Just finished a build of a Vitus 979, took it out for its inaugural ride, and found the seat post kept slipping under my weight.

Most Vitus 979s (and most vintage Vitus bikes in general) have a grub screw that secures the seatpost. This one doesn't, it has a traditional clamp.


The slit in the seatpost next to the clamp doesn't get narrower when the clamp is tightened to the max, which I assume means the seat tube isn't constricting at all.

​​​​​​I know that 979s officially take 25mm seatpost, which is what I'm using - but then again officially they also secure the post with a grub screw, which this one doesn't. So it could be a case of trying a 25.4mm seatpost - but I don't have one lying around and I'm reluctant to buy one before I've ruled out this being a clamp issue.

What should I try next? I could try adding more washers to the clamp bolt in order to see if it gives more of a squeeze. I don't have any in the right size hanging around, but I could get some easily enough.

Any and all wisdom thankfully appreciated!