Hi have a Cotic BFe frame which takes a 31.6 mm seatpost. I already have a race face 27.2 seatpost. My question is, should I buy a shim for £6 and use the 27.2 post or should I buy a new seatpost. Will a thinner post with shim make any difference?

Cheers.

Can anybody help with this?

I’m about to have the same dilemma – going to try the shim first see how it goes and then take it from there.

I use aliuminium shims on 2 of my bikes and both work perfectly.

Advantages of shims + skinnier post:

Cheaper, simple, more post flex (usually) so slightly more comfort

Disadvantages:

Let water into the seat tube when wet (Particuarly if the seattube slot is facing back), sometimes slip, need to make sure the shim is long enough to avoid frame damage.

I’d be inclined to swap the post if you’re planning to keep the bike for any length of time.

go for a new post i’ve used a shim in the past and a pain in the arse when you need to adjust the height of your post

I had the same issue on my new mmmbop and pitch. Since they share a gravity dropper on big rides and I already had a 27.2 Thomson it was the easiest option to use a shim. No problems for me.

If the seatpost is flexy and comfortable then its likely to be moreso than 31.6 posts.

Disadvantages:

You forgot PITA for moving saddle up and down or taking post out for transport etc.