cheap seatpost replacement options - 20.5mm

[jasong]

Picked up a clunker 10speed for small commutes. Since it cost $10, don't want to invest much at all in it or put anything on it that would attract attention.

The seatpost is, as usual, the limiting factor being about 8" long. It's the style the seatpost clamp (bolted to the seat chassis (no rails)) bolts onto a still straight tube. This straight tube is 20.5mm (13/16) outter diameter at the frame and gets slightly larger (to 22.0mm) to attach to the clamp, and has no bends. Wall thickness is roughly 1.6mm.

What's a typical cheap replacement or solution in this situation? ie. raw pipe lengths. Rust resistance isn't an issue. I could even weld a pipe to bring the seat to where I need it. Future adjustability isn't an issue.

'Was thinking that a galvanized conduit or gas pipe might be an adequate replacement, and I could shim the smaller size to fit it to the seatpost bolt. Intermediate Metal Conduit (IMC) is available in OD 20.7mm thickness 1.8mm.

But, what method is used to determine if this is safe/adequate? What formula might be used that gives minimum wall thickness (for a given material) as a function of material type, exposed seatpost length, and outter diameter?

Searching for cheap seatpost replacements before, I never found anything less than around $10 at normal retail price. Nashbar became the exception with their sale recently on house brand items where they sold their posts in various diameters at around $6. But 20.5mm seems an oddball size. I also have curiosity in doing this in the future in trying to restore these clunker bikes for small distance commuters. Seatpost length often seems to be the limiting factor for taller people.

Would also be interested in hearing other things you guys do to take a clunker commuter bike and improve it, without investing much (>$20 is a lot), and without making it look more attractive. The idea behind this "class" of bike is to have something that can be stolen or locking it up remotely for infrequent use.

 

[[email protected]]

I like your thinking here.

The fact that you are concerned about the strength of "galvanized
conduit or gas pipe as an adequate replacement" makes a lot of sense
but this material is, in fact, a good idea for what you are trying to
do.

The limiting factors are that the rider is not extremely heavy, or
using an extremely long post.

Sometimes I have found electrical conduit that will fit a seat tube but
it has a much thinner wall than gas or water pipe and is only suitable
for a short seatpost.

Lewis.

*****************

 

[Lou Holtman]

<[email protected]> wrote in message
news:[email protected]...
> I like your thinking here.

>
> The fact that you are concerned about the strength of "galvanized
> conduit or gas pipe as an adequate replacement" makes a lot of sense
> but this material is, in fact, a good idea for what you are trying to
> do.
>
> The limiting factors are that the rider is not extremely heavy, or
> using an extremely long post.
>
> Sometimes I have found electrical conduit that will fit a seat tube but
> it has a much thinner wall than gas or water pipe and is only suitable
> for a short seatpost.
>
> Lewis.
>

I like your thinking?? Compromise your safety for a lousy 10 dollar seems
not so smart to me. You want to go safe? Take a solid rod and make in fit on
a lathe. It will be on the heavy side though..



Lou

 

[jasong]

<[email protected]> wrote in message
I like your thinking?? Compromise your safety for a lousy 10 dollar seems
not so smart to me. You want to go safe? Take a solid rod and make in fit on
a lathe. It will be on the heavy side though..

Lou, you may have missed the part of my post asking for a means for determining how to do this safely. It wasn't implied that an arbitrarily tough item be stuck in the hole and secured and wishful thinking maintained during the rides. Though I think that this could be assumed for the constrained case of a very small post extension (which won't be the case for me, I need some 30cm sticking out of the frame) which was what the poster had suggested. Shear forces on something that small, to result in destructive failure, would be very difficult to reproduce in the context.

Not even the idea of turning a solid rod down to this diameter is a convincing enough argument to be safe. Maybe the post that came with the bike isn't safe.

The intent was to motivate the stress/strain equations that manufacturers rely on to spec these parts out, placing several variables in a function and seeing what wall thickness pops out. I've got a post pending the frameforum as well.