I am thinking about replacing my Ultegra 6700 Rear Derailleur. I haven't been able to note any markings on mine showing it to be a short or medium cage. From the pictures of each on the Shimano site I think it's a medium cage.

My chain rings are the compact 50/34 with an 11/28 cassette. The Shimano info shows the 11/28 cassette to be acceptable to both the SS or GS, but I'm not positive what they mean by: "Front chainwheel tooth difference" of either 16 (SS) or 22 (GS) teeth or less. Is that the difference between the big ring and small ring? If so, my 50/34 chain rings with a difference of 16 teeth should work with either, correct?

Also, what does Shimano mean by "Total capacity" 33(SS) or 39(GS) teeth or less?

My last question is, given a choice, which is better the Short or Medium cage, will either shift faster or smoother?
I'm sure that I'll never go larger than the 28 tooth cassette.

Thanks in advance.

Bob

Front chain wheel difference is exactly what you said, 16 for a compact, but it would be much larger for a triple.

Total capacity is the difference between (big+big)-(small+small), ie how much the derailuer needs to take up. If you exceed the total capacity the chain will go slack in small-small(ish) combinations.

Your required capacity is 33, so you would be fine with the SS, assuming the chain is correctly sized.

So long as your RD is functioning properly, you would gain absolutely nothing by replacing it, whether it's a GS or an SS. You won't notice any change in shifting performance between the GS and SS Ultegra. With your 11-28 cassette and 50/36 double, you could use either GS or SS, so if you actually need to replace the RD, look for the best price.

The shifting is acting up a bit, it intermittently hesitates shifting to the next easier cog, usually from the 13 tooth cog to the 14 tooth cog, sometimes it's several seconds before it shifts, which drives me crazy. So I am gathering all the possible parts needed to get it back to perfect shifting which it was for over three years.

I did buy a new short cage (medium cage was out of stock) from the lowest price provider, in this case it was Ribble cycles in the UK.

Turn the barrel adjuster out 1/2 turn.
Clean & lube cables.

Enjoy your new derailleur! Don't forget to service your cables when you switch out the old unit.

Done and done. Dealer has lubed and adjusted it a few times and replaced the inner cable. I'm tired of fooling around. Will now replace all cables inner and outer and the cable sliders on bottom bracket and handlebars(?). I'll have the rear derailleur if needed. The shifters seem fine, but if needed they'll be next.