# 7800 STI Shifters with RD-7900



## 45leopard (Jul 25, 2010)

Can I use RD7900 rear derailier with 7800 STI shifters - I have a 34/50 compact and want to put a 28-11 cassette on the rear (currently run a 11-23)? I understand that this is 2 cogs too many for the 7800SS rear which I currently use.


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## DaveT (Feb 12, 2004)

45leopard said:


> Can I use RD7900 rear derailier with 7800 STI shifters - I have a 34/50 compact and want to put a 28-11 cassette on the rear (currently run a 11-23)? I understand that this is 2 cogs too many for the 7800SS rear which I currently use.


You more than likely can use your 7800 rear derailleur with an 11/28 cassette with no other changes other than a possible adjustment to the B screw.. It would be worth buying the cassette and trying it with your current derailleur before you buy the 7900 derailleur.


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## ukbloke (Sep 1, 2007)

In my experience 50/34 by 12-27 is close to the safe limit for the 7800SS RD. It depends slightly on your chain-stay length (how close it is to a multiple of the chain pitch), and how close you want to run to the absolute limit. I wouldn't feel comfortable going beyond this 31T which is already significantly over the Shimano spec. I feel that the bike should work in all gear combinations and not self-destruct if you happen to choose big-big by mistake. I also had to fully max out the B-screw - I think I even put it in backwards.

As for the RD7900 with the 7800 shifters, Shimano says no on their compatibility chart.

I'm not completely convinced by the 10 speed 11-28 cassette either. How often do you really need that 11T cog? Can you really tell the difference between 28T and 27T on a climb (about 4% lower)? Is it worth the increased spacing (4T gap from 24 to 28), and "missing" gear ratios (eg. no 16T)?


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## Clevor (Sep 8, 2005)

ukbloke said:


> In my experience 50/34 by 12-27 is close to the safe limit for the 7800SS RD. It depends slightly on your chain-stay length (how close it is to a multiple of the chain pitch), and how close you want to run to the absolute limit. I wouldn't feel comfortable going beyond this 31T which is already significantly over the Shimano spec. I feel that the bike should work in all gear combinations and not self-destruct if you happen to choose big-big by mistake. I also had to fully max out the B-screw - I think I even put it in backwards.
> 
> As for the RD7900 with the 7800 shifters, Shimano says no on their compatibility chart.
> 
> I'm not completely convinced by the 10 speed 11-28 cassette either. How often do you really need that 11T cog? Can you really tell the difference between 28T and 27T on a climb (about 4% lower)? Is it worth the increased spacing (4T gap from 24 to 28), and "missing" gear ratios (eg. no 16T)?


Hmmmm, I started a thread about using the 7900 rear with 7800 shifters and two posters said it works fine. I wonder if Shimano is just saying that to get everyone to upgrade to 7900?

When mention is made of using the B screw adjustment in this thread, I presume it's not nescessary if one is willing to adjust the chain length correctly? In my case, I am going from a 53/39 and 12-25 rear to a 50/34 and 11-28 rear. Would adjusting the chain length be necessary, if I wanted to do things right and have the rear derailleur positioned properly at big-big and small-small cross-chain combos?

I think the reason why guys want the 11T is a 50x11 is about the same (just a bit taller) than 53x12 so they think they are gaining the high end back. My complaint with Shimano is the lower end of the range; I don't like the cogs offered there. I mean if you need lower gearing than a 12-25, the 11-28 only offers a 24T and then 28T, which is the same with the 12-27 cogset. That is only one gear lower than what you got with 12-25 and the next lowest gear is TALLER that a 25T, albeit by a tooth. We need lower gears, damn it! At least Campy offers a 25T and 28T with their 12-27, and better yet, a 26T and 29T with their 12-29. Granted, the Campys do have 11-speeds to work with.

Of course, to get lower gearing with Shimano you can always go the triple route, with the 30T granny ring. Shimano still offers triples with Ultegra and 105.


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## ukbloke (Sep 1, 2007)

Clevor said:


> Hmmmm, I started a thread about using the 7900 rear with 7800 shifters and two posters said it works fine. I wonder if Shimano is just saying that to get everyone to upgrade to 7900?


It may be that practice is different to Shimano's strict chart. However, if the cable pull ratio is different, then it won't work right. I don't have the practical experience on this, sorry.



> When mention is made of using the B screw adjustment in this thread, I presume it's not nescessary if one is willing to adjust the chain length correctly? In my case, I am going from a 53/39 and 12-25 rear to a 50/34 and 11-28 rear. Would adjusting the chain length be necessary, if I wanted to do things right and have the rear derailleur positioned properly at big-big and small-small cross-chain combos?


B-screw is not related to chain length. It allows the distance between the top pulley of the RD relative to the cassette to be tuned. A shorter distance gives quicker rear gear changes. A longer distance is needed to accommodate a larger cassette to avoid the pulley bumping on the cog. You will likely need more B-screw when changing from 25T to 28T largest cog. You might need to increase chain length to ensure that big/big does not overly stress the RD. Of course you are losing 3T on the big ring and gaining 3T on the cassette so it might wash out.

At the end of the day these gear ratios are simple math. There is only so much you can do to cover the desired low to high end range with reasonable gaps. Often people want it all in terms of range, but don't consider the minimal amounts of time that they likely spend at the ends of the range, and then they pay with gaps and missing ratios where they could really need them. 11 speed would help of course. SRAM Apex and MTB have some wide monster cassettes for those who really need the range and can tolerate the gaps. Then there's triples as you mention.


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## frdfandc (Nov 27, 2007)

Here is the skinny.

You can use 7900 rear derailleur with 7800 shifters without issue. The issue is using the FD. The 7900 FD will work ok with the 7800 shifters, but the 7800 FD will not work very well with the 7900 shifters - revised cable pull.

The 7800 rear derailleur will accomodate the 11-28 tooth cassette without issue also. The max stated rear size for the 7800 is a 27 tooth, but a 28 tooth will be fine with a proper B-screw adjustment.

Shimano says no on the compatibility because they want you to purchase the group as a "system. Not just individual pieces. If they can get you to purchase a set of shifters/FD/RD and crank instead of just shifters, who is that better for? Them. More money into their wallets.


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## ukbloke (Sep 1, 2007)

Cool - thanks for the clarification on the cable pull compatibility.



frdfandc said:


> The 7800 rear derailleur will accomodate the 11-28 tooth cassette without issue also. The max stated rear size for the 7800 is a 27 tooth, but a 28 tooth will be fine with a proper B-screw adjustment.


My concern was RD capacity when using the 11-28 in combination with a 50-34 compact. I think it is right on the hairy edge - it might work for some, but I wouldn't do it. I'm confident that it is fine with a standard crank-set. though.


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## 45leopard (Jul 25, 2010)

*How about MTB rear derailleur*

Interesting discussion. So far I understand I could just go ahead and change my cassette for a 11x28, leaving the RD7800 rear derailleur. The downside is it it on the ragged edge and I will need to keep off the big/big combo. I will also need to adjust the B screw so that the derailleur cogs do not interfere with the large ring.

So far I have not seen a definitive statement whether I can replace the RD7800 by RD7900, using the 7800 shifter. I have seen a few posts to say you can, but cannot find the geometric dimensions anywhere for the 7900 set. the following link was useful -

http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946#Shimano

It says that Shimano 10 speed is 3.95mm sprocket pitch with 2.3mm cable pull (1.7 ratio). Does anyone know whether the 7900 group is a lot different than this - i.e. does it still use the same 1.7 pull ratio?

Another alternative I was considering is the 10 speed MTB group from Shimano (e.g. XT GS or SGS which have 35 and 43 tooth capacities respectively). Anyone have any experience with using MTB derailleur on a road bike? I understand that the Shimano MTB derailleurs use the same pull ratio (1.7), so 7800 shifter plus XT should work.


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## ukbloke (Sep 1, 2007)

I'm not sure about the 10 speed mountain bike group (Dyna-Sys). It has very limited compatibility with the existing 9 speed MTB, and again there is mention of different cable pull.

Another possibility would be the new Ultegra 6700 rear derailleur, but this is a similar situation to 7900, except that Shimano have not yet included 6700 information in their compatibility chart. Reading between the lines I'm starting to think, like frdfandc, that either 6700/7900 RD will probably be OK.

Finally, you could go with a road-bike long-cage derailleur (Ultegra 6600, Ultegra SL 6600-G, or even DA 7800). This will certainly have the capacity and most likely the B-screw will allow the 28T. The down-side is slightly slower rear shift performance, an uglier RD, a longer chain, and a few grams of weight.

Having said all that, I'm still not convinced that 11-28 is really worth all this hassle and expense over 12-27. Does that one extra tooth at the top and bottom really matter? Are you prepared to live with the bigger gearing jumps (eg. 24T to 28T)?


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## frdfandc (Nov 27, 2007)

You will only need to keep it off the big-big combo if your chain is too short. With the proper length chain, you can go big-big. 

The long cage road derailleurs have the same max tooth sprocket size, but the total capacity is more due to the application of this RD on a triple.

As for the XT/XTR 10 speed stuff, since it is still new and not yet on the market at most shops, it would be hard to say if its compatible with the road stuff.


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## Clevor (Sep 8, 2005)

On this thread:

http://forums.roadbikereview.com/showthread.php?t=162475

One guy has run a compact with 11-27 and one guy with 11-28 with no problems on the DA 7800SS derailleur. Except some B-screw adjustment in one case.

I have not had a chance to install my 50/34 yet. I may just use it with my existing 12-25 if I find the gears low enough, so not sure I'll actually try an 11-28.

I believe the correct term for the DA 7800GS rear derailleur is medium cage, as Shimano does not make a long cage. But it's called long cage by a lot of people. It's quite apparent the max tooth cogs specified by Shimano is rather conservative, so a med cage has no problem with triples. Campy did make a short, med, and long cage at one point.


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## Argentius (Aug 26, 2004)

*We're confounding*

two issues here.

One is using a bigger cassette, 27 or 28 big cog, with an SS (short-cage) Shimano rear derailleur. IME this CAN work, ymmv.

The other is using 7900 shifters, with older (any) derailleurs.

The basics are that the REAR will work fine -- Shimano now calls this "B compatibility" and I know several who use it just fine.

The FRONT, however, will not shift well / at all IME. Ultegra 6700 supposedly will, but the cable pull ratio for the left 7900 shifter is different than all other Shimano.

Know several racers with 7900 shifters / rear derailleurs, 7800 everything else.

The 7900 chain and crank together are pretty much magical, though.


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