ruirui

last christmas, i built my wife a nashbar bike using dura ace 7800 groupset i got from my buddy.

it's 53/39 front and 12/25t back.

my wife doesn't race and finds that very difficult to ride even when we hit some hills.

the front will be swapped to a 7900 compact crank, so i think and hope the 7800 FD won't have issues with it.

as for the rear, since the 7800 RD is short cage, i see per manual, the smallest to largest it will support is 11/27t.
https://www.bike-components.de/bedien.../rd-7800.2.pdf

i googled and found ultegra 7900 has them in 11/27t.

so would my 7800 RD have issues if i'm running an ultegra 7900 11/27t in the rear?

any compatibility issues?

many thanks...

Antieverything

The 12-27 will work fine, if you wanted you could fit a 28t cassette on there. I routinely fit 30t on my short cage rd's but that requires flipping the b screw and/or getting a longer one. As far as the crankset it should work although the curve on the cage may not be ideal. There may be a little more rubbing of the chain to derailleur. If that sort of thing bothers you or the wife a new fd may be a future purchase. Keep in mind you may need to adjust the chain length for the swap. Probably shorter with the compact crankset.

achoo

I'd say don't pay extra for an Ultegra cassette. 12-28, $20:

Shimano 10 Speed Tiagra 4600 Cassette, Cassettes, CASSETTES

The 12-28 will work just fine with a DA7800 RD.

HillRider

The "7900" is Dura Ace, not Ultegra. The current 10-speed Ultegra cassette is 6700 and is available in 11x28 or 12x30 format. I agree that the Ultegra cassettes are more expensive than needed and either the 105 (CS-5700) or Tiagra cassettes will be more than adequate. Also, if your wife has problems climbing, she has absolutely no need for an 11T small cog and a 12xXX cassette will give her a useful interior cog in place of the useless 11T.

Donnie Johnson

What wheel set are you using? I have a set of Dura Ace 7801 wheels and there are compatibility problems with the some cassettes and the 7800/7801 hubs. According to Shimano They are a special one off model and require a deep spline 10 speed cassettes. (I am unsure if 11 sp cassettes will fit as I have never fitted one.) The only 10 speed cassettes that fit are 10 sp Dura-ace cassettes and the 6700 Ultegra cassette. 105 and anything lower don't have deep enough grooves to fit onto the hub. I believe that this is only a problem for Dura Ace 7800 and 7801 hubs. If you use something else this should not be a problem.

Good Luck

Donnie

ruirui

today a riding buddy of mine gave me his 12/30 ultegra cassette that he doesn't need anymore. can you tell me more about how you go about in flipping the b screw and/or where to get a longer one?

thanks...

FastJake

Look up what the "b-tension" screw is and what it does. Basically you need to take the RD off, remove the screw and thread it in from the opposite side, effectively making it "longer."

Might I suggest a triple crank instead of a compact? It would require more parts to change but if you can afford DA7800 then certainly you can afford appropriate gearing.

ruirui

FastJake, the same buddy sold me his 7800 DA groupset for $500, since it was going as a build for wifey.

Ok, I get the reverse screw in.. But not sure how to adjust it to accept the 12/30 before everything goes on.

FastJake

You can adjust it now and put the 12-30 on later. Keep in mind you will almost certainly need a longer chain for the new cassette (it's good to use a new chain on a new cassette anyway.) Check the chain length before you use the bike. Derailer Adjustment

ruirui

Ok, I was able to install it and run it with the Ultegra 6700 - 12/30t.

I inverted the B screw and extended it as much as possible but noticed the RD cage cog (the closest one to the cassette) is very close to the 30t when chain is cycling through.

Also, another thing I forgot to mention is the crank I have is the Dura Ace FC-7950 compact crank. And also noticed the chain rubbing against the 50T in front. see photo

please help.. thanks.

ruirui

i checked with the previous owner for the dura ace wheelset that i have. it came off of a 2006 Specialized Allez Pro (Yellow). i checked specialized website and only found: REAR HUBShimano Dura-Ace, 20 hole, aluminum freehub body, Shimano QR

Specialized Bicycle Components

ruirui

Bump