34.9mm seatpost solutions

[vincenz]

My current saddle post is a 34.9mm diameter alloy item, I have been looking at moving to a carbon seat post but I have found very little in 34.9mm diameter, a friend suggested a shim to take it down to a 31.6mm diameter however when I went to purchase one the guy in the shop said he wouldn't recomend it on a carbon frame.
Was he talking balls or are there genuine reasons not to have a carbon-alloy-carbon interface between frame, shim and post?
Thanks in advance

[neil_bolton]

Re: Carbon and Aluminium - IIRC there is a chemical reaction that is caused by carbon on aluminium - however I never really saw any problems in my time.

34.9 is huge though, but have you tried Bontrager - or even USE - as they use shims on their posts (IIRC)..?

[vincenz]

I'll have a look thanks Neil!
If i'm being honest i've just bought a Ritchey stem and handlebars and they do a matching seatpost but its over twice the price in 34.9 as opposed to the smaller diamter hence the interest in a shim.

The guy in the shop (Evans cycles) seemed to suggest that frames with a 34.9mm bore have this for a reason and that using a smaller diameter causes excess fatigue (in theory he is right) but I would have thought the seat post would have gone first as the shim is making up the diameter loss to the frame.

The more I think about it the more balls it sounds.

[BOR]

Vincenz,

I have a 34,9 carbon seat post from Syntace. Don't know price or how easy to get. Seems OK.

Syntace P6 Carbon.