Quill seat posts for MTB?

Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?

Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.

- Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it.

Google brandt+pretension

Certainly the pinch bolt is tightened. But is the pinch bolt, in some
manner independent of all else? Just floating there in space absorbing
all stresses and strains imposed on it? Or is it simply a clamp that
transfers loads imposed on it to the bicycle frame.

Is the fact that all seat posts have some minimum insertion limits
important? Given that the seat clamp tension bolt "absorbs the
tension"?
The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post.

Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right?

I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt.

Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube.

And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it.

On Fri, 02 Dec 2016 21:07:37 -0600, AMuzi wrote:

On 12/2/2016 5:58 PM, John B. wrote:
On Fri, 02 Dec 2016 16:36:45 -0600, AMuzi wrote:

On 12/2/2016 3:04 PM, Gregory Sutter wrote:
On 2016-12-02, Joerg wrote:

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Joerg,

99.9+% of all bicycles, made for all uses and types of loads, clamp
the seatpost at the top. I submit that you are on a wild goose
chase, and have several misconceptions.

A longer post usually means nothing, as the only area of the frame
that clamps down on the seatpost is the top 7-12cm. The post just
has to be inserted to its minimum-insertion line.

For similar reasons the quill seatpost is probably not a good idea
for your frame: the seat tube won't be shimmed as far down as the
quill will clamp, and isn't meant to take the forces that the quill
will apply.

On the seatpost clamp, too tight is often worse than too loose,
because while too loose results only in slipping, too tight can
damage the clamp (strip threads or snap the band), the seatpost
(crimping) or possibly even the frame. If the creaking you hear
has recently started, perhaps it is because the post-clamp interface
is no longer correct due to overtightening or an impact.

(Anecdote time: I once lightly dented a Thompson post (!) at the
clamp from an impact while riding. My butt didn't like it either.)

Therefore I recommend to you that you ensure your existing seatpost
clamp is still round and smooth; that your seatpost in the clamped
area is the same; that you grease the threads on the clamp's bolt;
that you lightly grease the seatpost where it touches the clamp and
the frame shim; and that you use a torque wrench when you tighten the
clamp.


All good points but I'll add that in modern thinwall steel
TIG frames, seatpost flex can be a significant problem.
Builders now recommend post insertion 2 diameters below the
lowest weld as excessive flex can lead to weld cracks.

Seat Post flex, or Seat Tube flex?


Pithy observation.
Seat post flex would have no significance if not for seat
tube flex in the hard areas adjacent to the weld

http://www.yellowjersey.org/photosfr...st/wiltaxi.jpg

Interestingly cracks these thin hard steels seem to progress
perpendicular to the weld bead.

Theoretically at least the weld bead should be the stronger than the
tubing so it might be possible that it is tube itself failing. Perhaps
due to over heating during welding.

It isn't supposed to work that way but in any production process there
is an emphasis on higher production and one way to weld faster is to
raise the amperage and increase the travel speed.

--
cheers,

John B.

On Fri, 2 Dec 2016 19:09:33 -0800 (PST), Doug Landau
wrote:


Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?

Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.

- Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it.

Google brandt+pretension

Certainly the pinch bolt is tightened. But is the pinch bolt, in some
manner independent of all else? Just floating there in space absorbing
all stresses and strains imposed on it? Or is it simply a clamp that
transfers loads imposed on it to the bicycle frame.

Is the fact that all seat posts have some minimum insertion limits
important? Given that the seat clamp tension bolt "absorbs the
tension"?
The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post.

Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right?

I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt.

The pinch bolt simply compresses the seat tube to clamp the seat post
in place and resists further distortion to some extent but force
applied to the seat post is absorbed by the seat tube. Can't be any
other way.

Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube.

That is a possibility but tube manufacturers are aware of this and
make their seat tubes with a straight section to fit the seat post,
usually at least 130 - 150mm long. In some cases seat tubes are single
butted in which case the thicker section normally is fitted to the
bottom bracket.

This is not to say that some idiot couldn't cut the wrong end of the
tube to fit it but probably not on a production frame.



And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it.

--
cheers,

John B.

On 12/2/2016 9:09 PM, Doug Landau wrote:

Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?

Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.

- Because if clamped at the top, the pinch bolt will be under tension, and able to absorb the tension put on it by the post, before the welds can experience it.

Google brandt+pretension

Certainly the pinch bolt is tightened. But is the pinch bolt, in some
manner independent of all else? Just floating there in space absorbing
all stresses and strains imposed on it? Or is it simply a clamp that
transfers loads imposed on it to the bicycle frame.

Is the fact that all seat posts have some minimum insertion limits
important? Given that the seat clamp tension bolt "absorbs the
tension"?
The minimum insertion limit is so that the seatpost doesn't come busting thru the seat tube at its (the seatpost's) bottom edge, not having enough leverage, when pivoting where it exits the seattube, when lateral load is applied at the top of the post.

Remove the pinch bolt, push sideways on the seat, or even forward or back, and the ears that the pinch bolt goes thru will be spread a little bit, right? In other words, the post is trying to make the opening at the top of the seattube an oval, by placing tension on the walls, at the opening, from the inside, right?

I suggest that if tight, the pinch bolt will absorb this tension, which then cannot be experienced by the tube itself, until it exceeds the tension in the pinch bolt.

Of course the seattube might be butted, which will mess things up three more ways; the I.D. is larger so the quill clamp won't work as well, and the quill clamp might land right where the butt is, where the wall is not parallel, and also if the quill clamp clamps to where the wall is thinner in the middle, then it is at a point where the wall thickness was already deemed by the manufacturer to be too thin to be used for anything other than stretching between the ends of the tube.

And yes, I'm making this up as I go, as if I need to tell anyone here that, and am qualified to assert none of it.



A double butted seat tube has a smaller ID at the top, not
larger. In traditional format that's 26.8mm post versus 27.2mm

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On 12/3/2016 2:13 AM, John B. wrote:
On Fri, 02 Dec 2016 21:07:37 -0600, AMuzi wrote:

On 12/2/2016 5:58 PM, John B. wrote:
On Fri, 02 Dec 2016 16:36:45 -0600, AMuzi wrote:

On 12/2/2016 3:04 PM, Gregory Sutter wrote:
On 2016-12-02, Joerg wrote:

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Joerg,

99.9+% of all bicycles, made for all uses and types of loads, clamp
the seatpost at the top. I submit that you are on a wild goose
chase, and have several misconceptions.

A longer post usually means nothing, as the only area of the frame
that clamps down on the seatpost is the top 7-12cm. The post just
has to be inserted to its minimum-insertion line.

For similar reasons the quill seatpost is probably not a good idea
for your frame: the seat tube won't be shimmed as far down as the
quill will clamp, and isn't meant to take the forces that the quill
will apply.

On the seatpost clamp, too tight is often worse than too loose,
because while too loose results only in slipping, too tight can
damage the clamp (strip threads or snap the band), the seatpost
(crimping) or possibly even the frame. If the creaking you hear
has recently started, perhaps it is because the post-clamp interface
is no longer correct due to overtightening or an impact.

(Anecdote time: I once lightly dented a Thompson post (!) at the
clamp from an impact while riding. My butt didn't like it either.)

Therefore I recommend to you that you ensure your existing seatpost
clamp is still round and smooth; that your seatpost in the clamped
area is the same; that you grease the threads on the clamp's bolt;
that you lightly grease the seatpost where it touches the clamp and
the frame shim; and that you use a torque wrench when you tighten the
clamp.


All good points but I'll add that in modern thinwall steel
TIG frames, seatpost flex can be a significant problem.
Builders now recommend post insertion 2 diameters below the
lowest weld as excessive flex can lead to weld cracks.

Seat Post flex, or Seat Tube flex?


Pithy observation.
Seat post flex would have no significance if not for seat
tube flex in the hard areas adjacent to the weld

http://www.yellowjersey.org/photosfr...st/wiltaxi.jpg

Interestingly cracks these thin hard steels seem to progress
perpendicular to the weld bead.

Theoretically at least the weld bead should be the stronger than the
tubing so it might be possible that it is tube itself failing. Perhaps
due to over heating during welding.

It isn't supposed to work that way but in any production process there
is an emphasis on higher production and one way to weld faster is to
raise the amperage and increase the travel speed.



Stronger is perhaps a one dimensional description in that
the weld and surrounding heat affected zones have bands
which are usually less ductile.

https://qph.ec.quoracdn.net/main-qim...to_webp=t rue

short answer = it's complex.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On 2016-12-02 10:03, AMuzi wrote:
On 12/2/2016 11:38 AM, Joerg wrote:
Folks,

In the old days they were available but can't find modern
ones for MTB.

https://www.youtube.com/watch?v=5EkigpkVXLY

Fixing a seat post only at the top (kind of) works for
regular bikes that aren't used on harsh turf but not for MTB
with heavy loads. Mine started creaking to no end and that's
probably not good for the frame. So I am looking for a
longer seat post but ideally with quill.

Any hints who still carries that?


We make them. We didn't come up in your web search?


Surprisnigly not, I keyed in the string "seat post" and then "quill".


http://www.yellowjersey.org/yampost.html


Thanks! A bit pricey but since you guys make every post in the shop
that's to be expected. I was hoping to find one from stock since I had
one on the 80's on a used bike but I guess they are no longer made. IIRC
it was from Japan.

My MTB buddy is a machinist, maybe he can machine a cone and I can slit
a standard post for an inch or so. It is just to prevent the slight
rocking. If this doesn't work it'll be time for a Yellowjersey order.

--
Regards, Joerg

http://www.analogconsultants.com/

On 2016-12-02 16:38, John B. wrote:
On Fri, 02 Dec 2016 09:38:30 -0800, Joerg
wrote:

Folks,

In the old days they were available but can't find modern ones for MTB.

https://www.youtube.com/watch?v=5EkigpkVXLY

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Any hints who still carries that?

Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?


It needs to be clamped at the top _and_ the bottom. If you clamp it only
in one place there will always be an ever so slight rocking. This
produces creaking, wear and most annoyingly a gradual sagging.


Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.


"Minimal" is the key word here. It cannot be zero and even if it was it
would wear with use. A minimal gap is required in order to get the seat
post in and out and also because of the usual factory tolerances. This
causes minimal rocking back and forth. A quill at the bottom in addition
to the usual clamp at the top will avoid that. Not a new concept, I had
that on an old bike.

Aside from the creaking, no matter how hard I adjusted the top clamp the
only way I could prevent a gradual sag when riding a lot of washboard
turf was to smear toothpaste around the top of the post where it clamps.
Not the gel type for electric brushing but the regular stuff with some
grit in there. Coincidentally that's also how I prevented the MTB stem
from coming loose without exceeding the 5Nm torque limit.

--
Regards, Joerg

http://www.analogconsultants.com/

On Sun, 04 Dec 2016 08:11:18 -0800, Joerg
wrote:

On 2016-12-02 16:38, John B. wrote:
On Fri, 02 Dec 2016 09:38:30 -0800, Joerg
wrote:

Folks,

In the old days they were available but can't find modern ones for MTB.

https://www.youtube.com/watch?v=5EkigpkVXLY

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Any hints who still carries that?

Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?


It needs to be clamped at the top _and_ the bottom. If you clamp it only
in one place there will always be an ever so slight rocking. This
produces creaking, wear and most annoyingly a gradual sagging.


Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.


"Minimal" is the key word here. It cannot be zero and even if it was it
would wear with use. A minimal gap is required in order to get the seat
post in and out and also because of the usual factory tolerances. This
causes minimal rocking back and forth. A quill at the bottom in addition
to the usual clamp at the top will avoid that. Not a new concept, I had
that on an old bike.

Aside from the creaking, no matter how hard I adjusted the top clamp the
only way I could prevent a gradual sag when riding a lot of washboard
turf was to smear toothpaste around the top of the post where it clamps.
Not the gel type for electric brushing but the regular stuff with some
grit in there. Coincidentally that's also how I prevented the MTB stem
from coming loose without exceeding the 5Nm torque limit.

As you mention a friend who is a machinist I suggest that the only
problem with making a seat post is how to tighten the bottom clamp. A
through bolt running from the top of the post to the clamp nut at the
bottom is certainly doable just as the old 1 inch quill stems. Added
to a conventional "top clamp" that should hold the seat post even if
an elephant should sit on it.

The problem comes about because the seat covers the top of the seat
post and makes tightening the quill bold difficult but given that once
the "correct" seat position is determined one doesn't usually
re-adjust he seat frequently it should be easy enough. Just install
the seat using the normal clamp and adjust to fit. Then remove the
seat, tighten the quill bolt and reinstall the seat.
--
cheers,

John B.

On 12/4/2016 7:12 PM, John B. wrote:
On Sun, 04 Dec 2016 08:11:18 -0800, Joerg
wrote:

On 2016-12-02 16:38, John B. wrote:
On Fri, 02 Dec 2016 09:38:30 -0800, Joerg
wrote:

Folks,

In the old days they were available but can't find modern ones for MTB.

https://www.youtube.com/watch?v=5EkigpkVXLY

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Any hints who still carries that?

Why is clamping the seat post with a clamp at the top actually
different from clamping the seat post at the bottom?


It needs to be clamped at the top _and_ the bottom. If you clamp it only
in one place there will always be an ever so slight rocking. This
produces creaking, wear and most annoyingly a gradual sagging.


Theoretically at least the seat post is supposed to be sliding fit in
the seat tube with minimal play so clamping at one end is very much
the same as clamping at the other.


"Minimal" is the key word here. It cannot be zero and even if it was it
would wear with use. A minimal gap is required in order to get the seat
post in and out and also because of the usual factory tolerances. This
causes minimal rocking back and forth. A quill at the bottom in addition
to the usual clamp at the top will avoid that. Not a new concept, I had
that on an old bike.

Aside from the creaking, no matter how hard I adjusted the top clamp the
only way I could prevent a gradual sag when riding a lot of washboard
turf was to smear toothpaste around the top of the post where it clamps.
Not the gel type for electric brushing but the regular stuff with some
grit in there. Coincidentally that's also how I prevented the MTB stem
from coming loose without exceeding the 5Nm torque limit.

As you mention a friend who is a machinist I suggest that the only
problem with making a seat post is how to tighten the bottom clamp. A
through bolt running from the top of the post to the clamp nut at the
bottom is certainly doable just as the old 1 inch quill stems. Added
to a conventional "top clamp" that should hold the seat post even if
an elephant should sit on it.

The problem comes about because the seat covers the top of the seat
post and makes tightening the quill bold difficult but given that once
the "correct" seat position is determined one doesn't usually
re-adjust he seat frequently it should be easy enough. Just install
the seat using the normal clamp and adjust to fit. Then remove the
seat, tighten the quill bolt and reinstall the seat.

Sounds workable to me.

--
- Frank Krygowski

On Friday, December 2, 2016 at 12:38:22 PM UTC-5, Joerg wrote:
Folks,

In the old days they were available but can't find modern ones for MTB.

https://www.youtube.com/watch?v=5EkigpkVXLY

Fixing a seat post only at the top (kind of) works for regular bikes
that aren't used on harsh turf but not for MTB with heavy loads. Mine
started creaking to no end and that's probably not good for the frame.
So I am looking for a longer seat post but ideally with quill.

Any hints who still carries that?

--
Regards, Joerg

http://www.analogconsultants.com/

SURFACE AREA. MIX LINSEED WITH ALUMINUM ANTI SEIZE APPLY TO TUBE. if SEAT POST SLIPS TAKE POST OUT N ADD A DOLLOP OF CLEAN SAND.