When I first installed a Campy 11 drivetrain, I put a 10 speed rear derailleur on the bike, and hooked it up to the 11 speed shifters, without a chain, just to see if the pulleys lined up with the cogs after all 10 shifts. The upper pulley was positioned about half way between the 10th and 11th cogs after 10 shifts, so I then knew it had a different actuation ratio than the 11 speed RD.
To make the RD travel further, the length of the lever arm needs to be slightly shorter. I expected the required amount would be small, so I tried a little experiment. I got an M5 socket head cap screw and cut it to the same length as the Campy cable clamp bolt. I removed the stock aluminum clamp washer from the RD and replaced it with two thin stainless steel washers having an OD of 10mm. To reduce the lever arm length slightly, I ground away the threads in the area where the cable would rest, just down to about the root of the thread.
I installed the 10 speed RD on an 11 speed bike making sure that the cable rested against the reduced diameter of the clamp bolt. I first checked the travel without the chain. This time, the upper pulley was almost perfectly aligned with the largest cog, after all ten shifts. I then installed the chain and checked the shifting. On the workstand, it shifted very well. With a little cable tension adjustment, the shifts were quick going up or down the cassette. If you have a relatively new 10 speed RD and want to upgrade to an 11 speed drivetrain at a lower cost, it’s worth a try.
Here are a couple of pics. The RD is the only 10 speed model I've got left, a medium cage that came off my winter/trainer bike. If I was doing this on a more permanent basis, I'd modify the original cable clamp washer. It has one arm on it that prevents rotation and another that guides the cable. I would cut off the cable guide arm, since it affects the length of the lever arm.
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