Any question answered...

Are you sure you’re not just feeling the brake pads shift within the caliper?
I can create similar movement on my bike but it’s just the pads moving.

The splines on the hub and rotor look completely fine. The play in the first clip with no lock ring is normal.
But at 50nm I really doubt that it’s the rotor moving.

I’d also do up the qr really tight and see if it goes away.

Sorry if the last video clip was a little confusing.
Look at the two white dots, one on the disc rotor and one on the hub. They move relative to each other with the brake applied and rocking the bike back and forth.

If you can rotate the disc centre relative to the hub, it's not being clamped by the lock ring. The rotor centre should be pressing on the hub shoulder with an axial load of about 10kN (27×1 thread at 40Nm tightening torque), acting as a clutch which would require about 60Nm to slip, which would be 750N tangential force at the rotor perimeter for a 160mm rotor.

Look at the two white dots, one on the disc rotor and one on the hub

Ah ok I see that now.

But yeah, what tester said!

The rotor somehow isn’t being clamped down properly.

Does the lock ring have a thin aluminium spacer (shim?) on the back of it? It should.

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It's probably not this, but torque wrench calibration and the mismatch of tightening torque and axial load due to thread friction are both potential confounding factors.

Try a different torque wrench, and clean the threads of hub and lockring.

(I say this for completeness, not actually expecting this to cause your issue)

Yes, mine also has the thin metal washer.

Thanks for all the replies.
I think I'll try to get a Centerlock spacer/washer to rule out that my lockring is bottoming out before reaching the necessary clamping force to keep the rotor in place. The thin metal washer gets crushed when tightening down the lockring though, so I'm not convinced that's the problem.

I had the torque wrench calibrated last year. Both threads are clean and had a small amount of grease applied so friction in the thread shouldn't be an issue either.

Centerlock shims and a different style lockring are what's left.

It’s worth taking the shim off the rear and trying the front lock ring with two shims.
If it goes away then you know what’s happened.

You could also take the (term?) existing washer off the rear lock ring and add it to the front, then tighten it down to 40 Nm and see what the result is.

Little update:
I tried two different lockrings, with internal and external splines.
I took a 0.7 mm HG washer, split it to get it over the spline and installed it behind the rotor to remove any chance of the lockring bottoming out.
Then I tried more grease on the threads and also grease on the washer of the lockring to prevent friction and hopefully actually reach those 50 Nm I want to tighten it with.

I'm still feeling and seeing the radial movement.
I'm out of ideas to be honest. Can't really explain what's happening and why.
I guess I'll just have to live with it.

Have you tried the rotor on a different hub, and a different rotor on the original hub?

My other wheelset is 6-bolt. Should have stuck with that for this build as well. Don't have any other Centerlock rotors to try unfortunately.

I had a similar issue last year, SRAM (ofc) Paceline rotors on 240EXP hubs. Zyro said "send it where you purchased it from [you such and such]" so I bought another pair from where I got those from, Centerline XR this time. It worked.

The play in the video is not normal because it's just too much. The lockring can apparently hold up up to a point. Trying with XTR rotors it's minimal.

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The play in the video is not normal because it's just too much

I think it is normal. There has to be some play in the splines with the lock ring off. Otherwise it would be a press fit and you’d have difficulty getting the rotor on and off.

I think the next step for @wenzovic is to try another rotor or take try the lock ring with two shims, take the shim from the rear lock ring.

Edit - which brand of lock ring are you using?

As tester said, the force to overcome the friction of the lockring would be high.
Even if there is slop in the splines, if the lockring was doing its job and holding the rotor at 50nm, you wouldn't be able to move it by hand

Exactly. There’s just no way the rotor could move if it’s being clamped down correctly at the proper torque.

Think about a 6 bolt rotor, there’s loads of slop before the bolts are tightened. Much more slop than the centerlock spline interface.

A second rotor could help ID A manufacturing defect. If it still moves, it’s the hub. If it doesn’t, it’s the rotor.

does anyone have any the dimensions for VIA iso disc hubs, front and rear ? i cant find anything online.

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I tried the "two washers under the lockring" thing as well.
Same result as before.

I guess it comes down to the question, why my lockring can't seem to create enough clamping force to hold the rotor. I'll try to contact some people in my area (like Trickstuff) and see what they have to say. Over in the German forum "MTB-News" the Centerlock issue is well known and Trickstuff commented on it a couple of times. After reading all those posts, the explanations and possible solutions given are a bit inconclusive.

Can you get a fine feeler gauge into the gap between the lock ring and the rotor spider?

Just what is the point of centrelock, besides being yet another standard for its own sake?

Looks nicer

what is the point of centrelock

Separating fools from their money

Is all helitape much of a muchness, or does someone have a particular recommendation?