Just got meself a giant xtc 08 c2 bike, Have got an older xtc (alu) and used a 27.2 carbon seatpost with a 30.9 U.S.E shim. Is this safe to use on the carbon frame or is using shims a big no-no? Also my (carbon) s/post is quite shorter than the original one on the new frame, 350mm as opposed to 400mm.I just dunno where i stand as to wether its safe or not. Any help would be much appreciated. Thanks

Seatpost length will be irrelevant if you use a shim

oh i know that, but the thing is i dunno if its safe to use.

As long as the shim meets the minimum insertion length that giant specify then the frame won’t know the difference (I run a shim on a carbon scott).

Yeah i think ill ring up local dealer in morning find out. Just wasnt sure wether you could use a shim or not at all.

My 08 XTC Composite had a 27.2mm shim

My old 2005 aluminium scott genius had a 34.9mm seat tube and had a small warning sticker no, actually it was under the lacquer specifically saying not to use a shim+narrower seatpost. I never knew if this was because at the time scott made the only carbon ‘upgrade’ seatpost in that size (others do now make them) or if there was an engineering reason for it.

On the other hand, those rivette frames used to supply a narrower seatpost and a shim to assist with internal cable routing, and with the theory that a slimmer seatpost made the ride more forgiving whilst allowing them to use larger diameter tubes at the seat tube/toptube/seatstay junction.

Whatever happened to rivette anyway? I rode one once and it was ace.

Giant have always supplied thinner posts with shims on a lot of their frames – the old NRS’s were like this.

As above – as long as the shim is longer than minimum insert for frame (buy a bit of margin, I’d say) then you’ll be fine.

Having said that – CRC and Merlin are both doing some good deals (< £40) on 30.9 Easton posts currently, I think.