jwb

Did some searches and found some older posts that suggest you can use the Century 12 rear cluster from Harris without changing the short cage RD (that's also their claim). But I guess I need some more recent feedback. I've got a 9 spd Trek 2100 with 53/39 chainrings and want to use the Century 12 cluster that has a 30T rear cog. I expect to have to lengthen or get a new chain, but is my short cage Ultegra RD gonna work? Lots of conflicting info from Shimano and others. So will it work? And can I use the SRAM PC-951 chain (recommended by Harris) on my Trek? Thanks for the help.

jwb

Retro Grouch 

There are 2 separate rear derailleur capacity issues.

The first is large cog capability. Shimano doesn't recommend using rear cogs larger than 27t with it's road rear derailleurs. Virtually every bike mechanic, however, says you can safely "cheat" on that by dialing in the "B" screw until the upper pulley doesn't rub anymore. The downside is you'll have less chain wrap around the smallest rear cogs but that doesn't seem to be a problem.

The second is chain slack take up. Shimano short cage road derailleurs generally are rated to take up 29t of chain slack (add the differences between the smallest and largest front chainring and rear cog). 32t means you'll be "cheating" on this one too. If you decide to keep your short cage derailleur, my strong recommendation is to make sure that you have enough chain to safely cover the big/big combination and fight with chain slap in the little/little combinations. Chain slap is much less of a problem than the damage that shifting a too short chain into the big/big can cause.

Honestly, if it was my bike, I'd install a mountain rear derailleur and eliminate both issues for sure.

FBinNY 

There's a bit of fudge in the range limits on most RDs. The manufacturers publish a same upper limit which will always work but you can usually go beyond that a bit depending on the chainrings, and by optimizing the "B" screw adjustment, and chain length. It's a gray area, but can be made to work.

HillRider

Your gearing will require your rear deraileur to "wrap up" 32 teeth ((53-39) + (30-12) = 32) worth of chain. Shimano says the current short cage Ultegra has a capacity for 33 teeth so you are ok. Even if it was a bit under capacity, it could be used by avoiding the small-small gear combination.

However, check to be sure the 30T cog will clear the derailleur's jockey pulley. Shimano claims a maximum rear cog of 27T for their older designs and 28T for the newer ones. Experience indicates that they usually clear 30T and sometimes 32T but you have to be sure it will work on your particular bike.

jwb

Hmmmm.......It seems to be a "try it on for size" issue. I was hoping someone had already crossed this threshold.

Retro Grouch,...what mtb rd would you recommend?

Thanks for the great responses.

jwb

Retro Grouch 

What other components are on your bike?

I like to match the quality level that's on the bike. If you have an Ultegra equipped bike, I'd step up to an XT derailleur. If your bike has 105 components, I'd install an LX rear derailleur. Make sure that you get the traditional, high normal version or your shifters will work backwards.

jwb

[QUOTE=Retro Grouch;10376580]What other components are on your bike?

Mixed bag.....105 on the front and Ultegra on the rear. Guess I can take my pick? So far things have been absolutely trouble free.

No one mentioned the chain? Guess it was a real noob question?

Thanks for proper version heads up.....my life is backwards enough!

zzyzx_xyzzy

Watch out for the difference between "rapid rise" and "normal rise" rear derailleurs. The rapid-rise work the opposite way your road derailleurs work, so high gear becomes low and vise versa.

barturtle

I'm using a short cage Tiagra on a bike with the Century Special (13-30) and 46/34 compact.

davidad

It'll work. You may have to increase the chainlength to keep from having the big-big combo damaging the rear der.

helicomatic

I've installed a number of these cassettes, and it really does depend on the particular bike how well it works. Some derailleur hangers sit further back or lower than others relative to the axle in the dropout, meaning you can get away with using a larger cog on certain bikes easier than others. I haven't encountered a bike with any Shimano 9 speed road parts that it didn't work on. Sometimes you don't even need to screw in the B-screw all the way, but occassionally you need to replace it with a longer one.

No matter what you do, though, the derailleur won't shift as well as using the "approved" parts, because as you tighten the B-screw, the upper pulley moves away from all the cogs, not just the big one you're trying to fit in there. In practice this is not a big deal.

My commuter bike has the Century 12 with the same Ultegra 6500 short cage derailleur, and it works just fine.