# DA 7800 questions: cogs and derailleurs



## Clevor (Sep 8, 2005)

Gotta start stocking up on 7800 parts as they are getting scarce.

If I run a compact crank with Shimano, I presume a standard double front derailleur will work, correct? In other words, there is no compact specific front derailleur as with Campy.

I think a short cage rear 7800 derailleur will work with up to a 27 tooth rear cog, correct?

If I run an 11-28 rear cog, I suppose I'd need a med cage? Would the long cage work and if so, the cons versus med cage.


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## PJ352 (Dec 5, 2007)

Clevor said:


> Gotta start stocking up on 7800 parts as they are getting scarce.
> 
> If I run a compact crank with Shimano, I presume a standard double front derailleur will work, correct? In other words, there is no compact specific front derailleur as with Campy.
> 
> ...


You are correct, A standard double FD will work with your compact crank. 

You may already know this, but be careful with the DA 7800 hubs. They were designed using higher splines and only 10 spd DA and Ultegra cassettes fit. 

You can run up to a 28T rear cassette with an SS cage, so if that's what you have there's no need to change out the RD.


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## Clevor (Sep 8, 2005)

PJ352 said:


> You may already know this, but be careful with the DA 7800 hubs. They were designed using higher splines and only 10 spd DA and Ultegra cassettes fit.
> 
> You can run up to a 28T rear cassette with an SS cage, so if that's what you have there's no need to change out the RD.


I did not know that, however I only use DA or Ultegra components anyway so I should be fine.

Yes, I did a search and there is a post on this forum about how a short cage will work with up to a 28 tooth cog, which is nice. It's pretty hard to find a med cage around nowadays. 

I guess I don't necessarily have to stock up on 7800 rear derailleurs, as I heard the 7900 rear would work fine with 7800 shifters, it's just the front 7900 that won't.


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## PJ352 (Dec 5, 2007)

Clevor said:


> I did not know that, however I only use DA or Ultegra components anyway so I should be fine.
> 
> Yes, I did a search and there is a post on this forum about how a short cage will work with up to a 28 tooth cog, which is nice. It's pretty hard to find a med cage around nowadays.
> 
> *I guess I don't necessarily have to stock up on 7800 rear derailleurs, as I heard the 7900 rear would work fine with 7800 shifters*, it's just the front 7900 that won't.


Actually, just about any RD will work. They're kind of dumb, with the shifter dictating the amount of travel. In fact, any 1997 or later Shimano Rear Derailer will work with 6,7,8,9,10 spd cassettes.


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## BikeFixer (May 19, 2009)

Clevor said:


> Yes, I did a search and there is a post on this forum about how a short cage will work with up to a 28 tooth cog, which is nice. It's pretty hard to find a med cage around nowadays.
> 
> I guess I don't necessarily have to stock up on 7800 rear derailleurs, as I heard the 7900 rear would work fine with 7800 shifters, it's just the front 7900 that won't.


The 7800 isn't designed to work with a 28 so it might not work perfectly just so you know.
You are correct on the 7900 frt and rear compatability :thumbsup:


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## ukbloke (Sep 1, 2007)

In my experience with DA7800, a compact 50-34 with a 12-27 cassette is as far as I would want to push the capacity of the short-cage DA7800 RD. In my case I needed a *lot* of B-screw to get it running smoothly in the 27. Now that was my comfort level and my requirement was that all of the gear combinations should be safe and usable (at least to the extent that they would not risk the self-destruction of the drive train).

Admittedly, a 28 is hardly any different and it does depend on chain-stay length and also whether you are prepared to push the limits. For example, can you guarantee that you will never ever use the big/big combination? But then the 28 is probably in an 11-28 cassette so that's really 2T more capacity.


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## PJ352 (Dec 5, 2007)

ukbloke said:


> In my experience with DA7800, a compact 50-34 with a 12-27 cassette is as far as I would want to push the capacity of the short-cage DA7800 RD. In my case I needed a *lot* of B-screw to get it running smoothly in the 27. Now that was my comfort level and my requirement was that all of the gear combinations should be safe and usable (at least to the extent that they would not risk the self-destruction of the drive train).
> 
> Admittedly, a 28 is hardly any different and it does depend on chain-stay length and also whether you are prepared to push the limits. For example, can you guarantee that you will never ever use the big/big combination? But then the 28 is probably in an 11-28 cassette so that's really 2T more capacity.


Not doubting your experiences and I agree that other factors can enter into this 'argument', but FWIW my '08 Tarmac Comp was OEM'd with the SRAM 11-28 cassette, a 105 (53/39) double and an Ultegra SS RD. I had no problems with that combo, but swapped it out for a 12-25 because that gearing suited me better.

Considering Shimano's specs set max capacity to 27T, 28 isn't 2T more, but that just my math.


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## ukbloke (Sep 1, 2007)

PJ352 said:


> Not doubting your experiences and I agree that other factors can enter into this 'argument', but FWIW my '08 Tarmac Comp was OEM'd with the SRAM 11-28 cassette, a 105 (53/39) double and an Ultegra SS RD.


And there's the key difference: your double contributes 14T while my compact contributes 16T. So there's the extra 2T that you need to run the 11-28 cassette "safely".

Yours has a total capacity requirement of 14+17 for 31, and mine is 16+15 also for 31. Both exceed Shimano's 7800/6600 short-cage spec by 4T. In my experience and judgement, this is very close to the limit. Adding yet another 2T is, IMHO, asking for trouble. At least for me, previous provisos apply, etc.


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## PJ352 (Dec 5, 2007)

ukbloke said:


> And there's the key difference: your double contributes 14T while my compact contributes 16T. So there's the extra 2T that you need to run the 11-28 cassette "safely".
> 
> Yours has a total capacity requirement of 14+17 for 31, and mine is 16+15 also for 31. Both exceed Shimano's 7800/6600 short-cage spec by 4T. In my experience and judgement, this is very close to the limit. Adding yet another 2T is, IMHO, asking for trouble. At least for me, previous provisos apply, etc.


Yup, after I posted I realized I had compared apples to oranges, or in this case a std to compact. For sure, the slight margin I had with the previously described setup vaporizes with the use of a compact, so my bad.


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## Clevor (Sep 8, 2005)

BikeFixer said:


> The 7800 isn't designed to work with a 28 so it might not work perfectly just so you know.
> You are correct on the 7900 frt and rear compatability :thumbsup:


Is the 7900 rear short cage designed to work with a 28th tooth?


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## Clevor (Sep 8, 2005)

PJ352 said:


> Not doubting your experiences and I agree that other factors can enter into this 'argument', but FWIW my '08 Tarmac Comp was OEM'd with the SRAM 11-28 cassette, a 105 (53/39) double and an Ultegra SS RD. I had no problems with that combo, but swapped it out for a 12-25 because that gearing suited me better.
> 
> Considering Shimano's specs set max capacity to 27T, 28 isn't 2T more, but that just my math.


I currently have a 53/39 double on my beater bike and 12-25 isn't doing it for me in Okinawa (at my age anyway). I went on a century ride and all I saw were compacts. Some guys were running 40+ tooth big rings. No triples so must be the stigma.

I want to salvage my current 53/39 setup with a 12-27 but haven't installed it yet. If that's not enough I was thinking 11-28. What's the drawback of this combo with a 53/39 except the humongous big gear; I guess gear spread?


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## PJ352 (Dec 5, 2007)

Clevor said:


> I currently have a 53/39 double on my beater bike and 12-25 isn't doing it for me in Okinawa (at my age anyway). I went on a century ride and all I saw were compacts. Some guys were running 40+ tooth big rings. No triples so must be the stigma.
> 
> I want to salvage my current 53/39 setup with a 12-27 but haven't installed it yet. If that's not enough I was thinking 11-28. What's the drawback of this combo with a 53/39 except the humongous big gear; I guess gear spread?


I don't think there's a drawback to using the std double with a 12-27 or 11-28. I changed mine out simply because the 12-25 better suited me and the terrain I ride. Two key points for choosing gear combo's, IMO.

I actually have more of an issue with the gear spread between 53/39, preferring the 'older' (as in mid 80's to early 90's) 53/ 42 combo. When switching to the 39, I don't like having to shift down 3-4 cogs at the rear to maintain cadence. Just a FYI, but if the gear spread of an 11-27 (or similar) isn't to your liking, Shimano offers a 16-27, but obviously there's more of a chance of spinning out on the decents. So again, choice hinges on terrain, etc. 

If you're not aware of this, here's a little 'toy' compliments of Sheldon Brown (RIP). You can plug some values in and get an idea of the results. It's especially helpful if you maintain a relatively constant cadence:
http://sheldonbrown.com/gears/

BTW, the 7900 SS RD _will _work with a 28T cog, but (as ukbloke posted) be careful of total capacity if you go with a compact crank.

It's listed under specs:
http://techdocs.shimano.com/media/t...02/SI-5X00A-002-ENG_v1_m56577569830651779.pdf


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## Clevor (Sep 8, 2005)

PJ352 said:


> I actually have more of an issue with the gear spread between 53/39, preferring the 'older' (as in mid 80's to early 90's) 53/ 42 combo. When switching to the 39, I don't like having to shift down 3-4 cogs at the rear to maintain cadence.


Hmmm, I was wondering why Campy, before they discontinued triple cranks in 2005, only made them in 53/42/30. They also made a 50/40/30, but never offered a 53/39/30.

If you ask me, the Shimano 53/39/30 with 12-25 rear is the way to go here in Okinawa. You basically have a double setup for when you are fit, and a granny gear if you need it. Beats walking (I've done my share of that here). 

I have both 7800 triple and double crank setups (parts, not installed) and the triple only weighs around 100 gms or a quarter pound more than the double setup. Only downside I imagine is the Q-factor and poorer aerodynamics of the crank sticking out farther. And of course, the stigma . I know C40 was also a staunch proponent of this triple setup, but even he has succumbed to the compact mania.


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## PJ352 (Dec 5, 2007)

Clevor said:


> Hmmm, I was wondering why Campy, before they discontinued triple cranks in 2005, only made them in 53/42/30. They also made a 50/40/30, but never offered a 53/39/30.
> 
> If you ask me, the Shimano 53/39/30 with 12-25 rear is the way to go here in Okinawa. You basically have a double setup for when you are fit, and a granny gear if you need it. Beats walking (I've done my share of that here).
> 
> I have both 7800 triple and double crank setups (parts, not installed) and the triple only weighs around 100 gms or a quarter pound more than the double setup. Only downside I imagine is the Q-factor and poorer aerodynamics of the crank sticking out farther. And of course, the stigma . I know C40 was also a staunch proponent of this triple setup, but even he has succumbed to the compact mania.


I'd go with what works and not fret over what others think, but I'm old and sometimes crotchety. 

I don't know the differences (if any) with Q-factors, but wouldn't worry about it - and certainly not aerodynamics. If you ride with a clipless pedal system many offer some adjustment for Q-factor. 

The compact does offer some advantages in that it's a pretty good compromise between the std and triple, and weighs a little less.


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## Clevor (Sep 8, 2005)

Clevor said:


> Is the 7900 rear short cage designed to work with a 28th tooth?


Well, let me answer my own question: I guess so. Here is some advertising material on the 7900 short cage rear derailleur:

"Shimano Dura Ace RD-7900 SS 10 Speed:

Wide link design for increased stiffness and better shifting accuracy 
Sealed bearing 11 tooth pulleys 
Carbon pulley plate for weight savings 
Enhanced cable pull method allows for a wider adjustable range for shifting and is less likely to be affected by external factors 
Wide gear-ratio compatibility allows gear ratios equivalent to a triple system without having to change to a GS rear derailleur 
10 speed compatible 
Max rear sprocket: 28 tooth 
Max front difference: 16 tooth 
Total capacity: 33 tooth 
166 grams 
UPC: 689228247420"

Now a question on triple setups:

I do have some 7803 triple cranks. So would I be able to use the 7900 SS derailleur for that rather than a 7800 GS? What would be the max rear cog, e.g. 28, 32, etc.?

Did Shimano ever make a long cage 7800 rear derailleur or did they call the GS (MED) a long cage?


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## ukbloke (Sep 1, 2007)

Clevor said:


> do have some 7803 triple cranks. So would I be able to use the 7900 SS derailleur for that rather than a 7800 GS? What would be the max rear cog, e.g. 28, 32, etc.?


According to their specs, no. The front difference on a triple is typically going to be 20 to 23T, which is way beyond their spec of 16T. For example, 50-39-30 is 20T or 53-39-30 is 23T. But, I don't see how the RD can really care distinguish the front difference from the rear difference, so it may be that you can get away with it. 

If you have a 53-39-30 then you have a capacity of 10T left for the cassette. This would only give you a straight block 12-21. With the 50-39-30 you have 13T left over which could get you a 12-25. It may be that you can get an extra 2T beyond the Shimano spec so that would be 12-23 or 12-27. However, I'm not completely convinced by this approach. I think in the vast majority of cases with a triple, you are not going to be happy with the capacity of the 7900 SS RD.



> Did Shimano ever make a long cage 7800 rear derailleur or did they call the GS (MED) a long cage?


I believe they offer only the SS and GS for 7800 (and now 7900 too).


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## frdfandc (Nov 27, 2007)

To run a triple, you will need the GS (medium cage) derailleur. The SS cage will not be able to handle the excess chain length needed when used with a triple.


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