Dura Ace 7800/7900 compatibility questions...

Discuss light weight issues concerning road bikes & parts.
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2ndgen
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by 2ndgen

I have a 7800 group now.

I'm looking to be able to integrate 7900 shifters and derailleurs into my drivetrain.
I know that if I go with 7900 shifters, I'll definitely have to go with a 7900 FD.
I also know that I can use the 7800 RD with the 7900 shifters.

The main question is "if" I decide to go with a 7900 RD, can I use the current Ultegra 6600 11-23 cassette
& 7801 chain and later upgrade to the Miche Supertype cassette and KMC X10SL chain I'm planning on?

Yes, I'm aware of Shimano's compatibility charts, but I've heard varying results in real world applications,
but I'd rather ask people who've actually tried, failed and/or have had success with mixing up said group
components.

by Weenie


eric
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by eric

The cog spacing is the same between 7800 and 7900 (and all shimano 10sp road), so you can mix and match cassettes and chains.

The newer asymmetric chains may shift better but the old stuff will work. I'm using KMC chains and sram or 6600 cassette on my 7800 with 7900 rear derailleur bike. It works just as well as the previous 7703 and 7800 rear derailleurs did.


The other change from 7800 to 7900 is the brake cable travel. I can't remember which way it goes. You'll either get more power but more lever travel, or less lever travel and less power. If you hate it you can always get 7900 brakes.

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BeeBee30
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by BeeBee30

You don't have to use a 7900 FD, I'm using a Campag Record on one bike, a Sram Red on another, both work fine.
Ti or dye!

The Weenie formally known as CAADHEAD

thisisatest
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by thisisatest

eric wrote:The other change from 7800 to 7900 is the brake cable travel. I can't remember which way it goes. You'll either get more power but more lever travel, or less lever travel and less power. If you hate it you can always get 7900 brakes.

the brake lever will feel firmer. most people dont mind the feel. also, you will gain some brake pad clearance from the rims.
you aren't upgrading the crank, which is the only "need" for the asymmetric chain. i agree with CAADHEAD about the front derailleur, especially since you arent using the 7900 crank...

Doolop
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by Doolop

CAADHEAD wrote:You don't have to use a 7900 FD, I'm using a Campag Record on one bike, a Sram Red on another, both work fine.


Don't have to, but should. Besides, 7900 FDs are cheap and very light.

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BeeBee30
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by BeeBee30

Doolop wrote:
CAADHEAD wrote:You don't have to use a 7900 FD, I'm using a Campag Record on one bike, a Sram Red on another, both work fine.


Don't have to, but should. Besides, 7900 FDs are cheap and very light.


Why should you? This is Weight Weenies, Red is lighter 8)
Ti or dye!

The Weenie formally known as CAADHEAD

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BeeBee30
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by BeeBee30

Works fine for me lol.
Ti or dye!

The Weenie formally known as CAADHEAD

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2ndgen
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by 2ndgen

Thanks for all the info.
:thumbup:

Valbrona
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by Valbrona

Gear compatability aside, a lot of people are not realizing that with 7900 brake levers Shimano changed the actuation ratio to 1:4. Shimano claim that these new levers should only be used with compatible brakes, like the new Dura-Ace or like the new TRP979SL, both of which use the new 1:4 ratio. See Shimano Tech Doc: http://techdocs.shimano.com/media/techd ... 746237.pdf

thisisatest
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by thisisatest

Valbrona wrote:Shimano claim that these new levers should only be used with compatible brakes, like the new Dura-Ace or like the new TRP979SL, both of which use the new 1:4 ratio. See Shimano Tech Doc: http://techdocs.shimano.com/media/techd ... 746237.pdf" onclick="window.open(this.href);return false;


look closer.
the doc clearly states that the 7900 brake calipers should not be used with older levers. it never suggests you can't do it the other way around.
the reason is that when using older levers and new-generation brakes, the leverage is too great, the braking power is too high, and it's easy to lock up the wheels.
when using 7900 levers with older brakes, the leverage is lower, not ideal but not "dangerous", lever effort is higher, the feel is firmer, it's harder to lock up, but there's the nice side effect of greater rim/pad clearance.
one way is a safety issue, the other is not.

Valbrona
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by Valbrona

Yes, you are right. I got my thinking back to front.

thisisatest wrote:when using 7900 levers with older brakes, the leverage is lower, not ideal but not "dangerous", lever effort is higher, the feel is firmer, it's harder to lock up, but there's the nice side effect of greater rim/pad clearance.
one way is a safety issue, the other is not.


But if I was matching levers with calipers my starting point would be to pair up parts with the same actuation ratio. FWIW I use Campag Record with TRP970SL (both 1:3 ratio), but I notice TRP have brought out a 979SL caliper that is 1:4 ratio and compatible with the new 7900 STI levers.

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DMF
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by DMF

So just to be clear: 7900 FD with say, 105 5700 shifters (for now) or say, Sram RED shifters (for later), will work just fine? ...even if Shimano seems to claim otherwise in their compatibility charts?

bikedoc
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by bikedoc

stick with 7800 it shifts so much better and for longer (rear).
yes the front shifting of 7900 is better, the brake are different, dont know for better or worse yet, cant decide. but the rear is awful, the cable routing under the tape was the worst thing shimano has ever done.

petepeterson
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by petepeterson

DMF wrote:So just to be clear: 7900 FD with say, 105 5700 shifters (for now) or say, Sram RED shifters (for later), will work just fine? ...even if Shimano seems to claim otherwise in their compatibility charts?


This is what I am wondering too. If anyone could clarify that the cable pull ratios haven't changed on the new 10 speed shifters it would be great.

by Weenie


oleosanxez
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Joined: Sun Oct 01, 2017 4:38 am

by oleosanxez

Hey I know this forum is for 78/7900 compatibility but does anyone know whether a DA 7400 front double crank (38t/52t) has any compatibility issues with a DA 7800 rear derailleur/DA 7800 25-12 cassette (on a DA 7400 hub and FH body, if it matters)?

The rings dont have the shifting pins but seem to shift ok (SRAM PC 1031 chain) up front but sometimes when in the big ring, the rear doesn't want to upshift (that is, shift rightward out to the smaller gears--fyi downshifts fine). But when in the small 38t ring it shifts great both ways. Seems like a chainline issue possibly but not sure. Any ideas? Thanks.

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