From: jobst.brandt@stanfordalumni.org
Subject: Re: Stuck Seat Post and LN2
Newsgroups: rec.bicycles.tech
Date: Sat, 06 Oct 2001 03:41:33 GMT

J C? Tallant II writes:

> You might try a can of Freon. It gets cold enough to for some
> thieves to break steering wheel locks. (Don't let the Feds or any
> enviro-whackos see you release it into the air though.)

>> My LBS doesn't stock liquid nitrogen. Not that I need it right now,
>> but where would I get some if I did?

Don't!

http://draco.acs.uci.edu/rbfaq/FAQ/166.html

I guess this one needs to be put in that file as well.  Differential
shrinkage when cooling is not enough to free anything but a clean
press fit.  The reason the seat post is stuck is that it corroded and
generated aluminum oxide that is twice the volume as the metal that
corroded.  This has high pressure and interlocking.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>

From: jonisaacs@aol.com (Jon Isaacs)
Newsgroups: rec.bicycles.tech
Date: 06 Oct 2001 12:16:42 GMT
Subject: Re: Stuck Seat Post and LN2

>I guess this one needs to be put in that file as well.  Differential
>shrinkage when cooling is not enough to free anything but a clean
>press fit.  The reason the seat post is stuck is that it corroded and
>generated aluminum oxide that is twice the volume as the metal that
>corroded.  This has high pressure and interlocking.

I tried it about 2 years ago and it did work.  However it did not free right up
but it did make enough difference so that the seat post could be rotated when
clamped in a machinists vice and with a second application of LN2 the seat post
did come out.

jon isaacs

From: jonisaacs@aol.com (Jon Isaacs)
Newsgroups: rec.bicycles.tech
Date: 06 Oct 2001 13:28:43 GMT
Subject: Re: Stuck Seat Post and LN2

>>I guess this one needs to be put in that file as well.  Differential
>>shrinkage when cooling is not enough to free anything but a clean
>>press fit.  The reason the seat post is stuck is that it corroded and
>>generated aluminum oxide that is twice the volume as the metal that
>>corroded.  This has high pressure and interlocking.

I might add that a simple calculation shows taking the steel seat tube plus the
aluminum seat post to Liquid nitrogen temperature (-195C) results in a
differential strain of about 0.003.

A quick estimation shows that this would result in a reduction in the hoop
strain in the steel which would result in a reduction of the contact pressure
between the seat tube and the seat post of about 6000psi.  (27.2 mm seat post,
1 mm tubing where contact is occuring.)

Now assume that 1 inch vertically is in a bind and that the coefficent of
friction is only 0.25 and the reduction in torque required to twist the seat
post is about 230 ft-lbs.

This is certainly a significant reduction and clearly could result in freeing
an otherwise stuck seat post.

Which I might add, it seemed to do so when a couple of us tried this technique
a few years ago.

jon isaacs

From: jobst.brandt@stanfordalumni.org
Subject: Re: Stuck Seat Post and LN2
Newsgroups: rec.bicycles.tech
Date: Sat, 06 Oct 2001 18:34:08 GMT

Jon Isaacs writes:

> I might add that a simple calculation shows taking the steel seat
> tube plus the aluminum seat post to Liquid nitrogen temperature
> (-195C) results in a differential strain of about 0.003.

That is an ideal condition that is not easily achieved, especially
with a bicycle frame seat cluster and seat post.  To reach LN2
temperature requires immersion, something that is not readily done.
How did you cool the frame?

> A quick estimation shows that this would result in a reduction in
> the hoop strain in the steel which would result in a reduction of
> the contact pressure between the seat tube and the seat post of
> about 6000psi.  (27.2 mm seat post, 1 mm tubing where contact is
> occurring.)

I haven't checked the accuracy of this but it sounds possible if the
temperature were reached.

> Now assume that 1 inch vertically is in a bind and that the
> coefficient of friction is only 0.25 and the reduction in torque
> required to twist the seat post is about 230 ft-lbs.

I'm not convinced that the LN was as much part of the removal as your
fixturing and strength.  I can tell of similar experience without LN.

> This is certainly a significant reduction and clearly could result
> in freeing an otherwise stuck seat post.

I don't believe that the method is practical for most of these cases.

> Which I might add, it seemed to do so when a couple of us tried this
> technique a few years ago.

I would like to hear how the cooling was done.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>

From: jonisaacs@aol.com (Jon Isaacs)
Newsgroups: rec.bicycles.tech
Date: 06 Oct 2001 19:34:38 GMT
Subject: Re: Stuck Seat Post and LN2

Jobst wrote:

>That is an ideal condition that is not easily achieved, especially
>with a bicycle frame seat cluster and seat post.  To reach LN2
>temperature requires immersion, something that is not readily done.
>How did you cool the frame?

We had already cut the seat post top off.  So we filled the seat tube partially
(below the seat post)  with water (not a good idea in retrospect) and started
filling the seat tube with LN2.  The water froze forming a plug and then we
just continued to fill the seat tube with LN2 through the center of the seat
post.

We waited until the LN2 had pretty much stopped boiling, the normal indication
that we had reached equilibrium.  Then we put the seat post in a vise and went
for it.

>I haven't checked the accuracy of this but it sounds possible if the
>temperature were reached.

I just made the simple hoop stress calc and it shows a pressure of about 6000
psi would be required to achieve this strain.

>I'm not convinced that the LN was as much part of the removal as your
>fixturing and strength.  I can tell of similar experience without LN.

We had tried the same technique with the same tools prior to the application of
LN2.  Furthermore during the removal, the frame warmed up, the seat post stuck
again and a further application of LN2 was required to free it.

>I don't believe that the method is practical for most of these cases.

It is probably only practical for those of us who have access to a nice machine
shop and a large supply of LN2.

It was clear at the time that the Liquid Nitrogen did make the difference.

My calculations also show that if one assumes larger coefficients of friction
(I used only 0.25) then the LN2 temperature may not be necessary.

I would only use this technique as the final last resort, once all other
alternatives were exhausted.   The paint on the frame was damaged in the BB
area.

It think it is better than cutting the seat post out with a hack saw, that is
risky IMHO.

jon isaacs

From: jobst.brandt@stanfordalumni.org
Subject: Re: Stuck Seat Post and LN2
Newsgroups: rec.bicycles.tech
Date: Sat, 06 Oct 2001 22:37:25 GMT

Jon Isaacs writes:

> We had already cut the seat post top off.  So we filled the seat
> tube partially (below the seat post) with water (not a good idea in
> retrospect) and started filling the seat tube with LN2.  The water
> froze forming a plug and then we just continued to fill the seat
> tube with LN2 through the center of the seat post.

I retract my earlier critique.  With the seat post sawed off, a Dremel
tool will do the job, but if you can fill the seat post tube with LN
you are even better off than immersing the unit because the aluminum
oxide corrosion is a good insulator giving an even better shrink
differential than otherwise.

> We waited until the LN2 had pretty much stopped boiling, the normal
> indication that we had reached equilibrium.  Then we put the seat
> post in a vise and went for it.

>> I haven't checked the accuracy of this but it sounds possible if
>> the temperature were reached.

> We had tried the same technique with the same tools prior to the
> application of LN2.  Furthermore during the removal, the frame
> warmed up, the seat post stuck again and a further application of
> LN2 was required to free it.

>>I don't believe that the method is practical for most of these cases.

> It is probably only practical for those of us who have access to a
> nice machine shop and a large supply of LN2.

My shop just saws it off and uses a Dremel tool to reduce the wall
locally to zero and it practically falls out.  That goes for stems
too, except that hey must first be drilled to thin the wall.

Jobst Brandt    <jobst.brandt@stanfordalumni.org>

From: jonisaacs@aol.com (Jon Isaacs)
Newsgroups: rec.bicycles.tech
Date: 07 Oct 2001 03:10:34 GMT
Subject: Re: Stuck Seat Post and LN2

>I retract my earlier critique.  With the seat post sawed off, a Dremel
>tool will do the job, but if you can fill the seat post tube with LN
>you are even better off than immersing the unit because the aluminum
>oxide corrosion is a good insulator giving an even better shrink
>differential than otherwise.

This was my thinking as well, the real world case is actually better than the
easiest to calculate case.

The frame was actually a very nice fillet brazed Bill Holland custom steel that
was I was given due to the unique paint job and the stuck seat post.

>My shop just saws it off and uses a Dremel tool to reduce the wall
>locally to zero and it practically falls out.  That goes for stems
>too, except that hey must first be drilled to thin the wall.

Sometimes I think the actual corroded area is far enough down the tube that a
dremel might be difficult to use.

We did this mostly as an experiment because I had the frame at the lab and we
had the LN2 just sitting there boiling away.  We did verify that it worked for
this one case so our curiosity was satisfied.

jon isaacs