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cheap seatpost replacement options - 20.5mm



 
 
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  #1  
Old July 13th 05, 07:49 PM
jasong
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Default cheap seatpost replacement options - 20.5mm


Picked up a clunker 10speed for small commutes. Since it cost $10, don'
want to invest much at all in it or put anything on it that woul
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 (n
rails)) bolts onto a still straight tube. This straight tube is 20.5m
(13/16) outter diameter at the frame and gets slightly larger (t
22.0mm) to attach to the clamp, and has no bends. Wall thickness i
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
pipe to bring the seat to where I need it. Future adjustability isn'
an issue.

'Was thinking that a galvanized conduit or gas pipe might be a
adequate replacement, and I could shim the smaller size to fit it t
the seatpost bolt. Intermediate Metal Conduit (IMC) is available in O
20.7mm thickness 1.8mm.

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

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

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

--
jasong

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  #2  
Old July 14th 05, 01:10 PM
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Default cheap seatpost replacement options - 20.5mm

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.

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

  #3  
Old July 14th 05, 01:45 PM
Lou Holtman
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Default cheap seatpost replacement options - 20.5mm


wrote in message
oups.com...
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


  #4  
Old July 15th 05, 07:48 AM
jasong
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Posts: n/a
Default cheap seatpost replacement options - 20.5mm


Lou Holtman Wrote:
wrote in message
I like your thinking?? Compromise your safety for a lousy 10 dolla
seems
not so smart to me. You want to go safe? Take a solid rod and make i
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 fo
determining how to do this safely. It wasn't implied that a
arbitrarily tough item be stuck in the hole and secured and wishfu
thinking maintained during the rides. Though I think that this could b
assumed for the constrained case of a very small post extension (whic
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 tha
small, to result in destructive failure, would be very difficult t
reproduce in the context.

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

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

--
jasong

 




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