Seat tube size

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

I have recently learned that the interior diameter of the seat tubes of both of my bikes are 0.2
mm smaller than the seat posts in them. How much of a problem is this? The one frame is steel and
the other is carbon with aluminum lugs. Thank you.

Peter

Then how did you get the seatpost in them?

Rob Strickland

"Robert Strickland" <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Quoted message said:

I have recently learned that the interior diameter of the seat tubes of both of my bikes are 0.2
mm smaller than the seat posts in them. How much of a problem is this? The one frame is steel
and the other is carbon with aluminum lugs. Thank you.

Peter

Then how did you get the seatpost in them?

Rob Strickland

Dear Rob,

I'm curious, too.

I once dealt with a bike with a similar mismatch and the answer was that the previous owner had
taken advantage of a good deal of force and the slit down the back of the frame.

A more sophisticated technique might involve reversing the old motorcyclist's home method for
popping a bored-out iron cylinder liner back into an aluminum engine block.

We put the iron liner in the freezer, left the aluminum engine in the oven (it helps to wait until
the usual owner and operator of the freezer and oven is off visiting relatives), and then dropped
the cold, contracted iron liner into the warm, expanded aluminum block. Moments later, you'd need a
press to remove the liner.

Pop the seat tube in the freezer and stuff the frame into the oven--well, I see there may be
difficulties. Lord knows what would happen with a carbon-fiber frame or seat-post.

Carl Fogel

Actually, a moderate amount of force (along with the small amount of grease that usually is applied
to seatposts) was all that was required. I only realized the difference when measuring them with my
vernier calipers. I then discovered how easily the right-sized post goes in and out. Both of these
bikes were bought second-hand and came with these posts in them. I worry on the one hand that the
tube has been compromised in some way and that I should stick with the existing posts- which have
never given me a problem. On the other hand, I am concerned about further damage to the frame if
this situation persists.

[email hidden] (Carl Fogel) wrote in message
news:<[email hidden]>...

Quoted message said:

"Robert Strickland" <[email hidden]> wrote in message
news:<[email hidden]>...

Quoted message said:

"Peter" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Quoted message said:

I have recently learned that the interior diameter of the seat tubes of both of my bikes are
0.2 mm smaller than the seat posts in them. How much of a problem is this? The one frame is
steel and the other is carbon with aluminum lugs. Thank you.

Peter

Then how did you get the seatpost in them?

Rob Strickland

Dear Rob,

I'm curious, too.

I once dealt with a bike with a similar mismatch and the answer was that the previous owner had
taken advantage of a good deal of force and the slit down the back of the frame.

A more sophisticated technique might involve reversing the old motorcyclist's home method for
popping a bored-out iron cylinder liner back into an aluminum engine block.

We put the iron liner in the freezer, left the aluminum engine in the oven (it helps to wait until
the usual owner and operator of the freezer and oven is off visiting relatives), and then dropped
the cold, contracted iron liner into the warm, expanded aluminum block. Moments later, you'd need
a press to remove the liner.

Pop the seat tube in the freezer and stuff the frame into the oven--well, I see there may be
difficulties. Lord knows what would happen with a carbon-fiber frame or seat-post.

Carl Fogel

[email hidden] (Peter) wrote in message news:<[email hidden]>...

Quoted message said:

Actually, a moderate amount of force (along with the small amount of grease that usually is
applied to seatposts) was all that was required. I only realized the difference when measuring
them with my vernier calipers. I then discovered how easily the right-sized post goes in and out.
Both of these bikes were bought second-hand and came with these posts in them. I worry on the one
hand that the tube has been compromised in some way and that I should stick with the existing
posts- which have never given me a problem. On the other hand, I am concerned about further damage
to the frame if this situation persists.

Dear Peter,

If you have indeed managed a press-fit with seat-posts wider than the frame-tube, I expect that the
steel frame is safer, simply because metal is more likely to fail more gradually, while carbon-fiber
tends to fail catastrophically.

That is, the steel frame would probably just bend back and flange outward at the rear slit, probably
slowly enough that you'd have time to react and shift your weight automatically to the pedals and
handlebars. If a carbon-fiber frame failed, it would probably be more abrupt, dramatic, and
dangerous.

Since the seat-posts are already inserted, it's tempting to say that any damage is already done and
that there's no need to worry. But if these are indeed over-size seat-posts driven down into the
frame, then they may be secured more by a wedge effect deep in the frame than by the traditional
compression collar. So it's possible that your weight on the seat is banging the post deeper and
deeper, or at least exerting a strain for which the frame is not designed.

It might be worth a trip to a local bike shop for advice, particularly if these frames are ridden
off-road where seats take more strain.

Carl Fogel

On 18 Oct 2003 21:32:39 -0700 (Peter) said:

I have recently learned that the interior diameter of the seat tubes of both of my bikes are 0.2 mm
smaller than the seat posts in them. How much of a problem is this? The one frame is steel and the
other is carbon with aluminum lugs.

Don't you mean the other way around? If the posts were actually larger than the bores of the seat
tube then you wouldn't be able to get them in there.

In steel frames, it is normal to use a post that is .2mm smaller than the theoretical tube ID, just
to give a minor amount of working clearance.

If these are measurements you actually made, then you may also be pushing the accuracy of your
measuring ability and instrument. It is easy to measure a seat post OD, but it is hard to precisely
measure a seat tube ID, not only because it may not be perfectly round, but also because ID
measurements are harder to make.

-
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Jim Adney [email hidden] Madison, WI 53711 USA
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(Peter) said:

I have recently learned that the interior diameter of the seat tubes of both of my bikes are 0.2
mm smaller than the seat posts in them. How much of a problem is this? The one frame is steel and
the other is carbon with aluminum lugs.

Frame bores are never so round as to allow a free sliding fit with no diametral clearance. There
must always be a few thousandths of an inch difference between the bore size and the post size if
the post is to move freely up and down within the frame.

.2mm equals 8 thousandths of an inch. Seatposts of any given nominal size will vary by 0.1mm or more
among different specimens.

Often, a post one size above that which will slide freely in the frame can be forced into the bore,
pushing the ever-so-slightly oval bore into a rounder shape. Doing this tends to scratch up the
post, and can make adjustment a nuisance.

If you wish to run the closer-fitting post in your steel frame, I'd advise cleaning up the existing
bore with a light pass of an adjustable reamer or berry hone for better fit. Rather that do this to
a carbon frame, I would just use the smaller post if I were you (sparing the frame and the reamer
any potential damage). You may be able to find a particular seatpost in one or the other size that
will give you a more perfect fit if that matters to you.

I doubt that you would have any problem with the smaller seatpost in either case.

Chalo Colina

[email hidden] (Peter) wrote in message news:<[email hidden]>...

[snip]

Dear Peter,

You're in luck. Ignore anything that I said and listen to Chalo.

Carl Fogel