Alu post stuck in alu frame

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gregoryoftours
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Alu post stuck in alu frame

Postby gregoryoftours » 6 Jan 2020, 10:17pm

Has anyone had success with getting a stuck aluminium seatpost from an aluminium frame? If so what worked? I've heard that penetrating oil (which I have tried) doesn't work very well in the case of similar metals. I tried a slide hammer (admittedly a pretty lightweight one) and fairly brutal twisting with a handlebar and extensions to the bar clamped to the seatpost. The seatpost is the right size for the frame, and the bike overall is fairly clean so doesn't look as if it should be too heavily corroded. I haven't got the bike with me right now to look at but as far as I can remember there is about 20cm of post inside the frame. What would be the best thing to try in terms of heating/cooling?

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fausto99
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Re: Alu post stuck in alu frame

Postby fausto99 » 6 Jan 2020, 10:35pm

Try filling the seat tube with ammonia for a couple of days before attempting any more force or heat-cool cycles. Fix the frame upside down and fill from the bottom bracket.

Bonne chance

gregoryoftours
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Re: Alu post stuck in alu frame

Postby gregoryoftours » 6 Jan 2020, 10:53pm

fausto99 wrote:Try filling the seat tube with ammonia for a couple of days before attempting any more force or heat-cool cycles. Fix the frame upside down and fill from the bottom bracket.

Won't that damage the frame also though?

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fausto99
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Re: Alu post stuck in alu frame

Postby fausto99 » 7 Jan 2020, 8:28am

Not after only a couple of days, whereas it should attack the fluffy white oxides which are holding the parts together so strongly.
Last edited by fausto99 on 7 Jan 2020, 9:32pm, edited 1 time in total.

Jdsk
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Re: Alu post stuck in alu frame

Postby Jdsk » 7 Jan 2020, 8:56am

I'd try soaking in penetrating fluid and then direct impact from below along these lines:
viewtopic.php?f=5&t=105655

And if that didn't work I'd send it to someone with lots of experience and pay them, such as The Seatpost Man:
https://theseatpostman.com/about/.

Jonathan

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Mick F
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Re: Alu post stuck in alu frame

Postby Mick F » 9 Jan 2020, 5:47am

Have you mounted the seatpost in a vice with the frame upside down and poured in the penetrating oil in through the BB and left it a week?

If so, have you tried gently turning the frame - alternate clockwise/anticlockwise?

If not, I suggest you do them. That would be the first thing I would do.
Mick F. Cornwall

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fausto99
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Re: Alu post stuck in alu frame

Postby fausto99 » 9 Jan 2020, 8:38am

Mick F wrote:Have you mounted the seatpost in a vice with the frame upside down and poured in the penetrating oil in through the BB and left it a week?

If so, have you tried gently turning the frame - alternate clockwise/anticlockwise?

If not, I suggest you do them. That would be the first thing I would do.

+1 for this method, but I found that even Plusgas A penetrating fluid did not work as well as ammonia on the fluffy white stuff that grows on aluminum.
I have to confess that gently did not work for me. I had to put my back into it - and I have the luxury of a bench mounted vice on a bench fixed to the wall!

gregoryoftours
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Re: Alu post stuck in alu frame

Postby gregoryoftours » 10 Jan 2020, 8:55pm

Thanks for your suggestions
fausto99 wrote:Not after only a couple of days, whereas it should attack the fluffy white oxides which are holding the parts together so strongly.

What type and strength of ammonia would you recommend? I'm worried about where to draw the line and causing damage to a fairly lightweight frame.

So far I've had the bb out and stood the frame upside down for 3 days with a penetrative releasing fluid (brunox in this case) filled past the end of the post in the frame.

Then tried a small slide hammer (about a kilo) on the post, then clamped a flat handle bar in the post clamp, with a couple of feet of scaffolding added each side and gave it a fair twist, also to no avail.

Brucey
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Re: Alu post stuck in alu frame

Postby Brucey » 10 Jan 2020, 9:04pm

in terms of heating/cooling, you don't want to mess the paint up, right? This method is not zero risk but it is still fairly low risk:

If you can do so, get hold of some liquid nitrogen and fill the seat pin up with that (i.e. from the inside). Then chuck warm water over the seat tube on the outside. For a few seconds (i.e. whilst there is still liquid nitrogen inside the seat pin, but liquid water on the outside of the seat tube) you have got a massive delta T and if the seat pin is going to move, it'll move if you haul on it at this moment.

cheers
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fausto99
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Re: Alu post stuck in alu frame

Postby fausto99 » 10 Jan 2020, 10:33pm

gregoryoftours wrote:What type and strength of ammonia would you recommend? I'm worried about where to draw the line and causing damage to a fairly lightweight frame.

I just bought a 500ml bottle marked "household ammonia" from the local hardware store.
The label on the back says "Ammonia solution 9.5%". I've still got 2/3 of the bottle left!

tim-b
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Re: Alu post stuck in alu frame

Postby tim-b » 11 Jan 2020, 6:56am

Hi
I'd be as wary of using ammonia as you are, and I agree that you have to break the bond/expansion of the oxidisation. You could try to use a hacksaw blade and carefully cut along the inside length of the seatpost. For ease, cutting relies on the shortest cut so try to determine how much is in the frame and don't cut flush to the seat lug because you'll need something to grip when the time comes. When cut you can roll the seatpost into itself like the top of a (now old-fashioned) sardine tin. If alu has corroded to alu then all bets on the internal state of the frame are off
Regards
tim-b
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Marcus Aurelius
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Re: Alu post stuck in alu frame

Postby Marcus Aurelius » 11 Jan 2020, 2:36pm

The problem is galvanic corrosion. It’s a bugger. Try repeatedly heating and cooling the point where it’s stuck. Heat it up with a brazing torch, then rapidly cool it with ice water, repeat until it releases.

Brucey
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Re: Alu post stuck in alu frame

Postby Brucey » 11 Jan 2020, 2:52pm

somewhere between 100 and 150C the paint will get wrecked. Transfers often at lower temperatures than this. Excursions above 200C (and especially above 250C) will irrevocably change the properties of many aluminium framesets so ought to be avoided.

BTW if you can get a 200C temperature differential (eg by using liquid nitrogen) then the seat pin could be about 0.1mm smaller diameter, in which case it ought to move easily.

You can also thermally shock/strain the corroded joint (with little or no risk to the paint) by tipping boiling water into the frame. Often after a few goes at this, the extra stress (the seat pin tries to expand inside the frame) causes the bonds of corrosion to weaken and the seat pin will start to move where it didn't before.

cheers
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Re: Alu post stuck in alu frame

Postby PDQ Mobile » 11 Jan 2020, 2:58pm

To avoid paint damage by getting it really hot which as others have suggested may well work on the end. Use oil too. It stinks as it burns but helps.


Hot oil (say around 90 degrees) poured over the frame tube is worth a try. Frame fixed level with ground somehow.

Immediately try to move the post.
The outer tube expands slightly before the post - if you're quick.
Or lubricate well first and use water.

Be careful with hot liquids.

Brucey
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Re: Alu post stuck in alu frame

Postby Brucey » 11 Jan 2020, 3:25pm

BTW I have reason to suppose that the seatpost man (amongst other things) may use a long jacking frame that pulls the seat pin from the top, probably secured through the frame at the bottom bracket. The reason this works can be seen easily if you stretch a piece of toffee; it can't get longer without also getting narrower, and this applies to elastic deformations as well as plastic deformations.

If the seat pin is made of moderately strong material it will stretch by a fraction of a percent elastically without significant danger of failure. If the strain is about 0.3% lengthwise (and the material is isotropic) the loaded parts of a typical seat pin will be about 0.1mm smaller in diameter too and ought to come out more easily. The other thing is that because the strain only appears where the seat pin is loaded, the bonds of corrosion don't have to be broken everywhere at once; the bond can kind of peel away, i.e. come free locally, a bit at a time, not all at once. A bit of delta T and a bit of penetrating oil (which will penetrate more easily too) and most seat pins ought to come free without too much of a struggle.

cheers
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