I picked up a second hand gen1 speed concept at the back end of last year and from the looks of it the previous owner had struggled with this problem. Post was covered in tape and you could see the indentations from over tightening. Took 2 shims made from a coke can in front of the wedge (not between the wedge and post) and park tool carbon assembly paste the full inserted length and I haven’t had a problem. I’m probably about 10kg heavier than the guy I bought it off and I’ve ridden some pretty rough roads.
One thing to check if it is an older speed concept, from what I remember when I looked into this, the wedge was updated to solve this problem. Pretty sure the update is he wedge with a 5mm hex and 7.2Nm torque requirement, the older one was different.
@jtuk Mine is also a gen 1 Speed Concept, but it does have the “upgraded” wedge and there are no visible issues with the post itself. It just sinks. I think it was a pretty poor design. I will try a shim in front of the wedge to see if that helps. Thanks! Pretty sure I’m going to end up with a combination of all of these things… shims, tape, emery cloth, and more!
An easy thing to try is to “grease” the bolts. It allows for a bit more torque to be applied when tightening it down.
I’d also consider thin skateboard grip tape. If other measures didn’t work then I’d consider more “extreme” methods. Possibly like wedge stopper piece inside the seat post (to keep from bottoming out) or possibly drill a hole and place dowel pins to keep it from moving.
Put a piece of wood or something inside your seatstay such that your seatpost sits on top of it. That is tricky to easy depending on how that cavity is shaped and bb/crank axle access.
Example, if you have a window in your house that won’t stay up/open one can use a wood stick to prop it up.