I think the correct answer is you can run the 10spd cassette with the 11 Deraileurs. But you'd also have to change chain and shifters.

Your cassette will work just fine. An Ultegra "11sp" RD will work as well. You may be able to futz with an 11sp Ultegra FD and get it to work, but it will not be optimal.

11sp is built around 11 cogs on your cassette in basically the same linear space as 10 cogs on the rear hub. That's why wheels are listed as 10/11 compatible. When the cassette is bumped to 11sp, the cogs, spacing, and chain are narrower to work in the more confined space. As a result, changes are made to the front rings and/or spider to optimize function with the narrower chain.

The 11sp FD cage is slightly narrower to match the changes with rings/spider. The changes are not dramatic, but will require precise adjustment on a 10s system to avoid excessive chain rub at the best and poor shifting performance at the worst.

The 11sp RD is a parallelogram on a spring. It has to cover basically the same distance for 11s vs. 10s. The indexed shifter is what controls where the RD stops in its travel.

FWIW, you won't experience much (if any) performance increase with the upgrade to Ultegra RD and FD (IMO).

If you want to improve your shifting I would forget the derailleurs and change the crankset. Chances are if your bike started with a mixed shimano groupset it will have an FSA crank or very low end shimano one. Changing the cranks will make a much bigger impact that the derailleurs although it may take some fiddling to get the chain line correct if you do it yourself. I would be amazed if you would be able to notice a difference between 105 and ultegra if nothing else changes.

http://forum.slowtwitch.com/...;;page=unread#unread

Thank you Burnt! I was looking for this topic for a couple of hours, going through similar questions, but not identical, so I started a new one. Glad you've found that!

Thank you all for your replies, I think I will gather a bit more money, and invest in a new crankset next year :)