Quill seat posts for MTB?

The Muzi fix is remarkable. We imagine considerable owner handwringing before development.

On 12/7/2016 1:52 AM, Doug Landau wrote:
On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote:
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

How deep does the thicker part at the top?


Any dimension you specify to the tube mill, probably.

The standard Tange Prestige traditional OD (28.6mm) DB seat
tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm
at the (trim this end) top of a 650mm. Champion CrMo touring
tubes the same except thicker wall. That said, every brand
offers optional length tubes, various materials, diameters,
shapes and gauges besides being very welcome to special
orders at some healthy manufacturer not hobbyist volume.

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

On 12/7/2016 7:45 AM, DATAKOLL MARINE RESEARCH wrote:
The Muzi fix is remarkable. We imagine considerable owner handwringing before development.


Fix? Women have viewed me as a fixer-upper project all my
life. I remain unreconstructed.

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

On 12/7/2016 10:00 AM, AMuzi wrote:
On 12/7/2016 1:52 AM, Doug Landau wrote:
On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote:
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

How deep does the thicker part at the top?


Any dimension you specify to the tube mill, probably.

The standard Tange Prestige traditional OD (28.6mm) DB seat tube of
0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this
end) top of a 650mm. Champion CrMo touring tubes the same except thicker
wall. That said, every brand offers optional length tubes, various
materials, diameters, shapes and gauges besides being very welcome to
special orders at some healthy manufacturer not hobbyist volume.

FWIW: Our tandem was custom built (oversized Reynolds 531) back in
1979. Among the things I didn't really like was this: the builder
brazed a shim inside the top of each seat tube to reduce the ID to the
size of the seatpost. That shim is only an inch or so long (axially
within the tube).

I was afraid it would give problems with the seatpost rocking, but it
never has. He did skimp on the width of the clamping slot at the back
of the seat tube, such that the slot closed just before firmly clamping
the seatpost. A hard tug on the stoker's handlebars (say, when carrying
the bike or loading onto a rack) could slightly pivot the captain's
seatpost. I eventually fixed that by widening that slot with a saw. But
otherwise, the shim kludge has worked fine.

--
- Frank Krygowski

On Wednesday, December 7, 2016 at 10:38:06 AM UTC-5, Frank Krygowski wrote:
On 12/7/2016 10:00 AM, AMuzi wrote:
On 12/7/2016 1:52 AM, Doug Landau wrote:
On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote:
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

How deep does the thicker part at the top?


Any dimension you specify to the tube mill, probably.

The standard Tange Prestige traditional OD (28.6mm) DB seat tube of
0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm at the (trim this
end) top of a 650mm. Champion CrMo touring tubes the same except thicker
wall. That said, every brand offers optional length tubes, various
materials, diameters, shapes and gauges besides being very welcome to
special orders at some healthy manufacturer not hobbyist volume.

FWIW: Our tandem was custom built (oversized Reynolds 531) back in
1979. Among the things I didn't really like was this: the builder
brazed a shim inside the top of each seat tube to reduce the ID to the
size of the seatpost. That shim is only an inch or so long (axially
within the tube).

I was afraid it would give problems with the seatpost rocking, but it
never has. He did skimp on the width of the clamping slot at the back
of the seat tube, such that the slot closed just before firmly clamping
the seatpost. A hard tug on the stoker's handlebars (say, when carrying
the bike or loading onto a rack) could slightly pivot the captain's
seatpost. I eventually fixed that by widening that slot with a saw. But
otherwise, the shim kludge has worked fine.

--
- Frank Krygowski

lot to be said on an adjustment allowance for play when butted hard.

On Wed, 07 Dec 2016 09:00:47 -0600, AMuzi wrote:

On 12/7/2016 1:52 AM, Doug Landau wrote:
On Saturday, December 3, 2016 at 5:34:10 AM UTC-8, AMuzi wrote:
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

How deep does the thicker part at the top?


Any dimension you specify to the tube mill, probably.

The standard Tange Prestige traditional OD (28.6mm) DB seat
tube of 0.7-0.4-0.7 has 80mm at the (uncut) bottom and 180mm
at the (trim this end) top of a 650mm. Champion CrMo touring
tubes the same except thicker wall. That said, every brand
offers optional length tubes, various materials, diameters,
shapes and gauges besides being very welcome to special
orders at some healthy manufacturer not hobbyist volume.

The Columbus "LIFE" tube set even offers an option of a seat tube that
is double butted but the upper end is enlarged on the outside to
furnish a straight 27.4mm I.D. nearly the entire length of the tube
:-)

--
cheers,

John B.