Maximum safe seatpost mismatch?

David Johnson said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0?

You may have trouble with the post slipping. Also, the binder bolt
may be more likely to snap if it has to bend.

If I had to use a smaller post, I'd shim it with sliver of sheet
metal, such as from a soda can.

JT

David Johnson said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? If I go that way, the 25.8
will be for a short rider who wants a different seat than my normal
tandem stoker. This post will never be up more than an inch or two,
(and it's max mark is only about 4 inches)

To tighten it, you'll need to deform the tube. Not much at all, not
dangerous in and of itself I imagine. But then when you go to put a
26.0 back in, no luck.

Use a shim. 0.2 mm is about .008 inch. Find some .004 shim stock and
wrap the post.

David Johnson said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? If I go that way, the
25.8 will be for a short rider who wants a different seat than my
normal tandem stoker. This post will never be up more than an inch
or two, (and it's max mark is only about 4 inches)

I used a 31.6 post in a 31.7 tube with no problems. Hell, my cheap seatpost
on my other bike isn't even a constant diameter... it will fall into the
seat tube until it gets 2 inches to the clamp, where it expands a couple
thousandths of an inch and gets hard to insert.

Add some tinfoil or sodacan shim, but don't sweat it if it's not quite
perfect.

--
Phil, Squid-in-Training

Dan Daniel said:

To tighten it, you'll need to deform the
tube. Not much at all, not
dangerous in and of itself I imagine.

I don't understand what is dangerous about this. Are you suggesting
the seat tube itself might crack or break? If so, you're mistaken

JT

John Forrest Tomlinson said:

Dan Daniel said:

To tighten it, you'll need to deform the
tube. Not much at all, not
dangerous in and of itself I imagine.

I don't understand what is dangerous about this. Are you suggesting
the seat tube itself might crack or break? If so, you're mistaken

JT

I think with a large difference in diameter, there is a chance of
ovalising the seat tube, but 0.2mm in steel doesn't seem enough for
this to be a problem. Slippage is likely to become a problem before
anything else. Tinned steel shim stock can be had from your local
model railway shop.

Kinky Cowboy*

*Batteries not included
May contain traces of nuts
Your milage may vary

David Johnson said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? If I go that way, the 25.8
will be for a short rider who wants a different seat than my normal tandem
stoker. This post will never be up more than an inch or two, (and it's
max mark is only about 4 inches)

You can wrench it down, but then you're likely going to deform the seat
tube. I have a used frame on which somebody did exactly this and now 27.0
can't even be inserted and 26.8 doesn't quite fit snug.

Do now what I must do on that bike. Cut a piece of aluminum to wrap
around the seat tube. Mine is a piece I cut from a PBR* can (and the
color scheme matches my bike!).

EK

*That's Practical Bicycle Repair, of course.

djohnson-<< How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? >><BR><BR>

Undue strain on the clamping mechanism that may cause it to fail.

Peter Chisholm
Vecchio's Bicicletteria
1833 Pearl St.
Boulder, CO, 80302
(303)440-3535
http://www.vecchios.com
"Ruote convenzionali costruite eccezionalmente bene"

John Forrest Tomlinson said:

Dan Daniel said:

To tighten it, you'll need to deform the
tube. Not much at all, not
dangerous in and of itself I imagine.

I don't understand what is dangerous about this. Are you suggesting
the seat tube itself might crack or break? If so, you're mistaken

JT

Ok, let me try again...

'...... not dangerous in and of itself I imagine.'

Any clearer? 🙂

The problem I see for the OP is, as I understand him, he wants to swap
posts for different people using the same bike. This is where the
deformation of the seat tube by using an undersized post will create a
problem- reinstalling the larger post.

Dan Daniel said:

John Forrest Tomlinson said:

Dan Daniel said:

To tighten it, you'll need to deform the
tube. Not much at all, not
dangerous in and of itself I imagine.

I don't understand what is dangerous about this. Are you suggesting
the seat tube itself might crack or break? If so, you're mistaken

JT

Ok, let me try again...

Yes, sorry.

JT

JT wrote in part:

<< If I had to use a smaller post, I'd shim it with sliver of sheet
metal, such as from a soda can. >>

A Twix wrapper is much easier to work with and just as
effective as metal shims. Can be folded over for
different thicknesses. And it comes with candy.

Robert

R15757 said:

JT wrote in part:

<< If I had to use a smaller post, I'd shim it with sliver of sheet
metal, such as from a soda can. >>

A Twix wrapper is much easier to work with and just as
effective as metal shims. Can be folded over for
different thicknesses. And it comes with candy.

Yeah, but then you get that annoying /crinkle/ sound as you mash.

Bill "ants for protein?" S.

David Johnson said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? If I go that way, the 25.8
will be for a short rider who wants a different seat than my normal
tandem stoker. This post will never be up more than an inch or two,
(and it's max mark is only about 4 inches)

That's .008" of slop, which is a lot. The clamp won't have much of a
grip unless you're overtightening it, its grip will be on a narrow
band of the post, the seat tube will be getting flexed at the top more
than usual, and the post will tend to slip down even if overtightened.
Worst of all, there's a good chance that the top of the seat tube will
become deformed in the process, making it difficult to install a post
of the correct size later.

I'd call it a bad idea. If there was as great a tolerance for
mismatch as this, the low-end component suppliers would be selling a
27.0mm post as a "multi-fit, good for 27.0 - 27.2mm" unit, and such.

Use the size that's meant to go there. Seat posts aren't expensive.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

David Johnson said:

Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0?

I used a 27.0mm aluminum post in an aluminum frame made for 27.2mm
posts for a long time. It never failed or slipped, and I've since
put in a different post with an appropriate shim (equaling a 27.2mm
post) without difficulty.

Look at a millimeter. Now, imagine one fifth of a millimeter. It's a
tiny, tiny, minuscule amount to request from most frames.
--
Rick Onanian

Rick Onanian said:

David Johnson said:

Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0?

I used a 27.0mm aluminum post in an aluminum frame made for 27.2mm
posts for a long time. It never failed or slipped, and I've since
put in a different post with an appropriate shim (equaling a 27.2mm
post) without difficulty.

Look at a millimeter. Now, imagine one fifth of a millimeter. It's a
tiny, tiny, minuscule amount to request from most frames.
--
Rick Onanian

Too late--The previous posters made me doubt enough to order a proper
seat post from the nearest good bike store. I'm not willing to risk my
Santana, and it was mostly laziness and the lack of a decent local bike
shop that made me want to use an existing post.

In article <[email hidden]>, [email hidden] says...

Quoted message said:

I'm not willing to risk my Santana

Gee, you have a name for it?

;-)

Rick

Werehatrack said:

On Sat, 17 Jul 2004 20:27:45 -0400, David Johnson

Quoted message said:

How close does a seatpost have to be--Is there anything wrong
(mechanically I mean, not aesthetically) with using a 25.8 (steel)
seatpost in a steel frame meant for a 26.0? If I go that way, the 25.8
will be for a short rider who wants a different seat than my normal
tandem stoker. This post will never be up more than an inch or two,
(and it's max mark is only about 4 inches)

That's .008" of slop, which is a lot. The clamp won't have much of a
grip unless you're overtightening it, its grip will be on a narrow
band of the post, the seat tube will be getting flexed at the top more
than usual, and the post will tend to slip down even if overtightened.
Worst of all, there's a good chance that the top of the seat tube will
become deformed in the process, making it difficult to install a post
of the correct size later.

I'd call it a bad idea. If there was as great a tolerance for
mismatch as this, the low-end component suppliers would be selling a
27.0mm post as a "multi-fit, good for 27.0 - 27.2mm" unit, and such.

Use the size that's meant to go there. Seat posts aren't expensive.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

Lot of suspension seatposts come with one or several shims to become universal fit.

Rick Onanian's point on 1/5 mm is well taken, but it's so easy to make your own shim, I would have used a shim if I wasn't in a pinch (pun intended). I'm familiar with people having difficulty getting proper sized seatposts to work after using a narrower post.