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Has happened twice now mid-ride, can rock the various cogs around the spline slightly but also in and out about a mm or two, very rattly sounding on the bike
1st time the lockring had come a bit loose but this time the lockring was still tight but the cassette was loose
It's an XT cassette and Stans (3.30) freehub
All spacers are on correctly, freehub is pretty new with no real wear on the splines, and threads on lockring and freehub are fine, cassette is new I can't spot anything obviously wrong
Any ideas?
Check hub for play.
Check cassette locking thread.
Are you using enough force when tightening? I use whatever is on the lock-ring with a torque wrench.
Are there any spacers missing?
None in hub (although bought 2nd hand they are effectively new, and I've put 90 miles on them)
Thread is A1
Yes, (although never felt the need to use the recommended 50Nm before and never had a similar problem
After it came loose first time I made sure to do it more accurately than the 'tight and a quarter turn' method this time and as I said the lockring was still as tight when I checked just now as it was when I tightened it, but the cassette behind it was all wobbly)
No (and cassette only just fits on freehub, when fitting smallest cog sits slightly proud until lockring is tightened)
It may be an 11 speed road freehub that needs a spacer behind the cassette to run 11 speed Mtb. If the lockring is tight and the cassette is still loose, and assuming you've not forgotten to put one of the spacers between the cogs, then you need to add a thin spacer behind the cassette.
You can usually check this before tightening by holding the cassette on the freehub with your palm and using the other hand to see if there is play. Not sure what you mean by "smallest cogs sits proud" - it is designed to sit proud, that's why the freehub notches in it don't go all the way through that cog like they do on all the other ones.
The other thing it might be is the cassette lockring not tightening down against the freehub body correctly. I've seen this happen when the outboard freehub bearing hasn't been seated correctly and so the lockring binds on that rather than the cassette. Try screwing in the lockring (by hand) with no cassette on the freehub
Finally... Check the rivets that hold the freehub together haven't failed. Last year I broke the rivets on 3-4 XT and 2-3 XTR cassettes and these were the symptoms (when it first happened I assumed the lockring had just come loose). Thankfully they were all warrantied and I am now on SRAM one piece!
I've had this same issue, but the problem was squeezing all the cogs on so the lock ring could get a good purchase.
As 'gee' suggests, it sounds like you may need a rear spacer if the cassette felt loose.
Be aware that rear spacers come in different thicknesses. I've just had a problem with loose cogs on my road bike. Turns out that the rear spacer that came with the hub was too thin. A slightly thicker spacer sorted the problem.
I had this once, the greetings body was cracked so the lock ring would never tighten properly.
Don't think it's a rear spacer required as it starts off tight and wobble free then loosens after about 20 miles
If it was a spacer or lock ring not gripping would it not be loose from the off?
I had this issue, and fixed it by adding an extra lock ring washer to the lock ring. Even though it's only .2mm thick it seems to have done the trick.