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Discussion Starter · #1 · (Edited)
I have viewed the Shimano Interchangeability Chart between 7800 and 7900 and spoken to a couple of local mechanics, but really can't seem to get a straight (unanimous) answer. Couple that with the fact that I have seen complete factory builts from everything from full 7900 to partial builds. Shop owners/mechanics telling me 7900 just too new and no significant feed back on it to know what is working with what yet....soooo....

The Question: Who has full Dura Ace 7800 and has partially "upgraded" to Dura Ace 7900? And what was the bare minimum that you were able to get away with?

The Problem: I am shallow enough to admit that I want the new DA 7900 shifters to get rid of the derailleur cables on my 7800s. Ok. (plus I have crashed them a couple times, replaced the crash plates a few times, and they are headed into their 3rd season - that's the rational justification ;)

Now I know I have to change the front derailleur. Ok

But what else?
- I am pretty sure that the FD 7900 will work with the 7800 rings, albeit "slow shifting may occur." But do I have to run a 7900 chain, or is a 7800 chain or 6600 chain ok?

- I think I can get away with the 7800 Rear Derailleur, but if not, will the 7900 RD work with 7800 cassettes? 6600 or 105s cassettes? (I have too much invested in cassettes and chains to have to swap over.)

- I know I can keep the 7800 brake calipers.
- Im pretty sure the 7800 rings (crankset) can stay.

So for those of you running partial builds, what say you? How is it working out?

thanks

zac
 

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Zac,

I bought a full 7900 group just about as soon as it was available as I didn't have the patience to wait until it was readily available piecemeal.

I initially put the whole lot on my bike, with the exception of the cranks. I have a DA SRM and I mounted the 7900 rings on the srm. Everything worked very well. The front shifting was extraordinary. It all worked well. The only thing that concerned me was chain noise. No matter how clean the drivetrain setup was, I couldn't shake the hunch that the 7900 chain and cogs were noisier than their 7800 counterparts. I changed them out and put a 7800 chain back on initially, and ran it with the 7900 cassette and rings. It was a lot quieter, and though the shifting was marginally slower in front, it certainly wasn't a show stopper by any means. I have since changed out the chainrings and put back on my 7800 rings. This was mainly due to sizing issues. The 7900 rings are only available in 53 right now, and I have run a 50 big ring for years now, and I wanted that back. Following a flat on my 7900 cassette equipped rear, I have a wheel on there that has a 7800 cassette. The shifting is great and smooth and very quiet.

I have always used the 7900 calipers and the braking is terrific. You can get away with using 7800 calipers with the 7900 levers but the braking won't be optimal as it would be with paired 7800 or 7900 components. This is all due to the changed cable pull ratio on the 7900 levers.

So at a minimum, you can get away with 7900 levers and FD. If you require optimal braking capabilities, then go for the 7900 calipers too. But that's all you really need. If you want the 7900 cassettes that top out with a 28 cog, you'll need the 7900 RD too and the 7800 RD doesn't have the wrap capacity to handle anything more than a 27 safely and reliably.

I have tried just about every combination of 7800 and 7900 components except the brakes, and it all plays nicely together. I think I am going to stick with the setup I have now - 7900 levers, derrs and brakes & 7800 rings/cranks, cassette and chain - since those consumable parts are so much cheaper in 7800. The prices for 7900 cassettes are huge!!!

If there's any other questions you have, don't hesitate to ask away. I hope I helped answer some for you.

Geoff
 

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Discussion Starter · #3 ·
Geoff, thanks yes.

I was initially thinking that I would do the shifters and just the FD. Everything else stays 7800. I may also do the rear if that plays well with the all the 7800 cassettes and chains. I just have too many cassettes in 7800 and 6600 and too many chains that I have no interest in the 7900 consumables yet. And as you say they are frick'n way too $$$$.

The range is not an issue. I mainly ride 11-21, 12-23 and occasionally I use a 12-25. I ride a standard double, but the only time I am in the small ring is on steeps or longs and on warm ups. So slightly slower shifting is not an issue. I guess I was more concerned with the asymetrical 7900 chain and how it played with the FD, and does the 7800 chain negate all the "benefits" that the FD bring? (No need to trim, etc.)

FWIW, having to trim is no big deal. Shimano has always had a nice half click trimming feature. I just don't want to loose it, if the FD doesn't play with the 7800 chain very well. Really, the only time I need it would be say 53 to big cog cross. Since my chain rubs the outer ring on the 39 to 11/12 cross due to the short chain stays, trim is not an option there anyway.

I realize the brake pull is different, but that would be easy enough to remedy if it is not satisfactory with the 7800 calipers. Do you feel that the 7900 STI coupled with the 7800 calipers will decrease performance, or is it just an adjustment/feel type of issue? For instance if it's easy to lock up the 7800 calipers from the hoods position with the 7900 levers, then it is just a matter of me getting used to the different pull.

Thanks again.
zac
 

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I don't know specifically how the 7900 levers and 7800 calipers play together, never having tried that combo. I would imagine that power might be okay but modulation would suffer, due to small lever movements translating into lots of movement at the caliper arms.

As for the FD, it works just fine with the 7800 chain and the trim action is unaffected. You don't need trim in the big ring and you don't really need it in the small ring, though there is a trim stop for the small ring in the 7900 levers. I also can't use the 39x12&13 and this is due not so much to FD rub, but the chain hitting the shift pins on the inside of the big ring. That said, I have a new frame on the way with longer chainstays than my current frame so I'll be curious to see how it helps the chainline.

Not needing to trim the FD when the chain is on the big ring is sure nice.
 

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Discussion Starter · #5 ·
bdaghisallo1 said:
I don't know specifically how the 7900 levers and 7800 calipers play together, never having tried that combo. I would imagine that power might be okay but modulation would suffer, due to small lever movements translating into lots of movement at the caliper arms.

As for the FD, it works just fine with the 7800 chain and the trim action is unaffected. You don't need trim in the big ring and you don't really need it in the small ring, though there is a trim stop for the small ring in the 7900 levers. I also can't use the 39x12&13 and this is due not so much to FD rub, but the chain hitting the shift pins on the inside of the big ring. That said, I have a new frame on the way with longer chainstays than my current frame so I'll be curious to see how it helps the chainline.

Not needing to trim the FD when the chain is on the big ring is sure nice.
Good info, big thanks.

Looking like I am going to do Shifters-FD-RD. But I am going to build it in stages just to see how everything plays out. I actually am going to try the 7900 shifters with the 7800 FD, just out of curiosity.
 
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