Would like to use a 850i seatpost on my Pitch (30.9mm). Would need to shim the 27.2 post to fit the 30.9mm frame. Never used a shim before any adverse affects on the frame before I buy said item(s)?

The shim needs to extend below any tube joins (i.e. top and seat tubes), otherwise no problems. Some posts are only made in one size, requiring shims by design. Keep them lightly greased inside and out, or they may become 'bonded' like a seat post to a frame.

Cant stand my shim, came with the frame (Giant) and the frame is a 30.8 – pain. It creaks like mad, pulls out when you pull the seat up, rotates etc. Clicks and creaks, seizes. Gease it and I can't tighten the clamp enough to stop the seat moving.

I wouldn't think a shim is likely to get stuck in the frame if it's the type with a slot in it – if it did get a bit hard to remove, it would be easy to prise it loose. What I do find with mine is that it has an annoying tendency to slide out with the post when you're raising the seat.

I wonder wether the thermoplastic ones are less likely to move or squeak – got one for my new roadbike but not fittedit yet :

have you tried putting the lip of the post under the collar?

USE Thermoplastic shim on my Stumpjumper, it's never moved, squeaked or done anything other than what it's meant to do in over 5 years. Recommended.

Used to use a slotted shim on my Pitch so I could use a 27.2mm post. Never caused a problem. There is a hole in the Pitch frame so you can see how far the shim has to extend down the seat tube

Sounds ok then, somebody on here had a split frame and blamed the shim for exerting more force or something like that. Cant see how it would but it sounds ok to use. Off to look for holes in my seat tube 😉

Thanks all.

sv I have a spare (never used) CNC x-lite shim 30.9 to 27.2 if you want. £3 posted?

SV – the hole is cunningly hidden behind the brace betwixt top tube and seat tube 😉

Never got on with one I used with a syncros post I used to have. Greased it, removed all sharp edges and it used to ping and click all the time. Gave up in the end and got a cheapo post off the classifieds. No noise since. Perhaps the plastic ones are better?

I have a slotted shim on my Trance – no problem apart from the occasional smear of grease needed to stop creaks.

Are they all a standard length? The xlite one might do well is it 30.8 or 30.9mm take it the 0.1mm wont matter too much?

Mr Garbett I thank you sir!

my mate kept complaining about his shim squeaking and coming out each time he moved the saddle so we araldited it in place. no problems sínce.

oh, this too is a Giant frame with a stupid 30.6/8 post siye that you can't find anywhere….

Ive got one of those plastic shims for my GF's bike she has a Giant XTC ex with a silly seatpost size its 30.8??wtf?? we had to cut her seatpost down as she is shorter than me! but when i want to use the bike i have to fit a 27.2 seatpost and shim, works so far no issues or squeeks.

My P7 came with a metal shim fitted, can't get the damn thing out so leaving it there!