I currently have a 105 11sp 11-28 cassette, with a Ultegra rear derailleur. Its what came on my Fuji. I would like to upgrade the cassette to a 11-32 Ultegra. What size derailleur and chain would I need with that?

Medium cage.
Assuming your chain is the correct length now, you will need about one more inch, but there are several common methods to get the correct length.
I prefer to wrap the chain directly (bypassing the rear derailleur) around the big ring and big cog, then add an inch, err on the side of too long (easier to shorten). Then install properly and test in all extremes.

You'd need a medium cage derailleur, or in Shimano parlance, "GS".

Here's why:

Your chain needs to be long enough to wrap around your biggest chainring and biggest cog. When you shift out of that combo, though, your chain would go slack if it wasn't for the spring-loaded cage of your rear derailleur. Rear derailleurs are sized by their ability to take up chain slack, and that ability is measured by a derailleur's "wrap capacity". To calculate your required wrap capacity:

1) Subtract the number of teeth on your smallest cog from the number of teeth on your biggest cog.
2) Subtract the number of teeth on your small chainring from the number of teeth on the large ring.
3) Add those two numbers together.

In your case, assuming you're running a compact double crankset: (32-11) + (50-34) = 37

So you need a derailleur that has a 37-tooth wrap capacity and can handle a 32-tooth cog. It just so happens that the Ultegra RD-6800-GS medium cage derailleur is rated for EXACTLY those specs.

As for the chain length, you'll have to measure that on the bike. catgita described one common method of doing so. We can't tell you a number of links because we don't know how long your bike's chainstays are. (And even if we did, it's probably easier to size it on the bike than to do the math.)

I just went from 11-28 to 12-32 and my 105 5700 SS works find. It's only rated for 28 but a member here said that he had had them work up to 32 and suggested I try mine. I'm glad I did because with a few turns of the adjustment screw it shifts from top to bottom with no issues. I don't know that the 11 speed will work but if you have th cassette I'd say it's worth a try. I didn't have to replace my chain, it was long enough to work big to big but I replaced it with a 1 link longer.

You can find the answers here:

Chain length sizing for bicycles with derailleurs

(how to size the chain, what to do in case RD capacity is not big enough... as well as how to calculate and what RD capacity is)

Just try what you have first even if not a GS. FYI, the rd-4700-GS is reported to have the same pull ratio as 11 speed ders and it accepts a 34t cog on paper. The same person who, somewhere around here, verified this already suspected compatibility, also though said that the ultegra rear der shifts smoother (on his 10 speed).

I went from 11-28 to 12-30 on a 5700 short cage derailleur. Obviously, there's a tad more needed with a 32t but with the B screw all the way in, it allows for more distance between the upper jockey wheel and the cassette teeth.

Someone on another forum gave me some great advice when I was looking at going 32t:

"Your RD will work with a 30 but the 32t will be challenging. Even with full use of B screw the issue is the clearance between the top jockey wheel and the bottom of the large sprocket. The diameter of a 28t sprocket is 113mm. The diameter of a 30t is 121mm - basically 2mm additional radius per tooth Measure what you have now and do the sums: is there 8mm spare space there ie to get a 32t in?


So, I guess the OP should measure the distance he has between the jockey wheel and cassette teeth and see if he has the required distance with the B screw all the way in. I have more than 8mm gap, so could probably get a 32t in there, I opted for a 30t in the end as I got a good deal on an Ultegra 12-30.

Here is the thread I posted my results in , post 46 has the pictures of the 32/SS
https://www.bikeforums.net/bicycle-me...ailleur-2.html