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Old 07-11-12, 11:23 AM   #1
lohump
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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.
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Old 07-11-12, 07:58 PM   #2
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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.

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Old 07-11-12, 08:30 PM   #3
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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.
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Old 09-21-12, 06:44 PM   #4
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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
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Old 09-21-12, 07:23 PM   #5
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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.
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