I'm heading to Spain in a month for work than a two week vacation. Two of the rides planned will include the Alto de l’Angliru and Puerto de Ancares and I'm afraid my 11/25 cassette is not up to the task.

I have an unused ultegra 11/28 which going by Shimano's documentation is not compatible with the 7970 derailleur. I have read about folks running as high as a 30 tooth cog by screwing the b-limit screw in all the way or installing it backwards.

Has anyone had any success with anything above a 27 tooth cog?

I've used a 28-tooth cassette with both the mechanical and Di2 versions of 79xx. It might depend on the frame, but it works on my bike anyway.

Gave it a shot over the weekend. With the b limit screw in all the way, there's about 2 mm clearance between the jockey wheel and the cassette when in the largest cog.

get the right [h=2]Derailleur.[/h]
too far to go with a funky b screw adjustment.

the problem is that the jocky wheel touches the 28 tooth cog.

Bah. It's only one tooth more than the rated capacity. I've been running 11-28s with DA7800 RDs on multiple bikes for several years now with no problems.

There probably aren't that many bikes out there that it won't work fine on.

But how does it work with the chain on? Shift OK or not?

Shimano is notoriously conservative on what they recommend. If they say it's compatible with an 11-28, then it needs to work flawlessly on all bikes with an 11-28, even without perfect tuning by the installer.

Dollars to donuts it will work just fine. Go out and ride it and see.

FWIW, I've run an 11-32 with a Shimano 105 derailleur with a maximum cog supposed to be 27, and it shifted acceptably.

(pardon the marginal thread derailment)

I've been thinking about trying this and was wondering if you have to be extra careful to not cross chain and whatnot with such a setup? I mean, if I accidentally went too low in the back while still in the big ring, couldn't I end up with a lock-up or ripping off the derailleur hanger or something?

It would depend on the size of your front chainrings.

I should have said that the 105 I used was a long cage. I didn't add that because long cage short cage affects wrap capacity, not maximum cog size, whic was the OP's question.

You've got to size the chain to run in the Big/Big because someday you'll inadvertantly cross chain under pressure and that's bad.

If you're pushing a derailleur beyond its wrap capacity, much better idea to let it be too big in the small/small combination, and stay out of the last couple of small cogs with the small ring.

I have a 105 long cage with a triple up front and 11-27 on the back currently.

I do realize that I am inviting derision by even discussing putting an 11-32 back there in the 41, but I live in a pretty hilly region (insert other excuses here.)

While I am pretty attentive and avoid cross-chaining, the possibility of accidentally doing it, chain wrap issue and having to get a longer b-screw, etc. have scared me away from trying the 11-32 cassette I have. I guess I just need to man up and pedal harder, eh?

Not true. It's working like a champ. I don't think there's enough room for a 30, but 28 definitely works on my frame without having to resort to putting the screw in backwards.

Shifts just as well as it did with the 25. Chain is a little too short for cross chaining, but other than that no problems.

I prefer as little "b" screw as possible for crisper shifting.

Also, a lot of B screw means less chain when on the smallest cog.

It'll work just fine, worst case is the pulley wheel rubs due to road vibrations and the wheel wears down slightly. I've had that happen on a 7800 SS cage and at least on that setup it made absolutely no difference in use. If I didn't look for it I would never have noticed, it's a very very small amount of wear. On that 7800 RD shifts are still of the swiss watch variety. It still has the same wheel on it.