I purchased a new bike a few weeks ago and could not get the shifting right. On closer inspection the b-screw on the 105 6800 rear mech was damaged and it could also have done with some new jockey wheels.
I'd taken the rear mech off already for cleaning and inspection, this gave me a good oportunity to upgrade to an Ultegra R8000 mech.
I'm using a 11-28 cassette and 52/36 front rings. Using teh calculatiojn from this link: https://guides.wiggle.co.uk/rear-derailleurs-buying-guide (52-36)+(28-11) = 33 which meant i needed a medium cage GS mech.
I couldn't find any marking on the 105 indicating cage length but presumed it would be the same as these are the same ratios the bike was supplied with. So I ordered an Ultegra R8000 GS.
It arrived on friday and I quickly fitted it so i could ride over the weekend. As the 105 was already off the bike i didn't have it immediatly to hand to compare.
After a couple of questions as to why I was running a mountaing bike rear mech I dug out the 105 again and teh cage on teh Ultegra is much, much longer. So I presume the 105 is a SS.
Have i got the right derailleur for my ratios? The link above states that SS only has a 29 tooth capcity which I am exceeding, and although the shifting performance of teh old mech was poor it didn;t look like it was down to capacity.