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  22  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.






rdmp2 [86 posts] 5 months ago

Ss cage should be ok for those ratios. 33 tooth capacity iirc.
I have ss cage for 50/34 11-28 and is perfect so you can switch it or ignore the snide comments and fit a 34 tooth if you ever do the Fred Whitton

kil0ran [1800 posts] 5 months ago

You're good to go. GS cage minimum max sprocket (if that makes sense!) is a 28T, and you're well within capacity. You could have specced a short cage mech and still been OK, but having the GS mech shouldn't cause any big issues - all you've got is a longer chain run through the jockey wheels than necessary, which I guess might possibly lose a tiny bit of drivetrain efficiency.

Always handy to have a GS mech available to my mind unless you're just doing flat crits as it means you can whack in a big cassette if needed. The R8000 GS cage supports a 34T cassette (officially) and you'll probably get away with bigger than that if needed.