. The guide's float helps make up for tolerances in manufacturing as well as wear. The tension pulley doesn't have this feature although both have some diagonal slop, dependent of the wear and grade these can overlap though. One of the reasons that after market pulleys have not won over the industry is because Shimano has done a real good job designing and producing their OEM ones and so many aftermarket pulleys lacked the same features.

Pulley diameter has nearly nothing to do with chain wrap. That's the job of the cage length and pivot geometry. Pulley diameter does affect pulley RPMs and thus wear and also will change the open gap between the guide pulley top and the largest cog's bottom. To a degree a smaller gap is better shifting *and thus why the extensions of the hanger length devices can lead to sluggish shifting).

Pulley width does need to be paid attention to if crossing between cog count (system "speeds") as more cogs is narrower pulleys.

Youi are experiencing the challenges of getting lower gearing while only changing one half of the system. What's your rings? Andy

jbucky1

The upper and lower pulleys are not normally interchangeable on newer stuff due to specific function. Guide at the top and tension at the bottom.
They are also directional.

www.buckyrides.com
ig: jbucky1

JohnDThompson 

What specific derailleurs are you referring to here? Side-to-side play in the guide (upper) pulley is a design feature with derailleurs intended for indexed shifting systems, where post-shift trim is not possible. The play allows the pulley to naturally settle into a neutral position after the shift.

Bill Kapaun

IF your RDER's are "older", you may have trouble with a 36T cog.
Model#'s may give a clue to capacity. Both max cog & chain wrap.

Sjtaylor

I’ve been told about compatibility issues between road bike and mountain bike indexing. I’m hoping older Ultegra brifter is compatible with the derailleur I plan to use. I’ve got a derailleur extender on the hanger now. I’ve yet to install and try the long cage derailleur but now that I know more about the idler pullies I’m ready to give the perspective new gearing a try.

I’ll use the old too short chain on the center front ring to check indexing and guide pulley clearance with the larger cogs. I’ll go large to large to determine chain length and hopefully have the complete shifting range.

I’m enjoying learning about this stuff and if I fail with this gearing change I can always go back. Reality is nothing is flat around here and popular routes have climbs steeper than I can handle with the 30+ GI on this road bike.

Thanks for the info.

Sjtaylor

The chain rings are Ultegra 52-42-30