Bit of a clumsy title but best I could think of!

I have a fairly new Shimano 1×12 equipped hardtail.  If I’m in a high gear and stop pedaling, particularly when standing up as I probably stop the cranks more abruptly, the cassette momentum momentarily slackens the top of the chain and then there is a thunk as the mech takes up the slack.

I’ve never had this before, is it just a symptom of larger heavier cassettes needing the chain to stop them, or is something not quite right?

Freehub with too much grease in…? Freehub bearings not in place properly?

Sounds like a freehub issue whatever.

+1 oceanskipper

What hub/wheel is it?

Is the cassette done up tightly?

Probably freehub. Might be binding slightly, bearing(s) could be on the verge of collapsing, etc. Might even be an iffy top jockey wheel.

Is the cassette done up tightly

Cassette lock ring not done up  properly wouldn’t create the symptoms the OP has described.

Clutch on mech knackered or turned off?

mine does the knock you describe, your analysis seems spot on, heavy cassette. i’ve had 2 years with zero problems so i think you’ll be alright. mines on a hope pro 4 and xt 12 speed cassette.

It could also be the clutch – is the clutch on (lever on the ‘up’ position)?

Pretty sure i commented on an identical thread last year sometime…

Combination of a freerolling freehub, low clutch tension and a heavy cassette.

Or a sticky freehub.

I think it’s a problem exacerbated by heavy cassettes but usually caused by something else.  In my case recently it was a draggy freehub bearing – one of the two bearings had become a little stiff to turn independently of the axle.

I’ve never had this before, is it just a symptom of larger heavier cassettes needing the chain to stop them, or is something not quite right?

Yes, my deore does this very noticeably and GX to a much lesser extent.

Does it regardless of freehub (one Fulcrum i.e. Campag, one DT Swiss, one OEM which is I think a KT).

Does it regardless of mech (I think the GX does it less because the cassette is lighter but I’ve not swapped that many parts to test that theory properly)

Interesting..

I had a similar issue with my prime road wheel running a road cassette at the weekend. Basically a loud ‘knock’ whenever I commenced the pedal stroke, which I assumed was the freehub engaging.  Stripped and regreased, bearings all fine. Problem still exists. Maybe it’s always been there and I’ve not noticed it, but don’t think so!

Well I think @jonnyboi nailed it, luddite singlespeeder here is late to the party with clutches and my SRAM Force one doesn’t have a lever! I thought the clutch was on but on removing the wheel it appears the lever needs to be raised to engage the clutch rather than parked towards the mech body.

A workstand test replicates the issue and with the clutch on, the chain sag when suddenly stopping the cranks is noticeably less (still does it a little).

To answer other questions, Bontrager wheelset with what I assume is a new fangled XD freehub (whoop they didn’t change the lockring standard!) an SLX 10-50 cassette and bearings seem fine, no drag on the freehub (like you would get when the freehub wants to spin the cranks or sag the chain whilst freewheeling)

It’s not an xd freehub with an slx cassette it’s a micro spline freehub if 12 speed

My road bike does this too, when in the smallest (11 tooth) at the back.   Just the combination of rotational inertia on the block, coupled with a derailleur that can’t pull the slack back quickly enough when I suddenly stop pedalling. Worse if I kick backwards on the pedals whe I freewheel too.

Any other rear cog is fine as there’s more torque applied to the cassette by the rear mech + chain when in larger cogs.

Most concerning bit for me is the momentary big dangle of chain that could snag or go into the wheel.

Done it since new ( 4 years ago)..not changed by changing the mech, cables, cassette (Ultegra so not top heavy).  Amd free hub is light to spin etc.

My road bike does this too, when in the smallest (11 tooth) at the back. Just the combination of rotational inertia on the block, coupled with a derailleur that can’t pull the slack back quickly enough when I suddenly stop pedalling.

I don’t think its quite that. There’s no extra slack to pull in when you stop pedalling, but the inertial force of the casette is battling the spring & clutch on the mech. If your clutch is worn there’s less stopping the casette, and if you’re in a higher gear the cassette is spinning faster, so there’s more force to stop.

Yes, it’s more common on bigger heavier hubs, but you should really only notice it when you stop pedalling and start backpedaling quickly.

IME (on hope hubs) it can be caused by the wheel (not freehub) bearings being worn & causing the freehub to bind inside of the hub. You don’t notice much play in the bearings because their is a tight tolerance between the pawls and spline. When the bearing collapses you have a fixie or walk home.

Is one of the hub bearings seized.
This happened to me and that’s what it was- the the one in the freehub.