scarleton

Shimano makes both the Ultegra 6870 SS rear derailleur and the 6870 GS rear derailleur. The GS is longer then the SS, allowing the GS to work with a cassette up to 11-32. The SS only works with up to a 11-28.

The question is: Considering the GS will work with the smallest range, a 11-23, what is the advantage in the shorter SS?

Al1943

Short RD = lighter weight.

rpenmanparker 

Perhaps better shifting on the range it covers.

bradtx

scarleton, Besides weight a RD with a short cage is touted to be a crisper shifting unit, something I can't prove or disprove.

Brad

chewa

+1 to the above. In theory the shorter (more rigid) arm should provide sharper shifts.

Also, this allows you to go through slightly deeper puddles without wetting the lower jockey wheel!!

cny-bikeman

Not sure I buy that one. As the shifting force is applied by the top pulley, and it appears to be no further from the pulley cage pivot on the GS than on the SS, I don't see how there would be a difference in stiffness. However, There are two more factors.

• The long cage by itself does not allow a larger rear cog - it only allows more total capacity (difference between small-small and large-large). The GS can only handle a larger rear cog because the top pulley is further down from the hanger. That means that although it may handle an 11 tooth OK the pulley won't be as close to the smaller cogs and therefore won't shift as crisply.
• The shorter arm also works better with a more compact cassette (smaller difference between small and large cogs) when on the smaller cogs. The long cage will not rotate as far as the small one to take up the same amount of chain, so again will not be in as good a position beneath the smaller cogs.

rydabent

Weight weenies strike again. If you lose a race because of the "additional" weight of a longer cage, maybe you should train more.

Drew Eckhardt 

The short cage looks more traditional.

I never noticed a shifting difference. Shimano derailleurs have about 10g separating long and short cage (ex: 2010 Ultegra 6700 SS and GS are 190 and 200g respectively) which is only a 0.03% speed difference for a 145 pound cyclist atop a 15 pound bike headed up the steepest hills. Campagnolo has about 10g between short and medium (good for doubles with all cogs and triples all but 13-29 and 12-30) then 10g from medium to long although that's still 0.06%.

bikebreak

My short cage RD 7700, rated for 25t iirc, will handle 27,28, and even 30, though there is some noise.

bradtx

You're close, 26T rating, but the best I could fudge one to was 28T. Oddly enough the 6500 RD was rated for a max. 28T cog and the 6503 was spec'd at 27T.

Brad

Bill Kapaun

I use 12/13-23/25 cassettes on both my 9 speed bikes w/22-32-36/38 rings.

Since I don't need the chain wrap capacity of the Altus level RDER's (mine are 29T or less), I thought I'd try a Tiagra RDER since it should "theoretically" shift a "road" cassette more crisply. I typically ride as a 1X9 using the middle ring.

I played with the B screw adjustment until the cows came home and couldn't get it as good as the Altus.

ksryder

Short cage less likely to get bent.

I know it's a small sample size and I'm a very amateur mechanic but in the past couple months I've dealt with autoshifting issues on 3 bikes (2 mine, 1 friend) and on the 2 bikes with long cages, after eliminating other possibilities, it was a bent cage.

fietsbob

if you only run 1 chainring you wont need as much chain wrap-up.. the function of longer cages is to make up for slack created in small-small vs Lg-Lg, combinations.

Anecdotal item. on a 13-34 t rear cluster, I used a short cage RD (type with the chain cage pivot concentric ,same axis as the upper pulley)
It worked Fine.

so a 1by (x whatever speeds on the back ) or very small front difference between 2 chainrings, 4~5t, not 10~12t.