Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 02-20-12, 10:36 AM   #1
guy2600
Senior Member
Thread Starter
 
Join Date: Nov 2009
Location: Boston area
Bikes: Sette Forza Elite Road, Sette Ace 26" MB, Specialized Shiv TT
Posts: 178
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
RD-7900 work with a triple crank?

I'm guessing it doesn't but figured I'd ask. I'm buying a new RD and can get a 7900 for the same price as a 7800 so would like to get the latest rev if possible. Thanks in advance...
guy2600 is offline   Reply With Quote
Old 02-20-12, 11:35 AM   #2
FBinNY 
Senior Member
 
Join Date: Apr 2009
Location: New Rochelle, NY
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Posts: 29,686
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 232 Post(s)
Rear derailleurs don't care about the crank set either way, so no automatic issues.

But there is the question of chain take up capacity, regardless if whether it's a double or triple. The RDs cage length determines the capacity which is listed in terms of the number of teeth. You need capacity equal or greater, than the sum of the differences between the largest and smallest sprocket front and back, ie. 12-26 & 28-38-48 = 14+20 = 34t capacity needed. The capacity requirement is the same regardless of whether you have a triple or double with the same inner and outer chainrings.

There is a bit of wiggle room on borderline cases so if you need 29t and the RD you want has only 27t you might get by with careful chain measurement, and possibly not using the small/small combinations (which most don't use anyway).
__________________
FB
Chain-L site

An ounce of diagnosis is worth a pound of cure.

“Never argue with an idiot. He will only bring you down to his level and beat you with experience.”, George Carlin

“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FBinNY is offline   Reply With Quote
Old 02-20-12, 11:54 AM   #3
BikeWise1
30 YR Wrench
 
BikeWise1's Avatar
 
Join Date: Aug 2005
Location: Oxford, OH
Bikes: Waterford R-33, Madone 6.5, Trek 520
Posts: 2,006
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Why not toss an Ultegra long cage on there for way less money, and less hassle, as well? If you MUST have bling, use a non-Dyna-Sis XTR.

Long cage is the right tool for the job.
BikeWise1 is offline   Reply With Quote
Old 02-21-12, 12:56 AM   #4
Chris_W
Gear Combo Guru
 
Chris_W's Avatar
 
Join Date: May 2007
Location: Switzerland
Bikes: Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
Posts: 2,241
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
It is possible to change the cage on many Shimano RDs. I have not tried to do so with a 7900, but you may be able to put a long cage on there from a different derailleur. Taking the cage off is not too hard, but putting one on often takes three hands plus a vice to hold it all in place and screw it down. However, as mentioned above, the Ultegra 6703 is a very good piece of equipment, and that would be my first choice.
Chris_W is offline   Reply With Quote
Old 02-21-12, 06:49 PM   #5
duckracer
Senior Member
 
duckracer's Avatar
 
Join Date: Aug 2011
Bikes:
Posts: 100
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
About a month ago, I picked up a new Ultegra 6703 for $64.00 at performance bike. Works great so I would just go that route.
duckracer is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 05:05 AM.