Anyone know where I can get one?
I don't mind if it's the old-style clamp so long as it's alloy, and not too expensive.

I don't know how long the Kalloy seat posts are
they are made in a number of diameters, and 22.2 at the top.

How long do you need for an extension, 12 inches above the frame perhaps?
I'd suggest a Redline /SBS 1" seatpost, its pretty darn long. [I used that on a old Mountain Bike and had a foot out of the frame]
it's an alloy : Chromium Molybdenum alloy steel. and then at the bottom of it .. get a 25.4 to 27.2 shim.

with that long a leverage under your backside I would not want to be atop a light seatpost.

an aside : you could get a thick wall piece of 7075T6 in 7/8" OD, to fit saddle clips at the top,
and then fit it with a machined shim to suit the ID of your frame...

20% of the length should be inside the frame..

The redline seat post is too short, if I'm looking at the right product.

Something like a butt buddy could work too though I can't find any on the web. I know that product was sold under a different name also but I can't remember what it was.

You can get one of these https://www.bikeman.com/ST4572.html It has a max height above the frame tube of 12" and the price is not bad.

It's got to be 500mm. . . perhaps a butt buddy type thing is the best solution. If only I knew where to get one.

Talk to a custom frame builder , to get them to weld something up HD,
get it Heat treated..

if its sticking out so far..


Brompton uses a pretty long seatpost but they are a 31.9 OD/ 1.25",
not 27,2 and add a 30% thicker wall tube for their extended height version.

.. or you could buy 2, have a machine shop re machine them
to fit one into the other , overlapping internally,
and then weld the joint and machine it back down.
(maybe have them do several it may fatigue bend over time.)

I got a Campag 26.6 OD post and a Kalloy one combined in this manner
so I got to fudge the max height line on the Campag one a little bit,
It was still 1" below the max height, but the stability of the stuff
buried inside the frame was maintained..

light, cheap, strong, pick only 2.

this one is only 410mm though......https://www.amazon.com/dp/B001C6BL8Q/...SIN=B001C6BL8Q

maybe not enough length for you.

this one is a butt seatpost: 520mm

https://www.cycle-frames.com/bicycle-...-TUBE-550.html

I highly suspect that 27.2mm being extended to 500mm is beyond the maximum flexural strength such a tubing would have.
there's a reason why many folding bikes come equipped with large diameter, long seatposts.

You're probably right. I acquired a somewhat odd second hand folding bike and I'm figuring out what would make it worth keeping. I think a butt buddy would be the best solution.

MMM...If you believe that the reason 500mm are not available due to over stress, then adding a butt buddy is not the answer. You are still sitting at hight that over stresses the tube. That the forces are transferred through the butt buddy rather than an extended tube matters not.

Thicker material compensates for that easily. I do think the real reason being they rarely need that length (folding bikes are about the only ones that needs long post and they have their own manufacturing for it).
I would worry more about the stress it can put on seat post clamp area of frame which actually bears brunt of torque.

say hiring someone to Make a steel , 4130 pillar is your best approach..

Ok it's not going to be cheap, unless you reside in a place that has low overhead costs.

Expect folding package will be larger,
Maybe your used purchase is only suitable for someone shorter..

no. Thicker walls do not help strength as much as larger diameter. Thicker walls would just add a lot of unnecessary weight.

As long as the bottom of the seatpost extends past the top tube joint, the seatpost won't crack prematurely from the extension.

best example I can give you is trying to bend various diameters of copper piping and rods.

the heat treatment of the finished part helps make for a less bendable length,
a mix of ductility and hardness, made by Heating and Quenching when that heat is reached ..
OP got a cheap Folding biker ,maybe he will send a picture..

Copper is a Non ferrous metal , piping is sold annealed for plumbing, to be flexible for fitting.
so a weak example .. metallurgical apples v oranges
when comparing with a tempered steel alloy., or high strength say 7075-T6 Aluminum.

You missed my point.
I was arguing for the ease of bending different diameters of tubing.

Larger diameter tubing is more difficult to bend and twist compared to smaller diameter tubing.
It's quite evident with the use of oversized tubing on aluminum frames or larger axle sizes (15mm and 20mm vs. 9mm and 10mm) for downhill bikes.

Now, just because you give the tubing a thicker wall, it doesn't make it stronger compared to making diameters larger. The main thing thicker walls help with are resistance to denting and punctures.

OP has a given OD 27.2 mm , so wall thickness and alloy of the material is all
there is to work with, the cost of remaking the frame tube to accept a larger
seat tube is out of the question here .

a spring tempered steel , a form of heat treatment after all,
is able to benefit significantly from wall thickness increase, when that is the only variable you can change..

anyhow OP may have gotten a deal, but probably needs to find a different machine that will fit their height.

This one is not It. sell it to a '5 foot 2 with eyes of Blue ' ... to quote a Song..

Go out and spend the money for a Brompton or a custom bike Friday

the one that is spoken of is a Cul De Sac, and is not suitable, apparently,
to the riders height.
several solutions offered , If they are not affordable ,
forget the project, sell it and cut your losses.

I have to agree with you.

my other foldingbike has a 28,6. seatpost.
for my height, it already tends to flex

That's not a seat post. It is the tubing that is used in frame making that accepts a 27.2 seat post.