Shimano ST-7800 shifters compatible with RD-6700A??

[biker.caliente]

So I tried to use my RD-7800 with the a CS-6700 12-30 cassette but the pulley and the largest cog still rub no matter how I play with the B-screw. I used a longer screw and even placed it backward.

I guess my choice are either a 9-speed mountain RD or the new RD-6700A. Does anyone know if the RD-6700A will even be compatible with ST-7800 brifters?

Thanks in advance for your help.

[aramis]

The long cage road derailleurs won't increase your clearance with the largest cog anyway. They just allow a greater difference between the shortest chain length and the longest chain length that is needed (like you get with triples, what they are made for).

I just put a deore 9 speed derailleur on my bike. That's what I did with my 10 speed shimano bike with an 11-32 on back and compact up front. Benefit is you can go with pretty much whatever cassette you want after that. Downsides are the less smooth shifting.

EDIT: didn't know the 6700A existed, thought you were talking about the regular long cage ultegra RD. Pretty cool but only 30T. Shimano is late to the game of big cassettes, I wish they would come out with a 105 and ultegra one that would do 32T and also a DI2 one I'd be happy. :P

If you go with a deore 9 speed rear derailleur they are super cheap but heavier. I think I paid $40 shipped for mine.

I also don't see why an ultegra 10 speed derailleur wouldn't be compatible. The indexing is set in the shifter and all the shimano 10 speed road rear derailleurs work with any of the 10 speed shifters.

[biker.caliente]

I should have been clearer with my question. I was told that the DA7800 and DA7900 are different mechanism and are incompatible. Thus I wonder of RD-6700A would be compatible with ST-7800, since 6700 is the lineage as DA7900.

Can someone share his/her experience?

Thanks

[dave thompson]

Both the 7900 and 6700 rear derailleurs will work perfectly with your 7800 shifters. It's the 7900 front derailleur that's different.

It's been my personal experience, on at least 5 different bikes using both 6700 and 7800 shifters along with 6700 short cage rear derailleurs, that the 6700 der will easily handle a 30t cassette and in my cases a 32t cassette without issues.

[aramis]

Both the 7900 and 6700 rear derailleurs will work perfectly with your 7800 shifters. It's the 7900 front derailleur that's different.

It's been my personal experience, on at least 5 different bikes using both 6700 and 7800 shifters along with 6700 short cage rear derailleurs, that the 6700 der will easily handle a 30t cassette and in my cases a 32t cassette without issues.

It depends on the individual frame and derailleur hanger, some just won't work.

Can you get yours to make the shift on a compact from the small ring to big ring when in the 32 without breaking stuff with the short cage? Can you still use all the gears (small/small and big/big) Or do you just remember not to make that shift or use those gears? Those were my fears with trying to get away with a small cage derailleur.

[irideslow8401]

Both the 7900 and 6700 rear derailleurs will work perfectly with your 7800 shifters. It's the 7900 front derailleur that's different.

It's been my personal experience, on at least 5 different bikes using both 6700 and 7800 shifters along with 6700 short cage rear derailleurs, that the 6700 der will easily handle a 30t cassette and in my cases a 32t cassette without issues.

+1 --7800 RD is not able to accommodate 28 cog or larger. RD works with 7900 or vice versa. FD is different between 7800 and 7900

[rice rocket]

So I tried to use my RD-7800 with the a CS-6700 12-30 cassette but the pulley and the largest cog still rub no matter how I play with the B-screw. I used a longer screw and even placed it backward.

I guess my choice are either a 9-speed mountain RD or the new RD-6700A. Does anyone know if the RD-6700A will even be compatible with ST-7800 brifters?

Thanks in advance for your help.

Works.

Ran this exact combo for ~3000 mi until I decided to strip down my bike and sell it.

[dave thompson]

It depends on the individual frame and derailleur hanger, some just won't work.

Can you get yours to make the shift on a compact from the small ring to big ring when in the 32 without breaking stuff with the short cage? Can you still use all the gears (small/small and big/big) Or do you just remember not to make that shift or use those gears? Those were my fears with trying to get away with a small cage derailleur.

My short cage 6700 rear der setup used with an 11-32 cassette has worked on a Time, 2 different Specialized, a custom steel, custom aluminum and a custom steel bike. In all cases I can shift any combination of gears, front and rear. Shimano changed the geometry of the 5700/6700/7900 derailleurs which allows them to clear the larger cassettes. I cut my chain to the big/big combo to ensure that I don't break something if I shift to that. I've had no issues in the over two years that I have been doing this. I don't have to reverse the B-screw either.

If the clearance between the upper jockey wheel and 32t cog is too close for your liking, you can replace the standard 11t jockey wheel with a 10t, a simple swap.

[oldpotatoe]

I should have been clearer with my question. I was told that the DA7800 and DA7900 are different mechanism and are incompatible. Thus I wonder of RD-6700A would be compatible with ST-7800, since 6700 is the lineage as DA7900.

Can someone share his/her experience?

Thanks

Not true for the rear derailleurs. 6700, 7800, 7900 are all cross compatible except for the FD. Gotta have a 7900 FD with 7900 shifters. I have done what you say, put 1 12-30 with 7800 stuff..I suspect your chain may be one link too long. Go to big/big combo and see how much 'slack' you have in the RD cage.

[oldpotatoe]

+1 --7800 RD is not able to accommodate 28 cog or larger. RD works with 7900 or vice versa. FD is different between 7800 and 7900

Not true, done that more than once..11-28 with 7800(and 6600) stuff.

[irideslow8401]

Not true, done that more than once..11-28 with 7800(and 6600) stuff.

That was with my experience. When i had compact 50/34 + 11-28 on my Tarmac 08, i could not seem to make the DA 7800RD work properly but the same combo worked with the newer 6700RD. Again shifters are the same 7800...perhaps my chain was not in proper length. I swapped the 12-27 cassette in..it works flawlessly with DA7800.

[sc53]

+1 --7800 RD is not able to accommodate 28 cog or larger. RD works with 7900 or vice versa. FD is different between 7800 and 7900

Not true. Mine is working with 7900 brifters and a 30-tooth cassette. But I had to reverse the B screw, and, as noted above, some frames and derailleur hangers just won't allow this combo to work. I have a small frame, 50 cm Ottrott, and the combo is working. YMMV.