Description of the situation
You have a carbon seat post and an aluminum frame. You’re wondering whether you should treat the seat post somehow before installation.
Condensed answer: Before installing a carbon seat post in an aluminum frame, it’s recommended to apply a thin film of carbon assembly paste on the seat post and inside the seat tube to facilitate installation, adjustment, and removal. Otherwise, the carbon seat post may get stuck in the frame due to corrosion of the aluminum seat tube.
Seized Carbon Seat Post In an Aluminum Frame
Contrary to popular opinion, a carbon seat can get stuck in an aluminum frame.
Unlike steel, aluminum doesn’t rust. However, when exposed to the elements, aluminum experiences oxidation which results in the formation of a thin top layer known as aluminum oxide.
The layer shields the underlying aluminum and protects it from further corrosion. If the protective oxide film is damaged, it regenerates quickly in the presence of oxygen. Hence why aluminum is known for its ability to resist deep corrosion better than iron.
However, the corrosion reduces the seat tube’s diameter and “traps” the seat post. Once stuck, the carbon seat post becomes very difficult to remove.
Aluminum alloy is highly prone to galvanic corrosion when coupled with carbon composite.
Galvanic corrosion is a deterioration process occurring when two dissimilar metals are in contact and in the presence of a conductor like water.
Carbon composite isn’t a metal, but it’s highly conductive and leads to galvanic corrosion too.
When the above conditions are met, a transfer of electrons begins from one of the materials to the other.
In this particular situation, the aluminum frame is acting as an anode giving up electrons whereas the carbon seat post is a cathode receiving electrons.
After the occurrence of galvanic corrosion, the seat post becomes permanently stuck into the frame.
For that reason, it’s of utmost importance to prevent the exchange of electrons between the aluminum frame and the carbon seat post.
The two most effective ways to prevent galvanic corrosion are:
1. Electrically isolating the two parts to stop the flow of electrons between them.
2. Limiting the moisture between the elements.
If you wrap the seat post in a material that doesn’t conduct electricity, galvanic corrosion will not occur.
This solution is not the most convenient because the seat post’s thickness will increase and prevent it from fitting inside the frame.
Also, some isolating materials could make the seat post extra slippery. As a result, the seat post will keep sinking into the frame.
Hence why the most common method to prevent galvanic corrosion is to apply some lubricating paste on the seat post.
What Substance To Use For Lubrication
Manufacturers of carbon seat posts recommend applying a thin layer of carbon assembly paste before installation.
The paste has two functions:
- Repel moisture
- Increase friction between the seat post and the seat tube
The extra friction reduces the clamping force needed to hold the seat post in place. As a consequence, the rider doesn’t have to tighten the clamp as much. This is beneficial because carbon has poor resistance to compression and can be easily “crushed”.
Regular Servicing Is The Best Policy
A carbon seat post may get stuck in an aluminum frame even when carbon paste is applied. The chances are low, but the outcome is not unheard of when the seat post isn’t removed over a long period.
To minimize the chances of a seized seat post even further, it’s advised to gently clean the seat post and re-lubricate it at least a few times a year.
What About Grease?
It’s not recommended to apply grease on a carbon seat post for two reasons:
- Grease can soften the outer coat and cause swelling of the composite laminate. As a result, the carbon seat post expands and seizes into the seat tube.
- Grease lowers friction and increases the torque needed to hold the seat post in place. To compensate, the seat collar has to be tightened to a greater torque setting which amplifies the chance of damaging the carbon component.
Below are quotes from manufacturers in regards to using grease on carbon:
No grease on carbon posts. Grease contains certain minerals that can attack clear coats, can penetrate the resin matrix and could cause swelling of the composite laminate. Can you say “stuck seat post?” Don’t use grease.
John G. Harrington
Vice president, bicycle products
Easton Sports, Inc.
No grease. In some cases it can be dangerous to use grease as the chemical composition can cause a reaction between materials. Besides, it increases the torque required to clamp the post.
Removing a Carbon Seat Post Stuck In an Aluminum Frame
The commonly accepted method to remove a seized carbon seat post from an aluminum frame is to heat the seat tube with a heat gun. Once heated, the seat tube expands and allows the removal of the seat post without extreme force.
If a heat gun isn’t available, one can also use a hairdryer or pour hot water on the seat tube.
Note: Heat methods damage the paint because it’s not resistant to high temperatures.
Ultimately, the best approach is to be proactive and prevent this situation in the first place by lubricating the seat post routinely.
If Nothing Works You Will Have To Salvage The Seat Post
If the bond between the seat post and the frame is too strong and heat doesn’t seem to help at all, the only solution is to destroy the seat post to save the frame.
The common method is as follows:
Cut the seat post across its length at multiple locations and pry it out piece by piece with a flat head screwdriver or another tool with similar characteristics. Once the seat post is out, gently sand the inner walls of the seat tube.
You Will Need a Torque Wrench
To avoid over-tightening and damaging the seat post, it’s recommended to use a torque wrench and tighten the seat post to the recommended torque setting provided by the manufacturer. If you go beyond, your risk harming the seat post and losing its warranty.
FAQ: What Will Happen If I Don’t Apply Any Compound?
Some people seem to experience no problems if they run the seat post dry for a while.
In the long term, however, the seat post is very likely to seize if you don’t remove and clean it regularly.
What You Need To Know
- You can install a carbon seat post in an aluminum frame as long as the post’s size is correct
2. Aluminum does not rust, but it is susceptible to corrosion. When exposed to moisture over a long period of time, aluminum forms a protective film. This reaction results in a stuck seat post because the diameter of the seat tube decreases.
3. When carbon and aluminum touch each other without being electrically isolated, galvanic corrosion (exchange of electrons) takes place. As a consequence, the two materials bond to each other making it close to impossible to remove the seat post out without damaging the components.
4. To prevent a carbon post from getting stuck into an alloy frame, apply carbon assembly paste on the seat post and remove it for cleaning and re-lubrication at least a few times a year.
5. It’s not recommended to use basic grease on carbon because the material is sensitive to it and can swell.