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My SRAM 980 9 speed cassette appears to have developed some rotational play between the two main halves. The lock not is tightened down hard. I can't remember, should there be a spacer behind the cassette perhaps?
I've not had to use a spacer on SRAM 9spd cassettes when fitting to Hope freehubs in the past.
Is it an alu freehub shell? If so it's possible the splines on the freehub have been cut into by the rings in the cassette it you're experiencing rotational movement.
Hope freehubs are notorious for this.
EDIT - Ignore that, didn't realise the 980 had the alu spider.
When the lockring is tightened up there should be no play in the cassette at all. Are they really loose, or does the lockring just need nipping up a bit?
They are not so loose that you can move them by hand initially (it's the cluster of sprockets on the alu spider, by the way), but after a few miles the knock returns and the two clusters can be moved relative to each other. The lockring is still tight.
I've had this same thing now with 2 different SRAM cassettes on 2 different hubs, a Hope and a DT Swiss 240
I couldn't sort it and ended up swopping cassettes to Shimano, wouldn't have another SRAM cassette as a result (and I used to swear by them)