Your derailleur may not work at 32

Ok. What is the "biggest" gear it will take? 11-28? It is an Ultegra Di2 6800 derailleur.

Provided your rear deraileur can handle a 32:

You may need to add another link to your chain.
You may need to adjust the B screw to allow clearance between the top pulley and the largest cog

Get an XT delore RD. You will need a new chain. I just updated all 3 of my bikes to this setup, plus the compact 50/34 in the front.

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Buddy, read the post above. It is Di2.

To be honest I haven't a clue. I tried the very thing you want to do and the derailleur couldn't handle it. You might need something with a long cage (something that works with a triple ring). But I'm not a wrench.

What are you running up front? You can do 11-28 for sure on modern Shimano stuff.. maybe if you go semi or compact up front, you can get enough gearing?

53-39 up front. Have a quarq so stuck with the 53-39.

If I remember correctly, you can take the b-screw all the way out and screw it back in on the opposite side. It will get you the range you need. It won't shift as nicely but will work. You will probably have to add a link to your chain.

You need the 6870 GS (medium cage) RD. And a new chain.

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You need to make sure you have a 6870 long cage rear derailleur. I think its a 6870-GS. The short cage version only goes up to 28, the long cage version will go up to 11-32. So if you don't have the long cage already you'll have to swap the derailleur, also you'll probably need to add some links to your chain.

I was typing a reply because most of the posts here were horribly wrong. P90Puma and artvark beat me to good info.

If your derailleur is the GS (that means the cage that holds the jockey wheels is slightly longer) version, you can throw a 32 on there no problem once the b screw is adjusted properly.

If not, you can buy a mechanical Ultegra 6800 rear derailleur that is a GS and swap the cage over to yours. Presto, it will work perfectly with a 32. Put the cage from your di2 derailleur on the mechanical one you bought and that one will work perfectly too. Re-sell it. Should be fairly cost neutral if you buy a used one and it's a 10 minute job. It's just a few bolts. It's very simple.

Good chance you will need to lengthen your chain but you can always test it out and see.

Here's a link to the GS version of the di2 Ultegra RD. I don't know where on the RD it says GS or SS. It might not at all.

hey dangle,
that's good info regarding swapping the cages! never thought of that.

I think jamming a 32 in there with a 28 max capacity is possible on some setups, but it's pretty case by case. If you're going to spend that much on electronic shifting, you might as well have it all work perfectly. The cage swap is the fastest and cheapest way to get there.

h2ofun was sort of right (had OP's derailleur been mechanical), but the very important clarification is that you need a 10 speed mountain bike rear derailleur if using 11 speed road shifting. The cable pulls are identical between those two groups. Yes, you can hit all 11 cogs on the cassette even if the RD is a "10 speed."

If somebody like OP (with a mechanical shifting setup) needed to keep their 53/39 front, they could pretty much put any cassette they wanted on the back with 11 speed road shifters and a 10 speed mountain bike RD. Maybe even the new XT 11-42!

I don't suppose Shimano is being nice and made MTB Di2 derailleurs compatible with road shifters?

Not that I know of, but I don't have a ton of di2 experience. It seems that it would be a software/firmware issue and not anything mechanical, so it could be possible in the future.

I'm curious what your application is. Are you looking for the 'clutch' feature of the offroad derailleur for a cx setup? I have heard that an XTR rear di2 derailleur will work with the most recent 11 speed Ultegra/Dura Ace di2 shifters if you run it 1 x 11. Just something I read once. Maybe it's possible you could throw on a wide range cassette with a clutched di2 rear derailleur for cx season since the di2 derailleurs don't have a higher cage tension setting like the mechanical ones do. Your cx shifters and drivetrain would cost more than my entire cx bike though...