New frame takes 30.9mm post, all of mine are 27.2mm. Rather than shell out for a new post, tempted to get a £3 shim. Is it likely to slip?

never had any problems with that combination on my Giant NRS.

I think if you have a dropped top tube you need to make sure the bottom of the shim is below the junction, though.

Ooh that’s a good point – even if the post itself is well below the junction?

Used that combination for 6 frequent-saddle-height-adjusting years and never had a problem.

The only issue I’ve ever had with shims is that they let water in – check your bike periodically and let out any trapped water (mine collected quite a bit – rear facing slots are obviously worse).

Ooh that’s a good point – even if the post itself is well below the junction?

Well, since the post will no longer be in contact with the frame, it’s irrelevant. The shim needs to be at least the same depth into the frame as you would be ok with if you had the right sized seatpost.

the post is only really supporting the seat tube where it’s in contact , james, so just for the length of the shim. As long as the shim matches your frames minimum insert requirements you should be ok though.

Ah thanks – looks like my shim extends about 10mm below the minimum insertion hole in my seat tube

i’ve only ever snapped one post and that was the only shimmed post i’d ever used. Sell 27.2 + buy a 30.9; the difference will be less than £3