I'm running a carbon post in my Al-frame beater bike (yes - I used assembly paste). I noticed Sheldon Brown advised against using carbon posts in metal frames. Is the information on his site just dated?

Many aluminum frames come standard with carbon posts. I had on on my Z85 and had absolutely no problems with it. As much as I love the late great Sheldon, I have to disagree with him on this one.

Yeah, its no different than running an aluminum post in a carbon bike which many people do. Even so, its probably a good idea to remove the post every so often to avoid any bonding.

My aluminum Tricross has a carbon post and my former 2006 Spesh Allez also had one. No problems on my end

Sheldon was the man but not sure why he said that, wish I could ask him

"Diameter is not the only factor affecting compatibility. Using a carbon-fiber seatpost in a metal frame, or vice versa, invites problems. Metal should be greased, but carbon fiber should not be. The seatpost clamps of metal frames easily damage carbon-fiber seatposts."

https://sheldonbrown.com/seatpost-sizes.html

proper torque is important of course
carbon post in alum

from a theoretical metallurgical standpoint, Sheldon Brown is correct.

But from a practical standpoint I would not worry about it. That being said, I personally believe a bike should be all or none. It isnt this magical cushion that everyone makes it out to be. and its expensive.

Just get a thompson post and check this website out


www.bustedcarbon.com

Yet another victim of the roof carrier system and garage combination and an absent-minded father.
The conversation went as follows:
Father: How was your day?
Son: Pretty rubbish actually.
Father: It’s about to get worse...
Son: Where is my bike??

Use some carbon paste on your post and you will be fine.

Sheldon is absolutely correct. Carbon and aluminum do not like each other. When they are put together, they corrode - it is kind of like rust on steel. This can result in the post becoming stuck in the frame and slowly deteriorating. And no, it does not take long for this to happen - I have seen it develop in less than 12 months.

The epidermis of carbon fiber isn't carbon, so much like squeezing (or fondling if you prefer) a silicone enhanced breast, you wouldn't be contacting any....

I think the reason for Sheldon not recommending carbon in metal frames is this was in the era when it was thought that greasing a carbon seat post was a bad idea. This has since been found to not be true. If it still concerns you, just use the carbon assembly paste. It is similar to grease but also contains a fine abrasive to increase the friction.

carbon is not metallic and will not rust nor will it cause aluminum to corrode.
There is no issue at all with the alloy clamps on shifters and carbon bars, front derailleur clamps on carbon down tubes nor alloy dropouts bonded to carbon tubes.
Carbon rims with alloy spokes, carbon seat rails with metal post clamps... list is endless and post to metal is no different.

The reason is a carbon post is less adept to withstand crushing forces and more likely to slip in a metal frame.
carbon paste and torque wrench are probably good recommendations.

I have a carbon post in my steel bike

It came that way. It looks good. I don't see any reason to change it.

I have a carbon post in my Al frame bike. I had a pretty good battle getting it out when I wanted to change my seat, but it eventually came out. It was in there close to 18 months, and was installed with carbon paste. I think the problem was water/mud getting between the two. I now loosen the clamp and move the post around when I think of it, maybe once every three months. Seems ok now, I was just fooling with it today.

I've used carbon seatposts with both steel and alu frames. Use a carbon assembly paste like the one Tacx sells - non-petroleum based and increases friction between surfaces and reduces the torque necessary to prevent slippage. Google Tacx carbon assembly paste and many vendors will be identified.

Look up galvanic corrosion (rust is a type of galvanic corrosion). That is what happens when raw carbon and raw aluminum meet.

In many of the cases you have mentioned, there is a clear coat over the carbon or the aluminum or both, so the raw carbon and raw aluminum do not interact. However, with seatposts, there often is no clear coat inside the frame, and many seatposts do not have clear coats where they insert, as the clear coat becomes scratched. Hence, there can be raw carbon touching raw aluminum, allowing for a chemical interaction resulting in galvanic corrosion. Also, sweat helps produce this corrosion.

Putting a layer of carbon paste on there provides separation between the carbon and aluminum.

Here's a nice discussion on an engineering forum about an aluminum frame mating to carbon laminate wall: https://www.eng-tips.com/viewthread.cfm?qid=186337

the new cannondale folder, cool.