On both of my bikes (Yeti SB140 and Santa Cruz Tallboy) the headset keeps coming loose. Both bikes have Chris King headsets, Fox Forks, Sram Maven Ultimate brakes (180mm) and both have Garmin top tube mounts.
Any ideas why?
my thoughts are the top tube mounts but can’t think why? Are the brakes too powerful?
Do you use an expander wedge in place of a star fangled nut?
We had issues on one of our road bikes where over time the expander had lost a bit of grip (I assume just thousands of miles of tiny vibrations) and so it was slipping up the tube instead of fully tightening the headset. We would clamp the stem thinking "job done" and actually the headset was never tight. About a turn on the expander plug fixing bolt had it firmly back in place.
Other thoughts (you've probably checked but assumption and all that)...
If they are SFN's are they on the move?
Have you properly torqued the stem bolts?
Also are you sure the loose is not in knackered bearings? Maybe a silly question but worth a check.
my thoughts are the top tube mounts but can’t think why?
Why would you think that something mounted on your (static) top tube would affect the (dynamic) headset?
Are the brakes too powerful?
No.
How are you torquing the headset and stem bolts? If it's happening a lot I'd be slightly concerned that you've now damaged the headtube (which could make it feel like a loose headset)
First remove the top cap (the round washer that you tighten to apply pressure) then, check that your steerer tube (the tube that is attached to the forks and turns them) is below the top of your stem or the Garmin mount, or any spacers. Round about 3 - 5mm below, this enables pressure to be applied via the top cap and top cap screw. If it’s not then add spacers to enable correct adjustment.
The top bolt of your steerer should be no more than half way above the top of the steerer but ideally below.
This is all dependent on your headset being assembled correctly and in the correct order!!!
Hope this makes sense but without a picture I’m trying to imagine different scenarios!
According to your other thread, the Tallboy is only a few days old. Has that come loose more than once in that time? I'd expect to have to retighten after a new headset installation as the first ride or two properly compresses all the headset parts down.
Other than that, Garage Dweller and Mr Wheels have it right.
Headset clamp inside the steerer would be my first check, happened on my road bike years ago, suddenly there was ~3mm gap not covered by spacers.
Numpty question but to tighten my headsets I have just been tightening the headset. Looking on YouTube it looks like I should loosen the the pinch bolts on the stem first, then the headset and then retighten the pinch bolts. I have now done this so hopefully it might resolve things? If this is correct why do you need to loosen the pinch bolts first?
Because the pinch bolts pinch the stem on the steerer, and it's the stem that holds it all together.
The pretension bolt, cap and expander can be taken out on some systems. They're only there to set it up.
Because the whole stem has to move downwards to put pressure on the bearings. If you are just tightening the top cap then that’s all that you’re doing.
If you’re anywhere near North Derbyshire then I don’t mind showing you, I am qualified but not working anymore!