Ok, I greased the post liberally when installing but was negligent in not removing it after particularly wet rides and now it is stuck (not a problem as I don’t raise and lower the post but somehow? it is off centre and I can’t see how as I’d surely have noticed).

I know about the destructive methods of removal but it’s a Thomson in a nice frame so I’d like to free it up without destroying/damaging frame and/or post (which may not be possible but thinking positively at present).

Would PlusGas or similar be a sensible route to take?

It’d be a good start.

I’ve only ever done it using brute force after liberal soaking in wd40.

I put on an old saddle and twist as you can get a lot of force in and it often doesn’t scratch the post too much.

Yeah, I’d suggest trying to twist it. Maybe clamp the post in a vice and try turning the frame.

I’ve also got one moving by hammering it into the frame a bit first with a piece of wood on top to save the clamp. That was enough to break the rust hold and allow me to start twisting it.

Thanks both, had already had a go at twisting with the saddle on plus fully loosening off the collar, removing the saddle clamp and the hammer/wood approach to no avail. Perhaps i’ll try a bit of oil and then ride with the seat clamp loose as my weight may help to break the seal.

Talking of which, shaky memory of second year Chemistry at school tells me that it’s oxidisation between the aluminium post and the steel frame rather than rust so guessing water isn’t the problem (although now I’m worried about what might be going on in the seattube). Not sure how good penetrating oil will be against oxidisation – oh well, off to find out!

Three other techniques: 1. remove the bb, you now have access to the seattube through the BB. With the bike upside down, spray a penetrating lube (Liquid Wrench, PB Blaster) into the seattube. 2. do the same as above, but use ammonia (which works especially well on aluminum to aluminum bonds). 3. Heat, use this with 1 or 2 above, in the form of a hair dryer or heat gun around the seat collar, careful of paint and stickers, do not heat the post (better yet stick the post stub in a bucket of ice water (the bike is upside down). If the frame is raw, you can use a soldering or welding torch (careful). Always have an extinguisher handy and do it outside, as penetrating oil is flammable and ammonia gas deadly (drain, rinse and air out the frame if it has been treated with ammonia and you are going to heat it).

In the instance of chemicals, time is your friend. I saw one post get unstuck after 3 weeks (and near daily checks) of sitting upside down with penetrating oil.

An old carbon dioxide fire extinguisher can be used on the seat post to cool it down (shrink), then put the seat post in a vice and twist and pull the frame.

Or Loctite freeze and release http://www.loctitefreezeandrelease.com/
or any of the many other similar products Wurth Rostoff Ice, CRC freeze-off http://www.youtube.com/watch?v=b0BaowQvL6c
Sol-X freeze ease etc etc

Thanks again chaps, good stuff but trying to avoid destructive methods at present.

As usual, Sheldon knows and some good stuff here http://www.sheldonbrown.com/stuck-seatposts.html It sounds like penetrating oil will be no good for an aluminium post in a steel frame but I’ve little to lose by trying. The CO2 freezing approach sounds like a possibility too.

Some innovative approaches I’ve heard mentioned by Rody from Groovy Cycles, including details of methods used by John Upcraft of Hubcap Cycles

http://hubcapcycles.blogspot.com/2009/10/metal-lathe-not-just-for-round-parts.html

http://groovycycleworks.blogspot.com/2009/01/friday-was-repair-day.html

http://hubcapcycles.blogspot.com/2009/11/dumpster-fat.html

However, as Sheldon points out, prevention is so much easier than cure 🙁

Is it the wrong sized seatpost for that particular frame?

If you want to heat the frame up easily without damage, use boiling hot water.

If you get bored trying to get it out and finally write off the seat post:
Caustic Soda will do the job

Took a couple of days outside during the winter past with the incredibly silly cold temperatures. I reckon one night would do it in the current weather spell

Is it the wrong sized seatpost for that particular frame?

Nope, size is definitely right, it’s more a slack maintenance routine that’s caused the problem. What I can’t figure out was that it was straight, I had a substantial crash in June 2011 and now it’s off centre – surely it can’t have got stuck that quick? I suspect the answer is that it was offcentre for some time and I didn’t notice (note to all – don’t be ashamed of staring between your pumping thighs as you ride as the result if you don’t may be frustrating).

If you want to heat the frame up easily without damage, use boiling hot water.

I don’t think that’ll work as aluminium expands more than steel but the opposite freezing method may work.

Thanks again for the thoughts – I’ve left it soaking in penetrating oil overnight and will ride tomorrow with the collar undone and see if that yields anything.

The steel will heat up before the aluminium as ithe steel seat tube is wraped around the seat post ( if you see what I mean) it has worked for me before. Just a thought.

Nuke it from orbit…

…to make sure.

Dynamite… might .. or might not.

http://www.screwfix.com/p/crack-it-freeze-release-spray-400ml/98823
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_274625_langId_-1_categoryId_165594
http://www.corgi-direct.com/crack-it-release-spray/-/ph15
Ambersil Rust Flash
etc
etc

When aluminium corrodes it can expand with enough force to damage whatever is surrounding it:
http://pardo.net/bike/pic/fail-001/FAIL-102.html

which is why when an aluminium seatpost corrodes into a frame, a fair bit of force or application of physics/engineering is required.

If you can get some liquid nitrogen. Then hold the frame upside down and put the seatpost in a container of liquid nitrogen (a pair of stout gloves might be a good idea). Take the seatpost out of the N2 and put it in a vice and twist, turn and pull and the seatpost should come out.

Tony Oliver has an idea in his book Touring Bikes. Put the frame / seatpost in a deep freeze for a few hours. Get some towels ready, that need to be soaked in some warm to hotish water. Take the frame out of the deep freeze put the seatpost in a vice, wrap the hot towels around the frame seat tube and the heat differential … might be enough to …. but I have never tried it.

The liquid nitrogen method is the easiest, if you can get some N2.

http://www.singletrackworld.com/forum/topic/steel-frames-and-wet-weather

Dont twist too hard though:
http://pardo.net/bike/pic/fail-001/FAIL-131.html

I had a good result with using diesel.

Cut it out.

This was the result of 8, yes EIGHT, hours cutting to remove an aluminium post from a carbon frame. Hacksaw blades used; 6…

SUCCESS! Well, mostly….

After loosening off the collar yesterday and gently prising apart the ‘ears’ at the top of the seattube I applied some spray oil (“with penetrative fizzing action” – yeah, right. Actually, it did fizz quite well) and left it overnight. I tried twisting the post this morning with no luck so went for a ride. Dismounting at the top of some wooden steps I gave the nose of the saddle a sharp backhand blow and suddenly it was almost straight! Further blows and attempts at twisting did nothing but from at least an inch out of straight it’s now barely perceptible. Certainly fits in with initial thoughts that it was moved by the crash back in June.

So, although still stuck pretty solidly it’s at least straightish and there is hope. Not sure whether the percussive blow, the fizzy oil or 15 stone of biffer did the job but likely a bit of each.

Thanks to all for suggestions and I guess the moral(s) of the story are a) regularly grease your post; b) try non destructive methods first as they may just work!