Advertise on Bikeforums.net



User Tag List

Results 1 to 5 of 5
  1. #1
    Rider lohump's Avatar
    Join Date
    Oct 2008
    Location
    Seal Beach, Ca
    My Bikes
    Trek Madone 5-Series Campy 10
    Posts
    108
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Largest Rear Cog for Dura-Ace 7900 RD

    Hi All,

    What is the Largest Rear Cog for Dura-Ace 7900 RD and a 50-34 Crankset that can be used?

    Thanks a bunch in advance.

  2. #2
    Senior Member VeloBrox's Avatar
    Join Date
    May 2011
    Location
    Oslo, Norway
    My Bikes
    1951 Armand Carlsen, 1969 DBS Deluxe, 1949 Diamant, 1978 DBS Winner Tandem, 1955 Herkules... to infinity and beyond!
    Posts
    378
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    According to Shimano specs, an RD-7900 can accept a rear cog of maximum 28 teeth. Usually you can exceed such limits by one or two teeth. My 105 rear derailleur works fine with a 30 tooth sprocket, even though it's rated for 28.

    Another thing to bear in mind is the total capacity, or the maximum chain wrap, of your rear derailleur. The RD-7900 has a total capacity of 33, meaning that with the 16 tooth difference between your smallest and largest chainring (50 minus 34), the difference between the smallest and largest rear cog can only be 17 teeth. Again, you can overstep the specs a bit as long as the RD doesn't double up on itself.
    Last edited by VeloBrox; 07-11-12 at 08:00 PM. Reason: Fixed link.
    -------------------------------------------------------------------------------
    1986 Peugeot PY10M
    1983 Trek 620
    1980 Peugeot PY10S (everyone needs two of these)
    1961 Monark 90320
    1953 Crescent modell nr. 11
    1942 Diamant modell nr. 11
    ...and a basement full of unrideable old stuff.

  3. #3
    Ride, Wrench, Swap, Race dddd's Avatar
    Join Date
    Jan 2010
    Location
    Northern California
    My Bikes
    Cheltenham-Pederson racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
    Posts
    3,323
    Mentioned
    21 Post(s)
    Tagged
    1 Thread(s)
    The 11-28t cassette is troublesome already with Shimano ten-speed.

    The B-tension needs to be jacked up so much that there is a large gap between the top pulley and the smaller cogs, so maintenance is frequent and chains and cables give shortened service life before shifting deteriorates.

    The smaller cassettes work much better with this derailer.

    I often substitute a non-floating top pully to restore the shifting resolution with 10sp 11-28t cassettes so as to extend the service interval. It makes a huge difference in terms of shifting response and cable adjustment range.

  4. #4
    Senior Member libero's Avatar
    Join Date
    May 2010
    My Bikes
    Viner, Klein, Caad9, Giant
    Posts
    146
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by dddd View Post
    The 11-28t cassette is troublesome already with Shimano ten-speed.

    The B-tension needs to be jacked up so much that there is a large gap between the top pulley and the smaller cogs, so maintenance is frequent and chains and cables give shortened service life before shifting deteriorates.

    The smaller cassettes work much better with this derailer.

    I often substitute a non-floating top pully to restore the shifting resolution with 10sp 11-28t cassettes so as to extend the service interval. It makes a huge difference in terms of shifting response and cable adjustment range.
    interesting. I've had an 11-28 put on my bike and wasn't completely happy with it - this may be contributing as I've already changed chains once.
    But what's a non-floating top pully ? It's a 3rd-party, non-Shimano product ? How does this help? Any recommendations for which one if I want to try this ?

    Thanks

  5. #5
    Senior Member
    Join Date
    Aug 2005
    Location
    Pittsburgh, PA
    My Bikes
    '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
    Posts
    25,759
    Mentioned
    6 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by libero View Post
    But what's a non-floating top pully ? It's a 3rd-party, non-Shimano product ? How does this help? Any recommendations for which one if I want to try this ?
    Just switch the two OEM pulleys in the rear derailleur, putting the lower fixed pulley on top and the upper floating pulley on the lower position. See if you like it but be sure to fine tune your limit screw and cable tension settings as there is no slack to make up for a near-miss.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •