Dura Ace rd-7800 with 12-30T cassette?

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eyedrop
Posts: 107
Joined: Wed Jan 27, 2016 5:33 am
Location: Prescott, AZ

by eyedrop

Hi everyone! Ive got a bike with DA 7800 rear derailleur, compact 34-50 chainrings and a 12-28 shimano cassette. I live in the mountains with some serious climbs where low gears are required.

Im pretty sure I can fit a 30T on there if I wanted since the B screw is only turned in about half way, but the LBS said I would sacrifice shifting quality. They recommended an Ultegra 6700 long cage and a 32T casette and said a Dura Ace at its absolute limits would probably shift worse than the Ultegra long cage setup. Is this true?

DCWNGL
Posts: 1
Joined: Wed Jan 27, 2016 4:15 pm

by DCWNGL

I have a Scott CR1-SL with DA 7800 rear derailleur, compact 34-50 chainrings, but with a 12-27 shimano cassette. I installed a 12-30 tiagra 10-speed cassette for the steep mountains. Unfortunately my bike does not have a replaceable derailleur hanger, and therefore, the upper pulley rubs the 30t cog a little (when using that cog). It shifts fine and I have never had to replace the top pulley yet. My understanding is a longer hanger will move the top pulley further away from the 30t cog and that a long cage derailleur will allow for more chain rap (which doesn't seem to be your problem). My understanding is different bikes have slightly different length hangers so you may need to try a 12-30 cassette to see if it works ok and then replace the hanger if the pulley rubs (if you have a bike with a replaceable hanger).

On a side note, there isn't much change in gearing between a 28t and a 30t. If you want the 32t cog the LBS recommendation may be the way to go (along with a longer hanger).

by Weenie


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Rick
Posts: 2034
Joined: Sat Aug 29, 2009 4:30 pm

by Rick

The Shimano XTR RD-M970 SGS Long Cage 10 speed Rear Derailleur works fine with road shifters, and will accept up to a 34 tooth, maybe even a 36.
At any rate, all your big cassette cog worries are solved.

Birdman
Posts: 283
Joined: Mon Aug 17, 2015 4:03 pm
Location: Chicago, IL, USA

by Birdman

Yes, is true. Shifting quality will not be at its best when you push it to the limit. The bigger question is: Will you even notice the difference? Maybe not. If you do notice a small difference, then the next question is: Is that small difference a deal breaker?

Lennard Zinn at VeloNews wrote many articles related to your questions. In case you haven't look there yet.

Also, consider this: RoadLink by Lindarets
http://www.wolftoothcomponents.com/products/roadlink
Cervelo R3 MUD ~ viewtopic.php?f=10&t=136544
Specialized Allez Gold ~ viewtopic.php?f=10&t=134419

oreoboreo
Posts: 510
Joined: Tue Aug 09, 2011 2:25 pm
Location: Aloha, Oregon/ Poipu, Kauai Hawaii

by oreoboreo

Roadlink, now that is a cool product. I sure solves a problem when people are looking for easy gearing with a short cage. :)

Scott
Let's finish the ride with a 20% grade.

2011 Scott Addict R1 DA 7900 Matt black
2012 Scott CR1 Pro Ultegra 6700
2015 Specialized SWorks Tarmac Da 9000
2016 Specialized SWorks Tarmac DA 9100

eric
Posts: 2196
Joined: Tue Nov 11, 2003 9:47 pm
Location: Santa Cruz, California, USA
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by eric

I've run a 30t cog with a 7803 and 7900 rear derailleurs without problem or significant degredation in shift quality, on Cervelo R3 and R3SL frames. 7803s have large jockey wheels. They have the same 27t max cog spec as the 7800 but may differ in reality. 7900's spec is 28t.

In the end it depends on your frame's derailleur hanger. Longer is better for fitting large cogs. You have to try it. If it doesn't work then a 7900 or Ultegra equivalent ought to do it. 28t-30t is a fairly small difference. If you want to go larger you'll need a MTB derailleur like Rick suggested.

Team Murray
Posts: 36
Joined: Thu Apr 30, 2009 11:19 pm
Location: Los Altos, CA

by Team Murray

I have had friends run their 7800 RD with 11-32 and had no issue. I ran a 6700 RD with 11-32 and it ran fine. If you have the cassette, just go ahead and get a new chain and try it. Or, I have a 6700RD and SRAM 11-32 that I would sell since I moved to 11-speed.

by Weenie


Visit starbike.com Online Retailer for HighEnd cycling components
Great Prices ✓    Broad Selection ✓    Worldwide Delivery ✓

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