Dura Ace Granny Gear Dynamics?
11-28-11, 02:40 PM
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Dura Ace Granny Gear Dynamics?
Anyone have any idea why this Dura Ace 7800 triple's granny gear spins just about half a diameter within the bottom bracket? What is the purpose of these tabs/inward protrusions? This is a Phil Wood Outboard BB, but its external dimensions are almost identical to the OEM Dura Ace bottom bracket.
11-28-11, 04:28 PM
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Just a wild guess, the tabs are to keep the chain from jamming between the chainwheel and the bottom bracket if it gets dropped to the inside.
11-28-11, 04:36 PM
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Haha--of course. Pretty good wild guess. Thank you!
Last edited by calamarichris; 11-28-11 at 06:52 PM.
11-29-11, 12:29 AM
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There are similar tabs on most of Shimano's triple cranksets. The thing that makes this Dura Ace 7800 triple crank special is that there are no mounting holes on the spider for the inner ring - the inner ring is bolted to the middle ring. They use a special bolt circle diameter for this (92mm), making Shimano your only source for spares. If you ever need to change your inner or middle ring, you are going to be severely restricted for choices. The Dura Ace 7700 triple and the new Ultegra 6700 triple also use the same system, which is why I do not recommend any of those cranks to anyone.
11-29-11, 12:40 AM
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Hmm... thanks ChrisW.
As long as my dual chainrings have lasted (12,000+ miles and still going strong) , and how infrequently I'll be riding this bike compared to my two-ring workhorse, I don't think it will be a problem in my lifetime.
As long as my dual chainrings have lasted (12,000+ miles and still going strong) , and how infrequently I'll be riding this bike compared to my two-ring workhorse, I don't think it will be a problem in my lifetime.
Last edited by calamarichris; 11-29-11 at 12:45 AM.
11-29-11, 02:07 AM
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I have to confess I'm a little disappointed. Shimano chainrings shift so well, that I honestly expected a more sophisticated answer involving centrifugal harmonics or some such thing.
Does anyone else have a 7803 triple? How does it shift?
I'm hoping to have this build finished in time for the weekend, and I'm dying to know if it shifts as well as my two-ringers.
Does anyone else have a 7803 triple? How does it shift?
I'm hoping to have this build finished in time for the weekend, and I'm dying to know if it shifts as well as my two-ringers.
11-29-11, 04:01 AM
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