Originally Posted by HandyBikesDC

This thread came up while looking for some other info and I just can't resist the urge to reply. I don't have a PhD in chemistry like some of the folks who posted seem to, but carbon paste should always be used when one of the involved components is carbon. Carbon paste is also great for metal components if you're having problems with slipping at all. If it's metal and it's not slipping, use grease (unless it is being mated with a carbon component).

@WELlbiked is right -- Chemistry is great, but anecdotal evidence is king on this one. Even when something "won't" react chemically, it can still get stuck for a variety of unforeseen reasons in the real world.

Originally Posted by RoadLight

Hi Sceadu498,

I recommend using nothing at first. Tighten the seat post clamp no more than the recommended max torque and ride your new bike a for a while and see if you have any slippage. If not, then don't use anything on it. If you do have slippage, then use a little assembly paste (also called "friction compound") near the clamping area. Do not use grease.

One of the reasons for grease when using two dissimilar metals (like an alloy post with a steel frame) is to prevent galvanic oxidation which occurs more rapidly than normal oxidation with metals of the same type. The galvanic process is a chemical reaction between two different metals which causes a small electric current to flow between them, greatly speeding the oxidation process (it literally creates a weak battery). Plus, grease is also helpful if either the post or frame seat tube are already oxidized.

But this is not a concern with carbon since it is not a metal and won't react chemically with a metal seat post. In fact, the epoxy resin matrix of your carbon frame will probably act more like an insulator than a conductor and will have no corroding effect. Plus, if your alloy seat post is new and its anodized finish is not scratched, it shouldn't oxidize for a long time. So the only thing you need to consider is slippage since the inside of your frame's carbon seat tube has a more slippery surface than metal and you have to use a lower torque on the seat post clamp with a carbon frame.

I've never had to use assembly paste with the seat posts of my carbon frames because I've never had any slippage. I started with an alloy seat post like you. Then I switched to a carbon K-Force seat post (it's amazing how much more comfortable it is because of the way it damps road vibration better then metal).

Finally, assembly paste is great stuff. It can allow you to get a quiet non-slip joint where two parts are clamped together at a lower torque which helps your parts last longer. And it's just as helpful for metal to metal joints as it is with carbon. It's use is becoming very common with alloy-to-alloy parts now. But there are two things to consider with it: First, assembly paste varies widely from brand to brand. Some brands are much more abrasive than others and will mar the finish of the parts more quickly. I use Park Tool SAC-2 which seems to strike a good balance. Second, if the parts will be adjusted frequently, I tend to try to avoid using assembly compound. The seat post falls into this category for me because I frequently fiddle with my seat height, depending on what I'm doing with my bike. And I have to temporarily raise my seat every time I clamp my bike in my maintenance stand because the seat post is usually the safest place to hold a bike with a carbon frame.

Kind regards, RoadLight