Why does my seat post slip down?

Two different bikes, seat clamps etc. but same symptom, post slips about 1cm per 1/2 hour. What am I doing wrong? Too much grease? too feeble with the clamp?

Are you sure you have the correct seat post clamp, lots of different sizes like 28.6, 30, 31.8, 34.9 and probably other sizes as well!! Make sure you buy a quality one as well, Hope are my favourite brand.

ARE EITHER/BOTH CARBON ?

One is a cotic frame, with the Hope clamp supplied with it, so should be the right size.
Other is a carbon whyte, also using the clamp supplied with it. Quite confident that clamps are the right size.

I had this problem till I found a tube of ‘fiber grip’ stuff.

NOT GREASE. its like a synthetic abrasive that gives added friction for assembling carbon parts. works in my steel frame/alu post

Tim try using some carbon assembly paste – made by Tacx and others – it will stop them sliping down. If the bottom of your posts are shiny you may have already worn it so may need replacing. Highly recommend trying a Thomson post will outlast the frame and much higher fit tolerance.

Maybe you need to go on a diet!

try running a spiral of insulation tape around the post

Cheers, I’ll give the fibre grip a try, sounds like a cheap option.

Perhaps a diet as plan b 🙂