Sceadu498

Putting together a new carbon frameset and will be using a aluminum seat post. Should I use a carbon assembly paste or grease when installing the seat post?

Thanks.

HillRider

Yes, you want something to protect the post and frame from corroding together inseparably and carbon assembly paste will do both that and prevent slipping. Grease is normally used on metal frames with aluminum seatposts but is likely to let the post slip in a carbon frame.

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

Andrew R Stewart 

Carbon fiber is electrically conductive. It can and I have seen it react with TI, AL and steel. This is why when building a frame of carbon fiber with metal inserts an insulating layer must be used (like fiberglass tow) or the carbon will react with the insert. Andy.

HillRider

Absolutely. There are many, many cases of aluminum components corroding in carbon frames and carbon is indeed a "metal" and will react with dissimilar metals. The epoxy binder is not a 100% insulator and relying on it to keep an aluminum seatpost from corroding in a carbon frame is a sure source of future problems.

RoadLight

Hi Andy,

I think you are confusing things here. I never said carbon fiber was not conductive. I said the epoxy resin in not conductive.

Carbon is conductive and galvanic oxidation is a huge problem when other metals come in contact with it. Makers of carbon composite frames must take great care that the various metal inserts (dropouts, water bottle cage bosses, front derailleur mounting brackets, etc) do not come into contact with the carbon fibers. This is especially true where milling and or drilling is done through the composite after the molding is complete. These are the only places where carbon fibers are exposed (unless the frame has been damaged). And that's the difference: These parts (or their screws or rivets) usually penetrate the epoxy resin and could contact the carbon fibers if proper measures are not taken. We are in 100% agreement about these facts.

But this is not true for the seat post. The portion of the frame's seat tube where the seat post resides has not been drilled or milled. There should be no exposed carbon fibers at the interior surface of the seat tube where the seat post is located. If there are, then the composite frame was poorly manufactured and the frame should never have passed quality control. Or the frame has been damaged. The seat post should not come into contact with any carbon. All that it will contact is epoxy resin.

It's my understanding that aluminum alloy is the worst metal with regard to galvanic oxidation with carbon (the problem exists with steel and titanium, also, but the process is slower). Yet, I've seen composite frames with aluminum seat posts that have no treatment whatsoever that show zero galvanic oxidation after a decade of use. I personally have one carbon frame that had an alloy seat post from 2008 to 2016 with zero problem and the seat post had no assembly paste, grease, etc.

After reading your and HillRider's post, I measured the impedance of the epoxy resin inside one of my carbon composite tubes. It was infinity---it showed no conductivity even on a megaohm scale. There is no doubt whatsoever that epoxy resin is an insulator. Since this is all the seat post should contact, there should be no risk of galvanic oxidation from the carbon fibers embedded inside it.

If you have ever seen galvanic oxidation of a seat post in a carbon composite seat tube, then you have seen a defective or damaged frame. I have never seen this happen.

Kind regards, RoadLight

RoadLight

Untrue. Epoxy resin is an insulator. Seat posts do not contact carbon fibers and do not undergo galvanic oxidation in carbon composite frames unless the frame is defective or damaged.

deacon mark

I had a carbon fiber frame with an AL seat post. I had huge trouble getting it out after a few years forget the science and use grease or CF paste.

HillRider

Tell that to deacon mark and many, many others who have had corrosion caused binding with carbon frames and/or seatposts. There must be a lot of defective or damaged frames and seatposts out there.

Edit to add other carbon/aluminum corrosion data. Years ago there were several carbon tubed/aluminum lugged bonded frames and the early ones had a high joint failure rate due to (wait for it) corrosion at the carbon/aluminum interface. Trek solved that problem by adding a wrap of fiberglass cloth to the tubes at the joints to act as a true insulator.

If you are still so convince it can't happen, feel free to experiment on your own bike.

dksix

What about CF seat post inside a CF frame? I'm having no slipping, even at the minimum torque. Should I still use some paste?

HillRider

That is a situation where there is truly no "dissimilar metal" contact so, if slipping isn't a problem, you don't need anything. However, some frames have a bonded in aluminum sleeve in the seattube where the seatpost is installed. If that's the case, the paste would be recommended.

RoadLight

Who manufactured your carbon composite frame and what was its model and year?

Kopsis

Galvanic corrosion needs three ingredients - anode, cathode, and electrolyte. In drier climates where sweat doesn't run down the seatpost, corrosion is much less common. Also note that aluminum oxide forms a reasonably effective barrier (more so than a painted seatpost) so if it doesn't get scratched on insertion, your risk is greatly reduced.

This test is not meaningful. To prove it, I just did the same test on a frame that had a seriously corroded alloy seatpost and got the same result. At a macroscopic level, carbon has a high electrical impedance ... that's why it's used to make electrical resistors. Galvanic corrosion happens on the microscopic, not macroscopic scale.

I have. Multiple times. In high end, undamaged frames. One of the "joys" of riding in a place where temperature and humidity race each other to reach the 90s and you can smell the salt spray in the air.

HillRider

Years ago there were several carbon tubed/aluminum lugged bonded frames and the early ones had a high joint failure rate due to (wait for it) corrosion at the carbon/aluminum interface despite the presence of epoxy as both the tube binder and joint adhesive. Trek later solved this problem by using a wrap of fiberglass cloth at the joints to act as an effective insulator.

If you don't accept this then feel free to leave the carbon/aluminum interfaces on your own bike dry and see what happens.

Andrew R Stewart 

"I think you are confusing things here. I never said carbon fiber was not conductive. I said the epoxy resin in not conductive." Post #6 RoadLight
"But this is not a concern with carbon since it is not a metal and won't react chemically with a metal seat post" Post #3 RoadLight

I only reply to what I read. The above two statements are at odds with each other. Were you not referring to carbon fiber with your post #3 use of the word "carbon"? If not then you have been confusing to your readers by not being more specific.

I also disagree with your post #6 "The portion of the frame's seat tube where the seat post resides has not been drilled or milled. There should be no exposed carbon fibers at the interior surface of the seat tube where the seat post is located" This flies in the face of many examples of after frame building finishing work that I, and it seems many others, have seen. To ream, grind, hone a carbon fiber frame for a proper seat post fit is not a sign of a defective frame or wrong manufacturing. To install a post is said frame without regard to galvanic reactions and to not periodically maintain said post fit in the frame IS WRONG.

The bottom line is what I just said. Proper insulating compounds with periodic maintenance is the true path to a long lasting and happy post/frame fit. Always has been and likely will always be so. Andy.

RoadLight

We're not talking about those joints. I agree fully on the above point. The OP was asking about one specific place---the portion of the carbon composite seat tube where his alloy seat post will reside. There should be no exposure to carbon fibers here---only exposure to the epoxy resin.

I have for over 8 years without a single problem. I've seen many other composite bikes with alloy seat posts that are "dry" that have exhibited no problem. I'm doing exactly what the manufacturers of the carbon composite frames that I've seen have done.

RoadLight

Respectfully, that sounds like poor measurement technique. To determine the conductivity (or the reciprocal, resistance) of a material, you don't measure its oxidized surface. You penetrate to bare material and measure. As for macroscopic vs microscopic---it's all microscopic AND macroscopic. It's simply a matter of how closely you view the phenomenon.

And it never occurred to you that the oxidation was due to these environmental factors (moisture, temperature, salts)? Why do you conclude it was an interaction between the alloy and the carbon fibers sealed inside an epoxy resin? It seems like you're avoiding the simple answer in favor of a more complicated one. I've seen aluminum alloys oxidize very rapidly due to the environmental factors you described and carbon composites where not present at all.

RoadLight

That's silly---parsing my words. Most folks refer to carbon composite frames as "carbon" frames. That's all I did above. I never meant to say that pure carbon is not a metal or is non-conductive. But carbon fibers are not exposed in the seat post area unless something is wrong. Rather, the seat post should be exposed only to epoxy resin. Surely, my point was clear in my post.

So why do so many high-quality carbon composite bike manufacturers like Specialized fit alloy seat posts to their carbon composite seat tubes dry? It sounds like they need your help very badly. Perhaps there would be a nice consulting fee for you if you present your case to them.

deacon mark

I am reasonable wrench and all the discuss really does not matter it is what happens in the end. My bike was a CF bike with no problems in the frame or seat post. The seat post was AL and once I got it set up I did not use anything and no slipping or course. Then maybe 2 years later I bought a CF seat post to use and new saddle. I had a heck of time getting the seat post out. I finally manage to get it to at least move by using vice-grips and turning the whole bike. I really do not care all about the science of metal and what can or cannot happen. A little grease or carbon paste will make things much better. Since using that I then take and loosen the seat about every 6 months just to make sure and it moves just fine and clamps back up. If your bike has nothing on the seat post and you have no problems then continue your practice. I know I am going to keep using something on my seat post.

HillRider

Maybe they expect the selling dealer to apply the paste when the bike is assembled and sized to the customer.

well biked

FWIW, when a bike is shipped to a bike shop from a manufacturer, the seatpost is not installed. As part of the assembly process, the bike shop, not the manufacturer, applies any grease, assembly paste, etc to the seatpost/seat tube. This fact alone doesn't prove or disprove anything, but I will note, for example, that when a steel framed bike is shipped, and it's to be equipped with an aluminum seatpost, the seatpost and seat tube are dry when it arrives at the bike shop in a box. Again, adding grease (in the case of the steel frame example) is part of the assembly process.

Kopsis

The difference between aluminum oxidation resulting from surface exposure to the environment and galvanic corrosion is pretty easy to recognize. I'm surprised you would even suggest this since I'm sure you're familiar with passivation. The cases I'm talking about are not subtle "surface" erosion, they are deep erosion in-your-face obvious examples of galvanic corrosion that will only happen in such short time scales when the CF frame is working as a large cathode.

However, we're not likely to reach agreement on this because your definition of a "quality undamaged" frame is one where not a single carbon fiber has it's surface exposed. Given that definition, you are correct and owners of such frames need not be concerned with galvanic alloy seatpost corrosion.

I submit that few (if any) frames actually meet that criteria due to the way they're manufactured and subsequent normal handling. Just the friction from inserting a seatpost is enough to abrade the micron thick layer of epoxy covering the fibers at the tube's inner diameter. If you want to call that "damage" then then nearly all CF frame owners have "damaged" frames and need to take appropriate precautions.

Given that there is no harm in using carbon assembly paste when installing a seatpost, and that it offers protection against galvanic corrosion if one has a frame that doesn't meet the "quality = absolute microscopic perfection" criteria, I'm going to continue to hedge my bets and I recommend others do the same. I've seen what happens when this practice isn't followed and it makes Al seatpost galvanic corrosion in a steel frame look like tame by comparison.

CliffordK

The price on carbon fiber seatposts isn't too bad. I'd seriously consider tracking one down, even used.

Squeezebox

Anyone want to say anything about an AL seatpost in an AL frame? I'm sure the alloys are different.

well biked

I would certainly advise a layer of grease with an aluminum frame/aluminum seatpost. Not gonna hurt anything, might prevent a nightmare situation later. There are a lot of variables with corrosion/potential corrosion. I work in a bike shop, I've seen situations with corrosion that are horrible, and sometimes even hard to comprehend how it happened. What I do comprehend is that it DOES sometimes happen, and an ounce of prevention in the form of grease or assembly paste (in the case of carbon) makes good sense.