RD-7900 work with a triple crank?
02-20-12, 10:36 AM
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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...
02-20-12, 11:35 AM
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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).
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).
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An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“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.
02-20-12, 11:54 AM
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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.
Long cage is the right tool for the job.
02-21-12, 12:56 AM
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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.
