I have a standard short cage derailleur (Ultegra Di2 6770 RD) and want to change it out the cage for one with higher capacity. I believe the RD-6800-GS cage would meet my gear capacity needs. Also, this road bike (a Calfee tandem) has a longer derailleur hanger, typically what you would see on a mtn bike, which automatically helps with jockey-sprocket clearance too.

As I understand it, the Ultegra RD-6800 GS (md-long) cage has a lowered top jockey wheel, which would allow for a bigger 32t sprocket than the RD-6700 GS does at only 28t.

Doing the above would be much more economical (sub-$100) than going with a K-Edge mod, as they just quoted $335 to do this with their cage. The Ki2 cage capacity can handle up to a 36t, but that exceeds my sprocket size needs.

There is a discussion of 6800 conversion in this post (https://velonews.competitor.com/2013/...ponents_299684), but nothing about mixing with 6700.


Plus a discussion & video about 6700 w/long cage on Di2:
https://velonews.competitor.com/2011/...ossible_199407

So, the thread title question: Is it possible to use an Ultegra RD-6800 GS (md-long) cage on a RD-6700?

Why not just use a mountain bike der. instead? https://techdocs.shimano.com/media/te...9830604989.PDF

uhm, because a full mechanical RD is not Di2...

My bad. I don't know anything about nor do I want to know anything about the electric shifting. I'm sorry.

The cage length does not affect max cog capacity. A longer cage takes up more slack, allowing a wider range (meaning, greater tooth-count differences between smallest and largest sprockets, both chainrings and cogs). Simple put, shorter cages are for double cranks and longer ones for triples. MTB RDs have a different upper paralellogram, which allows for larger cogs

In short, the swap won't make it any more likely a 32T cog will work.

Many folks exceed the large cog capacity of road RDs. You may get lucky, especially if the RD already sits further away from the cogs as you mentioned. Try flipping the b-screw around so that its screwed in from the back, just enough so the threads are secure, and the screw-head rests on the hanger. You can also try a longer screw, which may also need to be installed backwards or else the end of the screw may bypass the RD as it comes at it from a different angle.

^^^ Yup, I understand that cage length governs chain slack. I do need the long cage for my super compact chainring and wide range cassette, both of which add up to 39T capacity. Regardless, the question was not about the cage length, which is understood.

The key point about the 6800 GS cage is that it has a repositioned upper jockey wheel which DOES allow for a 32t sprocket with that RD.

Question is whether or not a 6800 GS 11spd cage will work on a 10spd 6700/6770 RD?

Don't know the answer to the OP.

If a 6800 cage won't work I have compared a 6770 and 6700A (30T version), without disassembling them, and it looks like the cages could be swapped. With the normal shimano wiggle room you should be able to get 32T. I picked up a 6700A thinking I would try it with a 12-30 cassette then 6870 came along and I am now thinking I will get a RD-6870 GS and 11-32 cassette. I already have an 11-speed DA compact crank.

I've used 6700 short cage derailleurs, on six of my bikes, with a 32t cassette, works great.

I would suggest, first install the 32t cassette that you're going to buy anyway, to see if it will work with your 6700 derailleur. If it does, you're golden.

You're in Coeur d Alene, I'm in Spokane. I'd be happy to show you how well the 6700 derailleur/32t cassette combo works.

Spent my first 19 years in CDA.

Thanks Dave.

I will be using this setup with a set of "super compact" chainrings that have a 18t gap size between them (yeah, I know that exceeds a typical 16t compact), so I believe the GS cage is needed to accomodate the chain slack. Total gear tooth range for my maximum setup will be 18t front + 21t rear = 39t total.

Do your setups approach 39t total?

Another thing that a lot of people overlook is the bike's rear derailleur hanger length. Not all road hangers are equal, and so longer hangers will allow for more top jockey wheel clearance automatically. I wouldn't be surprised if most people who find a shorter cage works ok for them, also have a slightly longer hanger.

The "total teeth" capacity assumes you will use all possible gear combinations from big-big to small-small. That's not necessary. The chain, for mechanical safety reasons, must allow big-big but there is no reason to worry about small-small. The worst that happens is that the chain goes slack but there is no damage and the combination is not necessary from a riding perspective. So, the upshot is the rear derailleur does not have to have enough capacity to cover the entire total teeth range.

My setups use the 11/32 cassette and 34/50 chainrings. I can't say if it would be successful using 'super compact' gears.

What I can say with some authority is that all the bikes that I tried the 6700 rear der and 11/32 cassette worked with no modifications or fiddling, beyond adjusting the B-screw so the top pulley would clear the largest cog. The bikes included a custom Serotta (Ti)' custom Tsunami (Alu), a custom Milholland (steel), two different Specialized, a Time and a Look.

Another fix, though I haven't needed it, would be to replace the standard 11t top pulley with a 10t top pulley to gain some extra clearance from the large cog..

I'd say, again, buy the 11-32 cassette, and maybe even a 10t top jockey pulley and try them with your existing set up. It may work as you want.

Beyond that, I've not seen any discussions regarding the swap of a 6800 der cage to a 6770 derailleur.

Let us know if you hear of anything with regards to your original question. I'm sure it will be of interest to some.

Shimano have a few different methods for attaching RD cages... looking at the pdf for the 6700, it seems to use the same method that was introduced (AFAIK) on the RD-7700. If you have a look at the pdf for the RD-6600 you can see it uses a different method; I've seen this mostly on MTB RDs.

Unfortunately there's no pdf for the RD-6800 for some reason, but I was able to dig up a pic of the reverse side...



You can just see the bolt there... I'm 95% sure that means it'll swap.

Assuming of course the lower knuckle on Di2 RDs is identical to those on mechanical ones. Looks like it.