I have a steel frame and have always had trouble with slipping posts.
I’ve tried different posts and they all seem to be a good fit (don’t come out without a bit of effort)
I’ve tried the original bolted clamp and a Hope QR (that is lubed and needs a lot of effort to close to have any chance of keeping the seat position)
It’s a 27.2mm post in a 30.6 tube, no difference with the plastic shim or an ali cane creek, though I sized up to a 30.8 OD to get a snug fit.
I’ve also tried two different carbon pastes (one specifically for gripping stuff).
I know the Hope clamp hasn’t got the best rep so I don’t know whether to change that, or get a 30.6 post to remove the shim from the equation.
Measuring the seat tube ID seems to be between 30.65 and 30.7mm.
Seeking opinions before throwing more cash at it. I don’t run a dropper on this bike so want to keep a QR collar on it.