Hi.
I have this problem on my current road bike. Cassette is a 10sp Ultegra, hubs are 10sp Dura Ace 7800 (I think it’s the first DA 10sp ones). The cassette seems to be slightly loose no matter how tight the lockring is tightened. I’ve tried putting a normal spacer behind the cassette (like you have between sprockets), but it would be too thick, and the cassette would not tighten properly. Because of the loose cassette the gear changes feel sloppy and not as quick as I would like from Ultegra and Dura Ace combo (DA shifters work as they should).
Should there be a very thin spacer I am missing? This would be pretty strange as it’s a 10sp hub and 10sp cassette. Only spacers I had to use was on a set of Ksyriums before putting a 9sp cassette on. Any advice welcome as the new setup performance is a bit disappointing compared to my old 9sp Ultegra.
