The top head of my thudbuster st started to rotate around the lower seat post. But it was still under the warranty period so they exchanged the whole unit. I didn't look closely at the old unit, but the new unit they sent me had a pin which passed through both the seat post and the head. I asked if this pin was holding the 2 parts in place, but they said it was a butted join and weren't sure what the pin was for. I assumed they meant that the head was pressed into the post. If in the future the head starts rotating again, I might drill a pair of holes and press in pins.
Regarding the travel Arc of the ST, it feels like the vertical movement is less than the horizontal movement. My back is sensitive to the change in position, so I decided to use the middle elastomer, which seems to move 1cm to the rear when I sit lightly on it. The soft one travels really far so I won't use it. The hard one dampens very little for me so I won't use that either.
I assume that Dahon seat tubes have slightly steeper angle since the seat tube passes behind the BB. This would shift the thudbuster movement slightly more horizontal than vertical. But the thudbuster is still very effective in absorbing shock even from cobblestone roads.
On another note, the clamp screw and nut stripped out and getting replacements is a pain because they're not standard fasteners. I have to wait a few days for the parts to arrive before I can ride again. The manual says 6.8Nm, but strangely the text printed on the clamp says 5 Nm. I hand tighten with a short wench so I assume I made it 6 Nm. But next time I'll check with a torque wrench. The problem is that I've been testing several saddles and have been readjusting the screw which wasn't turning smoothly. Maybe the washer isn't sitting level on the bearing surface. I wonder if I should grease the threads.
Having 2 screws on the clamp really helps with precise saddle adjustments.