Prevention is better than a hernia - stopping an aluminium seatpost sticking in a steel frame

KneesUp

Guru
What advice would you give to prevent an aluminium seat post sticking in a steel frame?

I've noticed mine becoming stiff before, so I greased it. This has pretty much gone now, but I regularly (as in three times a week) put the saddle down to fit the bike in the car (and then put it up again, obviously) - I had assumed that this would keep it free-moving but apparently not - I've just had a real struggle getting it back up again. I did manage to do it but it involved a lot of twisting and pulling with one foot holding the bike down. I've taken it out completely for now. (It occurs to me that this could be because the bike warmed up a bit in the car and the aluminium expanded faster, but the temperature range hasn't been huge today)

Is greasing it the only option? Because I move my up and down so often I ended up with a seat post that was covered in grease (and then muck stuck to grease) last time, which is why I didn't repeat it. I have some WD40 at work so I'll give it a squirt of that before I put it back in - but any other tips would be much appreciated.
 

Smurfy

Naturist Smurf
Are you sure the seat post is the correct diameter, and the seat tube has been correctly reamed? I mainly ride steel bikes, and I've never had this much trouble.
 

tournut

Active Member
Location
altrincham
What advice would you give to prevent an aluminium seat post sticking in a steel frame?

I've noticed mine becoming stiff before, so I greased it. This has pretty much gone now, but I regularly (as in three times a week) put the saddle down to fit the bike in the car (and then put it up again, obviously) - I had assumed that this would keep it free-moving but apparently not - I've just had a real struggle getting it back up again. I did manage to do it but it involved a lot of twisting and pulling with one foot holding the bike down. I've taken it out completely for now. (It occurs to me that this could be because the bike warmed up a bit in the car and the aluminium expanded faster, but the temperature range hasn't been huge today)

Is greasing it the only option? Because I move my up and down so often I ended up with a seat post that was covered in grease (and then muck stuck to grease) last time, which is why I didn't repeat it. I have some WD40 at work so I'll give it a squirt of that before I put it back in - but any other tips would be much appreciated.
Do with out the car. Dont mean that iv had same prob.
 
OP
KneesUp

KneesUp

Guru
Are you sure the seat post is the correct diameter, and the seat tube has been correctly reamed? I mainly ride steel bikes, and I've never had this much trouble.
Er, it didn't occur to me it might be the wring size! It's the one that came with it (secondhand) - I know you can't read the markings on it anymore.
 

screenman

Legendary Member
Aluminium warms up a lot slower than steel, which is why we use it as a heat sink in some cases. Get your hands on some copper slip, that will stay around, WD40 will not.
 

Smurfy

Naturist Smurf
Er, it didn't occur to me it might be the wring size! It's the one that came with it (secondhand) - I know you can't read the markings on it anymore.
I'm on my fourth steel frame, and not one of them gave me trouble with alumnimum seat posts. However, if the size is wrong by a fraction of a millimetre, it will always be a struggle. The post should slide easily, but be rock solid with minimal tightening of the clamp. I don't even have grease on mine and it's never any trouble.
 
OP
KneesUp

KneesUp

Guru
I have used WD-40 for now as it's all I have at work (that said, there is some Vaseline in the kitchen, so I may take up @raleighnut 's suggestion)

The WD-40 has created a load of grey slime - I presume it's a mixture of old grease, dirt, aluminium oxide and aluminium which has worn off the post as I've been putting it up and down?

Anyway, I've cleaned as much as that out as I could reach with the limited tools I have here, and it does seem to be a little easier. I noticed too that the 'slot' in the tube looked a little closed even with the QR removed - perhaps at some point someone has overtightened it? I'll see if I can tweak it apart with a long screwdriver or something when I get home.
 
Location
Loch side.
Aluminium seatposts regularly get stuck in all sorts of frames, especially if they are greased.
The mechanism by which this happens is galvanic corrosion. that produces hard aluminum salts which you can see as a white substance with the consistency of salt. The oxide molecule is many times larger than the parent aluminium molecule and thus swells the seatpost to a point where it is stuck. It is very, very common, even with carbon frames or aluminium frames with carbon posts.

A seatpost has a co-called dynamic joint holding it in place. In other words, just the top end at the frame is secured, the insides can and do wiggle around inside the oversized seat tube. This happens as you ride. Water enters by dripping down the seatpost and moving via capillary action (and even gravity once it is past the clamp). There the water gets massaged into the grease to form an emulsion. Emulsions are great at protecting water from evaporation and thus holds the water in place so that it can react with the aluminium.

By not greasing the post you deny the water an emulsifier and it dries out relatively quicker than when there is grease present. However, it will still get stuck and should therefore be removed periodically. Once it is really stuck, it requires hacksaw surgery to remove it, no chemical can undo the damage in there.
 
OP
KneesUp

KneesUp

Guru
By not greasing the post you deny the water an emulsifier and it dries out relatively quicker than when there is grease present. However, it will still get stuck and should therefore be removed periodically. Once it is really stuck, it requires hacksaw surgery to remove it, no chemical can undo the damage in there.
So I need to get it to fit without being greased? So a good clean with a bottle brush and see if I can open up the slot a bit.

(the chemical solution is caustic soda if it's a steel frame - not so great with an aluminium one ...)
 

raleighnut

Legendary Member
Location
On 3 Wheels
Grease contains Soap in its formulation, Vaseline does not, it therefore stops water getting at the aluminium (acting as a barrier)
Edit - I'd still take it out at least once a year and clean it, reapply a smear of Vaseline and replace it
 
Location
Loch side.
So I need to get it to fit without being greased? So a good clean with a bottle brush and see if I can open up the slot a bit.

(the chemical solution is caustic soda if it's a steel frame - not so great with an aluminium one ...)
Yup and no to the solution.

Whilst caustic soda readily interacts with aluminium oxides and easily removes even the toughest, ceramic-like anodizing, it is not a solution to a stuck seatpost. The reason is that you can't get it in there. The oxide creates such a tight seal that you won't get it in there and even if you do, it attacks just the first layer of the oxide and then neutralizes. Picture it as attacking a tube of oxide, not by dunking it but by removing it molecule by molecule from head-on. The procedure would thus be: get some caustic soda in there, wait for it to react, flush it out, dry it, repeat. Repeat until you've eaten through ten zillion one-molecule thick rings.

The hacksaw is the solution.
 
Location
Loch side.
Grease contains Soap in its formulation, Vaseline does not, it therefore stops water getting at the aluminium (acting as a barrier)
Edit - I'd still take it out at least once a year and clean it, reapply a smear of Vaseline and replace it
The soap of course, is not Dove but alkaline salts from calcium, lithium or whatever. It isn't the grease but it's ability to emulsify that's the problem. I don't know if Vaseline emulsifies under those conditions, does it?
 

Drago

Flouncing Nobber
Location
Poshshire
Moly anti seize paste, clean and re apply whenever the bike gets a good going over. Vaseline and grease work fine, but may as well use the proper shizzle.
 
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