# Another 7800/7900 Compatibility Question (7900/6700 Ultegra rear derailleur)



## JulesYK (Jul 2, 2007)

With apologies if this has been addressed before: Are the 7900/6700 rear derailleurs fully compatible with 7800 shifters? I'll be using either a 12-25 or a 12-27 cassette (and not the 11-28). I assumed it wouldn't be a question, but I found a Shimano compatibility chart that suggested that they are not fully compatible. (I know that the front shifting/chainring spacing is a bit different on the 7900/6700 groups compared to the 7800/6600 groups, but the cassette spacing is the same, so I'm not sure why there should be an issue.)

Many thanks in advance for your thoughts.


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## zac (Aug 5, 2005)

Yup, it works just fine, and will work with either of those cassetts on a standard double. The 7800 RD can even handle the 11-28, even though it is beyond the max.

EDIT: I just noticed that you are keeping the 7800 STI, and upping the RD...that works too. Note it is the RD that can handle the added range, so the 7900 RD can easily handle that 11-28. FWIW though, I think the 7800 RD is a better mech than the 7900. I know many guys who have differing opinions, but myself and several guys I ride with have all re-replaced our 7900 rear mechs with 7800s. Just throwing that out there for you to consider.

For future reference the RD is not indexed, it is the shifters that do that. 

Just be sure that you have nice easy radii on your rear shifter cable housings, so the cable slides as easy as possible.

The only things between the 7800 (and by extension 6600 and 5600) and 7900 (6700 & 5700) groups that must be within the same group are the Shifters and the Front Derailleur. Despite what Shimano says the rest can be mixed and upgraded at your leisure. Although I am not too sure about using 7801 chains on a 7900 crankset.

HTH
zac


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## JulesYK (Jul 2, 2007)

*Thanks - Thought that had to be correct.*

The Shimano compatibility chart was very confusing. I'm not a huge fan of the fact that the two generations are not interchangeable, but it could be worse (see Campy 10sp to 11sp...).

As for the chain, I have a small stash of the 7801 chains, and I hope that they work on the 7900/6700 systems because they have proven incredibly reliable, and all my minitool chain breakers work with them. Not so sure about the new asymmetrical chains, but I suspect they would work fine, as well.


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## DaveT (Feb 12, 2004)

JulesYK said:


> The Shimano compatibility chart was very confusing. I'm not a huge fan of the fact that the two generations are not interchangeable, but it could be worse (see Campy 10sp to 11sp...).
> 
> As for the chain, I have a small stash of the 7801 chains, and I hope that they work on the 7900/6700 systems because they have proven incredibly reliable, and all my minitool chain breakers work with them. Not so sure about the new asymmetrical chains, but I suspect they would work fine, as well.


Your 'standard' 10-speed chains will most certainly work very well on an otherwise 6700/7900 system. I'm using a KMC chain on my 6700 equipped bike, almost 1000 miles with no issues at all.


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## zac (Aug 5, 2005)

DaveT said:


> Your 'standard' 10-speed chains will most certainly work very well on an otherwise 6700/7900 system. I'm using a KMC chain on my 6700 equipped bike, almost 1000 miles with no issues at all.


Good to know, I think I knew that, but wasn't absolutely sure.


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## HazemBata (May 20, 2004)

Gentlemen,

I have a question along similar lines. I currently have a full Ultegra 9-speed drivetrain. I snapped the rear derailure hanger the the rear derailure got all bent out of shape about it. At the very least, I must now replace the rear derailure.

I would like to upgrade to the Dura-Ace 7900. So my question is this. How much of my 9-speed drivetrain do I have to upgrade in order to accommodate a 7900 rear derailure? Can I just swap derailures and be done with it?

Thank you in advance for your help.

Take care.


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