The bike won't rust, but the seat post will corrode. My carbon fiber Kestrel has a seat post which is permanently frozen; corroding aluminum can bond to carbon fiber quite tenaciously.

It happens all the time. As an automotive technician, I've had to beat on tires with a sledgehammer many times in order to get aluminum alloy wheels to break free from the steel hub. And just recently I purchased an aluminum bike with an aluminum seat post. The seat post wasn't stuck in the seat tube and came out easily, but it did have a bit of surface corrosion on it. I buffed it off with a wire brush and wiped a thin layer of grease on it before putting it back in.

That's a good idea. I'll have to remember to do that in the spring at the beginning of the riding season.

Should of got a CF seat post also.

At least once each year, on each of my bikes, I will get caught in a huge rainstorm - the sort that leads me to pull the seatpost and turn the bike upside down to drain water from the frame. That's a good time to regrease the seatpost, since it's been pulled anyway.

Most bikes that have internal cabling have some sort of hole at the bottom of the bottom bracket.

I believe one of my five bikes has that hole...Though it does not have internal cabling.

I keep a mangled seat post from my steel bike as a reminder of what a pain in the behind it is to remove a stuck seat post...and to grease every year.

Most bikes have internal cabling? No.

Read his post again.

Oops. Never mind! Too much coffee; I skipped the word "that."

a bike shop owner that knows way more than me told me to, so I do

while you're at it, put a bit of grease on any threaded part.

Let's see...

[MENTION=526701]Gdubeck[/MENTION] joins to ask his first question: "should I convert my Mongoose MTB to drop bars with 12-speed 105 components?"

and now...

[MENTION=526589]buchro[/MENTION] joins to ask his first question: "will a greased seatpost slip down and get stuck?"

Any similarities here?

Kroil and a pipe wrench aren’t going to do anything. The problem isn’t with lubrication but with the seatpost expanding. An aluminum post that oxidizes will increase in diameter and there isn’t any room. The material formed, aluminum oxide, is harder than aluminum and it’s volume is greater. Too large a peg in too small a hole and it won’t come out.

Yup. Or just galvanic corrosion.[size=14px] The metals can exchange electrons so there is a flow of electricity between the two metals. One will act as an anode and one will act as a cathode. Ya got a battery.[/size]

Yes but for different reasons. Steel in steel aren’t dissimilar and won’t galvanically corrode. There are other issues but I’ll address them in a second. Aluminum in steel or steel in aluminum will set up a electrical potential and will corrode. A layer of grease between them is enough of an insulator to stop the flow of electrons. Aluminum in aluminum won’t corrode either. Nor will titanium in titanium nor titanium in much of anything else. Titanium is fairly inert.

Carbon paste is used with carbon posts to keep them from sliding. They won’t corrode but they are slick and will slide. The carbon paste has grit in it to keep the post from sliding.

For steel in steel and aluminum in aluminum, grease keeps another kind of corrosion down. We live in a world of salt...sweat, salt used for road clearing, and seawater being the main culprits.. The chloride ion in the salt is very reactive with both aluminum and with iron. It plucks the atoms from the metal, forms a chloride, and then exchanges the chloride for oxygen forming rust (iron oxide) in the case of iron and alumina (aluminum oxide) n the case of aluminum. The oxides formed have a greater volume than the neutral metal so the seatpost effectively expands.

The chloride, by the way, is released to go back and pluck out more metal. The whole process starts over again. Chloride also helps speed up the corrosion process when two dissimilar metals are used. Aluminum will oxidize and the steel will reduce.

Except the outer electrons for aluminum and iron don’t add up to “7”. Aluminum has an electronic configuration of [Ne[color=#000000]] 3s2 3p1, and iron has an electronic configuration of [Ar] 3d6 4s2. Those don’t add up to 7.


It doesn’t “bond”. The aluminum oxide expands as noted above.

Bottom line: grease the post. It won’t hurt and it will save a lot of headache down the road.

Not by galvanic action, but some corrosion will still happen, as you mention further:

I recently bought an aluminum bike with aluminum seat post, and when I pulled the seat post out it had some surface corrosion in the form of aluminum oxide on it. I suspect this was because the former owner used the bike in triathlons, and going from dripping wet from swimming to a bike caused water to seep down the seat post into the seat tube. Luckily there wasn't much corrosion which I was able to buff off with a wire brush, and I reinserted it after spreading a thin layer of grease on it.

I would assume the same would go for a steel seatpost in a steel frame. Let some water get in,and rust starts to become a problem. Putting some grease on the seat post helps prevent aluminum from becoming corroded to aluminum, and steel from rusting to steel.

OP appears to be a seagull:

I also recommend adding a seat to the top of the seat post.

I still remember an insult Johnny Carson threw at his audience when a joke didn't go over well: "May you jump on your bike and find the seat missing."