With that in mind, does anyone know anywhere that makes, or would be willing to make, a longer RD cage? I contacted Fibre-Lyte, and they have no plans to make such a beast, and none of my searches have provided me with an answer.
Grill wrote: or you absolutely need a 32 to climb (in which case 30g makes no discernible difference until you shift some weight off yourself).
Some people actually race their light bikes in difficult climbing races where low gearing is a benefit even for very fit riders.
I know someone who used a 32t cog with 7900 DI2 derailleur for one of those climbing races. With 7900 Shimano moved the pivot a little to make room for larger cogs. While the spec is 28t, 30t is easily doable and 32t with the right combination of getting lucky on your frame's derailleur hanger and twiddling the B screw.
Can you swap the 6800's cage onto the DA derailleur? While the longer cage surely accounts for some of the weight difference it wouldn't be all of it. The DA body will be lighter than the Ultegra one.
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So, if you just move your RD a little further away from the cassette, you will be able to use larger cogs. If the B-tension doesn't do it for you, you could just have a new derailleur hanger custom made that does the trick.
If you're already researching custom made carbon fibre cages, this seems like a viable option.
Longer cages will most of all just let you chain cross more. If you're climbing 20% gradients I suspect you're an advanced enough rider never to venture into full chain cross big/big.
Unless you want to run a triple front, you don't need a long cage (to take up the slack chain), and it will not help you running a bigger cassette.
If you want to squeeze out the last bit of clearance after giving lots of B-screw, swap in a 10t top pulley cog.
Edit: Some clarification: the DA 9000 RD has a capacity of 33t. Take away 16t for the 50/34 compact front, and you have 17t capacity left for the rear. That is exactly what's needed to handle an 11-28 cassette. However, the capacity typically specs err on the conservative side, so you can usually handle a bit more. For a 32t bottom gear cassette, to be on the safe side with regard to chain tension, use a chain length as you would for 28t. That means you can not cross chain big-big, but who in their right mind would do that with a big cassette anyway.
If you run a standard 39/53 front you don't have those issues at all, because the smaller 14t front capacity leaves 2t extra for the rear, so you'll probably get away without running a short chain.
This is important on bikes with 52/36 or 50/34.
When you race you end up going back and forth on a compact chainring set more than you do on a 53/39. The mid cage and longer chain allows to to cross-chain better.
I have set up a few wi-fli and 6800 bikes with mid-cage derailleurs.
I am considering going mid cage myself. I will probably get a 9000 derailleur and
have K-edge turn it into a mid-cage.
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