icemilkcoffee 

I have a 25.4mm carbon seatpost together with an adapter shim in my road bike which takes a 26.8mm seatpost. It would gradually slip down over time. I am tightening it to 'scary tight' and I am using carbon paste, and it still slips. What gives? Is this how 25.4mm seatposts are ? Or are adapter shims prone to slipping somehow?

masi61

Adapter shims introduce a whole other level of imprecision to fitting a seatpost.
Is your shim for a 25.4 to 26.8 specifically? Is it aluminum or Delrin? When it comes to shimming a carbon post, you’ll have better success with a shim that is as long as possible and that fits somewhat snug as you go to set the post to your desired seat height.
I put a 27.2 carbon post in my 30.0 ti frame with the correct size sleeve in aluminum and it made an annoying “tick” sound and then my Hope seat collar bolt broke twice when I overtorqued it. It wasn’t until I went with a U.S.E. branded Delrin sleeve that was a bit thicker than I thought I needed. I pushed it in with copper antiseize and found a stronger bolt for my Hope seat collar and it never ever slipped or made any ticking noises again.

icemilkcoffee 

I use Cane Creek aluminum seatpost shims. U.S.E. doesn't seem to make a delrin shim for 25.4 seatposts unfortunately.

Troul 

wrap the post with one layer of carbon fabric laced with carbon paste.

Toadmeister

Ive had the same problems with 27.2mm seat posts, no shim. Unfortunately the problem is my 270# body.....

Phil_gretz

Why would you do this? Get a simple 26.8 mm aluminum seatpost and be done with it. What advantage do you believe your existing carbon seatpost provides, except that it's paid for?

_ForceD_

Fiber Grip paste is specifically for this issue.
Usually available at your LBS.

https://www.amazon.com/Finish-Line-C.../dp/B018FT9HOW

Dan

sarhog

The OP clearly stated in the first post that he is using carbon paste.

icemilkcoffee 

Carbon seatpost = better ride than aluminum seat post. I use carbon seatposts and carbon handlebars on both my bikes.

icemilkcoffee 

How do you get that to fit in the hole though? Wouldn't the carbon fabric just bunch up when you try to push it in?

Troul 

Just one layer leaving a slight gap from where it wraps around the post.

friday1970

I had the same issue. Carbon seatpost, shim'd to a larger seat tube. Kept slipping. I didn't want to overtorque it and crack the tube. I tried carbon fiber grease. This helped a little. But Michigan roads, you know, a big bump and down went the seat.
I just gave up and bought a larger seat diameter seat post from Bezos's shop. Haven't had a problem since.

Toadmeister

Appropriate GIF:

Pratt

Numerous good, maybe, great, suggestions.
But, how about beer?
Take a can of beer, empty it, and cut shims to fit out of the can.
If one can doesn't do it, add another.
Of course, some thought should be given to match the beer to the bike. Some trial and error may be required.

Troul 

what if grolsch is the only matching beverage?

Pratt

If Grolsch is the only match, that's great! Because he would have to go to Europe to get a can, or two. I'd call that "essential travel."

Melvang

But a 26.8mm seatpost?? Carbon slipping under load like that is really hard on them

Troul 

I was only aware it comes in a glass bottle.

indyfabz

Camilo

This. I have a 27.2 CF post and a 27.2 seat tube and I couldn't get it to stay put with CF assembly paste and proper torque. It would very slowly slip, just enough so I'd notice it after a ~ 1 hour ride. The post had worked in other 27.2 frames I'd used it with. I wasn't willing to tighten past the torque spec. So I cut a shim out of a beer can, put CF paste inside and outside the shim and now it stays put. Since both the seatpost and the frame are American, I used a local, American microbrew can for the fix. Or maybe a seltzer can since we seem to have a lot of those around too. I couldn't have made a shim out of the glass bottle Urquell even if it was a Czech bike.

Troul 

if the seat post has an opening at the top in the center, apply carbon paste to an expansion plug & insert it in the seat post section that is overlapped by the frame seat tube. Once the seat height is set, increase the expansion plug's tension.

rm -rf

Is the shim itself slipping, or does it stay in place and the post slides down?
On my titanium bike, I had to use some 320 grit sandpaper on the inside of the frame, it was machined way too smooth for the aluminum seatpost. No problems since.

Try scuffing the shim with some 220 or 320 sandpaper, then reapply the carbon assembly paste. The paste has tiny plastic grit in it for grip, but perhaps the shim is too smooth.

( And carbon assembly grit works great on aluminum to aluminum, like handlebars to stems. It allows a lot less torque on the bolts.)