NEW! More ways: -- carbon-fiber frames and seatposts; caustic soda
To update an old saying, 28 grams of prevention are worth 454 grams of cure. Stuck seatposts are always the result of improper installation. If you do it right, you will never have a seatpost get stuck after you install it.
Before you get serious about a stuck seatpost, you should find why it is stuck. There are two possible reasons: The seatpost may be stuck mechanically, for instance by being the wrong size and having been forced in; or, the problem may be chemical, caused by corrosion.
It is easiest to work with a fully assembled bike, sitting on the floor on its own tires. When pulling on the seatpost, the best way to hold the bike down is to put a foot on a pedal.
[Again the moving parts of a suspension seatpost are definitely not as strong as a single steel tube or forging. You will have to clamp a suspension seatpost below the suspension or you will very likely break the mechanism -- John Allen].
You can get the best purchase by standing behind the saddle, with your legs straddling the rear wheel. It may be helpful to have an assistant hold the front end of the bike steady. Even if you don't use an assistant, the front wheel should be restrained from turning from side-to-side. This may be done with a commercial handlebar clamping tool, or by strapping the wheel against the downtube with a toe strap.
Unfortunately, many seatposts will not lend themselves to this approach, because the saddle clamp may be pressed or bonded to the tubular part of the seatpost, and the joint isn't designed to withstand such force.
Or if you have access to a drill press with large drill bits and a way to align the bicycle frame precisely, you could drill out the seatpost to a thin shell and then crumple it up, pushing it away from the inside of the seat tube with a flat-blade screwdriver
I bought recently a used Kuota Kalibur carbon frame with carbon post. The post got stuck in the frame while being shipped by Fedex. Both the post and seat tube are not round but with a drop shape profile so you cannot even turn it. I tried to lubricate the post with soapy water and with oil. [Bad idea: oil expands the resin binder of the composite material and can deteriorate it.] I installed a piece of steel rod in the seat rail clamp, inverted the bike, stood over the inverted bike, stepped on the bar with both feet and pulled on the frame with all the force I had. Nothing happened. Tried to warm up the frame by friction (rubbing with clean soft cloth). Wiggled the post. Not even a sign of improvement. Then I have studied all the posts on the web in this matter. Tried the same exercises again, with zero results, except some pain in my back. Then I confronted the situation, that the only option left would be to cut out the post with a long straight blade which is absolutely crazy and risky on the carbon frame.
Then I stepped back to my engineering thinking (for last 20 years I design and build special prototype machinery). First I checked on the heating / cooling options and found that the thermal expansion of carbon epoxy composites is only 0.000001/F which makes it one of the materials with absolutely best thermal stability - forget about heating or cooling carbon - it will not help and may only damage the frame.
Then I started to analyze the physics of pulling one composite tube out of another. I was sure it was not glued together, as the frame is old enough to be perfectly cured and the tube was there only for a few weeks. It was only held there by friction. It was not inserted with a lot of force so it must have created some mechanism that locks the pieces together. My further conclusion was that if you pull on the frame trying to pull out the post you are causing the seat tube to elongate, which in turn reduces its diameter and causes even tighter clamping on the post. The more force you apply pulling on the seat tube, the It is exactly what happens when you put one piece of plastic tubing in another piece of tubing and then try to pull it out [like a Chinese finger puzzle -- John Allen].
The best way to do it is not pulling on the outer tubing, but pushing the big tubing off the small tubing, just pressing on the edge of the outer tube while pulling on the end of the inner tube. Now I knew I would be able to do it. I only needed some way to apply reasonably large force just on the top edge of the seat tube. I took two aluminum plates that just fitted nicely between the seat rail clamp plate and the top of the seat tube (one plate on one side of the seat post and the other on the opposite), with the seat rail clamp screws extended by a few turns. Then I gently turned the rail clamp screws in and the seat post just came out without any struggle :) :) :) I did not apply any more force then, just slight finger pressure on the Allen key while turning the screws. I am sure that a similar method may be used on metal frames and seat posts or any combination of the material. You just need the right length of spacer blocks or some kind of screw attachment to do the pushing off the seat tube from the seat post.
DISCLAIMER - don't attempt if you are unsure about anything and always use thick rubber gloves, face mask, overalls, boots. Sodium hydroxide will eat away at the aluminium but it will also eat off your skin faster than anything. [Getting it in the eyes can cause blindness. See precautions here. Have a source of running water, and for good measure, a bucket of water ready, in case of an accident.-- John Allen]
PREPARE THE BIKE:
Saw off the seat post at the top of the tube and bung it up with something plastic. You must do this because otherwise, liquid would flow out when the caustic soda eats through the seatpost).Make sure it is liquid tight. Remove bottom bracket. To avoid blowing out caustic lye under pressure when you pull things apart later, there must be a way for gas to get out -- generally, through the bottom bracket. I'd suggest removing anything else off the bike that is aluminium, and covering up any decals you may want to save. Take the bike outside and set upside down on a patch of dirt away from kids and animals.
PREPARE THE SODIUM HYDROXIDE:
Measure out 200g of sodium hydroxide powder into a plastic cup. Fill a 750ml bottle with 500ml of cool water.
Put on all the safety gear - the mix will get hot, produce fumes, and may splash. Don't let it touch your skin!
Place the bottle of water into an empty plastic bucket - the bottle may overflow so you want to catch that stuff. Slowly pour the 400g of powder into the 750ml bottle - use a thin wooden stick to stir it. It will get hot - make sure there is no powder left in the bottom of the bottle.
POUR IT IN!
Using a funnel and a thin hose pour in a small amount of the liquid mix into the frame of the bike through the bottom bracket . Maybe about 100ml at a time - wait about 5 minutes and it will start bubbling and gas coming out. When it starts to slow down pour another 100ml in Continue this so that the liquid is in the bike tube for about 1 hour. [No fire or flame nearby -- caustic soda releases hydrogen gas when it reacts with aluminum. -- John Allen].
Break the seal on the seatpost hole - warning! lots of liquid will come out. Be covered up and check where it is all going to flow to. Run fresh water through the frame thoroughly to make sure it is all out Clean the area - dilute any that is on the ground with lots and lots of water Throw away any containers and wash tools really really well
I found I had to do this twice. The second time I had to plug the top tube to stop the liquid flowing down there. [You might tilt the frame so liquid will not run down the top tube -- John Allen] Finally all that was left was a thin sliver of seat post. Hope this is good information. Cheers!
Andrew Livingston adds:
I've had practice cutting stubs of exhaust pipe out of mufflers - short stub, big diameter in which to twist and lever the stub out. Not much fun under a car but it works.
But if there's a lot of seatpost stuck in a bike frame (8" in my case), saw cuts don't work well. It's hard to cut down to the steel at the bottom of the post without cutting into the frame tube at the top. So it's tempting to try to pry out a section prematurely, and then it stays bent and in the way of further sawing and you're really stuck. At this point, Matt Duff's sodium hydroxide (caustic soda/lye) solution came to the rescue. After half-a-dozen doses the post completely disappeared. Washday magic! Multiple doses are needed, not because the lye isn't up to the job, but because the seat tube can't hold much. As soon as the seat tube cools down, run another dose through. The reaction is very energetic and will burp solution uphill into the head tube, so remove the forks first. BTW, in Matt Duff's description, the 200g of sodium hydroxide in 500ml water becomes 400g a few lines later. I mixed 100g in 250ml each time and rinsed after. With a funnel and hose on a stick taped to the upside down frame, and an inner tube plus clamp over a bucket at the other end, it was pretty easy. Especially compared to sawing.
"Free at last! Free at last! Thank God almighty, I am free at last!"
--Dr. Martin Luther King, Jr.
I have a related document, a Database of Seatpost Sizes available on this site.
Also see Jobst Brandt's advice on stuck handlebar stems.
Last Updated: by Harriet Fell