Quill seat posts for MTB?

[Joerg]

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/

[AMuzi]

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?

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


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

[Gregory Sutter]

On 2016-12-02, AMuzi <a...@yellowjersey.org> wrote:
> On 12/2/2016 11:38 AM, Joerg wrote:
>> So I am looking for a longer seat post but ideally with quill.
>

> We make them. We didn't come up in your web search?
>
> http://www.yellowjersey.org/yampost.html

Nice. I'll remember that the next time I get the urge to buy
one of those sweet Moser Leader AX frames.

--
Gregory S. Sutter Mostly Harmless
mailto:gsu...@zer0.org
http://zer0.org/~gsutter/

[Gregory Sutter]

On 2016-12-02, Joerg <ne...@analogconsultants.com> 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.

[AMuzi]

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.

[John B.]

Seat Post flex, or Seat Tube flex?
--
cheers,

John B.

[John B.]

On Fri, 02 Dec 2016 09:38:30 -0800, Joerg <ne...@analogconsultants.com>
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?

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.
--
cheers,

John B.

[Doug Landau]

- 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

[John B.]

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"?
--
cheers,

John B.

[AMuzi]

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/photosfromthepast/wiltaxi.jpg

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

[Doug Landau]

> >> 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.

[John B.]

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.

[John B.]

On Fri, 2 Dec 2016 19:09:33 -0800 (PST), Doug Landau
<doug....@gmail.com> 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.

[AMuzi]

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

[AMuzi]

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-qimg-ef947d7deb397ff004d57bd5d096876a?convert_to_webp=true

short answer = it's complex.

[Joerg]

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.

[Joerg]

On 2016-12-02 16:38, John B. wrote:
> On Fri, 02 Dec 2016 09:38:30 -0800, Joerg <ne...@analogconsultants.com>
> 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.

[John B.]

On Sun, 04 Dec 2016 08:11:18 -0800, Joerg <ne...@analogconsultants.com>
wrote:

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.

[Frank Krygowski]

Sounds workable to me.

--
- Frank Krygowski

[DATAKOLL MARINE RESEARCH]

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.

[John B.]

You know, there is another solution. Simply drill a series of holes in
the seat post, say 1/2" apart. A hole through the seat tube and Bob's
your uncle. Use a 1/4" grade 8 bolt, maybe 735 pounds tensile
strength, probably in the neighborhood of 440 pounds sheer strength.

If one used a "spring cotter" rather than a nut it could even be field
adjustable without tools.

--
cheers,

John B.

[DATAKOLL MARINE RESEARCH]

Bolt no good...no surface area (s)

[Joerg]

That is exactly what I was planning to do if I can't find an
off-the-shelf quill post.

>>> 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.
>

I'll see if there would be a spot in the top plate that can be drilled
without compromising its integrity. The cone that spreads apart the
newly slit section of the post bottom does not need much force if
shallow-angled and greased. So a 4mm or similar all-thread should suffice.

The MTB is only ridden by me and due to electrical wiring and such the
seat height can't easily be adjusted anyhow. So I'd just mount it like
usual, mark the post and then do the mods. That way the all-thread
pulling in the cone can be tightened and shortened while the seat is
off. If can still be adjusted up and down a bit but the seat would
always have to come off in order to access the tightening nut on the
all-thread.

> You know, there is another solution. Simply drill a series of holes in
> the seat post, say 1/2" apart. A hole through the seat tube and Bob's
> your uncle. Use a 1/4" grade 8 bolt, maybe 735 pounds tensile
> strength, probably in the neighborhood of 440 pounds sheer strength.
>

I am not going to drill the frame :-)

Voids the warranty and who knows what else it could do.

> If one used a "spring cotter" rather than a nut it could even be field
> adjustable without tools.
>

A spring latch like on walkers or market umbrellas would prevent the
post from sliding down but not from rocking. Rocking is the key problem
I have, resulting in nasty creaking and probably wear in the frame.

[Doug Landau]

On Saturday, December 3, 2016 at 12:30:15 AM UTC-8, John B. wrote:
> On Fri, 2 Dec 2016 19:09:33 -0800 (PST), Doug Landau
> >

> >> >> 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.

The pinch bolt, once under tension, will prevent whatever it is compressing from experiencing tension from being pushed from the inside. In this case, the lip and upper end of the seat tube.

>Can't be any other way.

You are not understanding. https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=pretensioned+steel+

[DATAKOLL MARINE RESEARCH]

due to electrical wiring ? the bike or yours ? use the sand stupid

[Doug Landau]

If it bears weight (your pin), it'll ovalize the holes in the seat tube, and be loose and rattle. It will bear weight because, as Gene pointed out, its flat head can't hold against the round tube.

[Doug Landau]

1/4" ?!? Way overkill. The shear strength of a #6 screw is >1500 Lbs

[DATAKOLL MARINE RESEARCH]

WELL, I have seen this bolt in post n the bolt bends, the hole ovals...no bearing surface area. 'Like' the post comes loose so....? where's one on the floor off the rack.... hole bolt post ?

Now here's a wannabe mountain ear who goes on abt hammering pins with rocks but a dollop of sand into the post lube ?

[DATAKOLL MARINE RESEARCH]

1/4" ?!? Way overkill. The shear strength of a #6 screw is >1500 Lbs

a .25 inch bolt in grade 5 is not taking 1500 pounds the way ura gonna use it in the field.

I doahn have the equation for this but that bolt under 75 pounds over 10 miles/700 miles will fail. more surface area. gotta replace bolt.

the old saw is a 6 legged sofa under 4 fat women...calculate pressure under sofa feet vs shinylvinyl/carpet or hardwood flooring.

[John B.]

On Mon, 05 Dec 2016 07:29:09 -0800, Joerg <ne...@analogconsultants.com>

I would really suggest something larger. Most
quill" stems use about a 6 mm bolt so perhaps there is a reason :-)

Another point, you seem to be talking about a hollow seat post, split,
with a cone shaped wedge to spread the bottom of the post. This is
going to take a hammer to drive out if you ever try to move the seat
post and I doubt that a 4mm bolt the length of the seat post is going
to suffice.

>The MTB is only ridden by me and due to electrical wiring and such the
>seat height can't easily be adjusted anyhow. So I'd just mount it like
>usual, mark the post and then do the mods. That way the all-thread
>pulling in the cone can be tightened and shortened while the seat is
>off. If can still be adjusted up and down a bit but the seat would
>always have to come off in order to access the tightening nut on the
>all-thread.
>
>
>> You know, there is another solution. Simply drill a series of holes in
>> the seat post, say 1/2" apart. A hole through the seat tube and Bob's
>> your uncle. Use a 1/4" grade 8 bolt, maybe 735 pounds tensile
>> strength, probably in the neighborhood of 440 pounds sheer strength.
>>
>
>I am not going to drill the frame :-)
>
>Voids the warranty and who knows what else it could do.
>
>
>> If one used a "spring cotter" rather than a nut it could even be field
>> adjustable without tools.
>>
>
>A spring latch like on walkers or market umbrellas would prevent the
>post from sliding down but not from rocking. Rocking is the key problem
>I have, resulting in nasty creaking and probably wear in the frame.
--

cheers,

John B.

[John B.]

Nope, if pushed from the inside the post will be under compression,
not tension.

>>Can't be any other way.
>You are not understanding. https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=pretensioned+steel+
>

--
cheers,

John B.

[John B.]

Good Lord! A flat surface and a round tube. An insurmountable
problem.... except that the problem was solved several hundred years
ago with shaped washers. Flat on one side, and concave to fit the
tube, on the other.
--
cheers,

John B.

[DATAKOLL MARINE RESEARCH]

uuuuuuuuuuuuuuuu

Nope, if pushed from the inside the post will be under compression,
not tension.

why then yawl need a thicker post !

non forsit

[Doug Landau]

On Monday, December 5, 2016 at 4:06:00 PM UTC-8, John B. wrote:
> On Mon, 5 Dec 2016 09:19:31 -0800 (PST), Doug Landau

I am talking about the seat tube. Remove the pinch bolt, wiggle the seatpost, and you will see it spread the ears that the pinch bolt pinches open a bit, and expand the diameter of the seat tube just a tiny bit at the top. Replace the pinch bolt and that can't happen, because the pinch bolt won't allow any expansion.

[John B.]

I think you better look again. From the tables I read a #6 screw
http://www.derose.net/steve/resources/engtables/bolts.html
has a proof load of between 55 and 85 KSI - which will be tensile
strength. The root area of the thread is 0.0091 square inches so the
tensile strength of the screw is ~637 pounds and sheer strength is
usually estimated at 60% of tensile strength so the actual sheer
strength will be ~380 pounds.
--
cheers,

John B.

[Doug Landau]

On Monday, December 5, 2016 at 4:32:57 PM UTC-8, John B. wrote:
> On Mon, 5 Dec 2016 11:21:32 -0800 (PST), Doug Landau

If the diameter of said #6 machine screw is .13", then the shear strength of one made from 50,000 PSI steel is ((0.13x0.13)x3.1415) / 50000, which is 2654.

[AMuzi]

Well, no.
Threaded fastener yield strength is taken at the minor
(root) diameter:

http://engineersedge.com/screw_threads_chart.htm

more:
http://us.misumi-ec.com/pdf/press/us_12e_pr1271.pdf

Also, you are assuming such a setup would be in pure shear
which may or may not be the case.

[Doug Landau]

Thanks. I thought something was wrong there. subbing 0.1 for 1.3 yields the expected 1570.

[DATAKOLL MARINE RESEARCH]

OK the field tested lots were standard Ace/TVal 6mm with shank bearing load bottom plywood rear rack carried 60-75 pounds to Raleigh carbon steel eyelets.

Bolts cracked at first threads 5-700 miles. Once every q2-18 months

Upgraded to 5's, same result longer use time...not much.

[John B.]

:-) You might be talking about the seat tube but you keep saying "pinch bolt" :-)
--
cheers,

John B.

[John B.]

If you won't mind the correction. While the nominal O.D. of a #6 screw
is 0.1380 the calculation for area of a circle is Radius squared times
Pi so the correct numbers are 0.069 x 0.069 x 3.1415 = 0.01495. and
that number times 50,000 = 747.

Unfortunately a screw has a thread cut on it and the diameter that
matters as far as strength goes is the root diameter of the thread
which is 0.0989 so the real calculations would be 0.04945 x 0.04945 x
3.1415 = 0.0076. Times 50,000 = 348.

The number "50,000" when applied to steel usually refers to the
tensile strength and the usual rule of thumb for sheer strength is 60%
of tensile strength so we end up with 348 x .6 = 230 lbs.

Hopefully you are not in the bridge design business :-)
--
cheers,

John B.

[Joerg]

On stemps it's different. The quill must be torqued in hard so the
handlebar won't come lose. On the seat all I want is to arrest the
slight back and forth rocking due to the clearance between seat post and
tube.

> Another point, you seem to be talking about a hollow seat post, split,
> with a cone shaped wedge to spread the bottom of the post. This is
> going to take a hammer to drive out if you ever try to move the seat
> post and I doubt that a 4mm bolt the length of the seat post is going
> to suffice.
>

Good point. Though I'd grease up the cone, all-thread and everything
down in there. The top where the regular seat post clamp sits would be
"toothpasted" as usual so it won't sag.

However, if it really got stuck in there that would be bad. Unlikely
that I'd ever have to adjust but who knows, with advancing age people
shrink.

[Frank Krygowski]

Yes, Doug used the formula "Area = pi * diameter^2", instead of the
correct " Area = pi * radius^2" and he also used tensile strength
instead of the correct shear strength. As John says, shear strength is
much lower.

--
- Frank Krygowski

[Doug Landau]

On Tuesday, December 6, 2016 at 3:02:52 AM UTC-8, John B. wrote:
> On Mon, 5 Dec 2016 17:35:10 -0800 (PST), Doug Landau
>

Right you are.

> Unfortunately a screw has a thread cut on it and the diameter that
> matters as far as strength goes is the root diameter of the thread
> which is 0.0989 so the real calculations would be 0.04945 x 0.04945 x
> 3.1415 = 0.0076. Times 50,000 = 348.
>
> The number "50,000" when applied to steel usually refers to the
> tensile strength

Actually it is yield strength

> Hopefully you are not in the bridge design business :-)

Likewise. Go ahead and put a bolt through your seat tube; I predict it will rattle after the first pothole.

[Sir Ridesalot]

If the seatpost is clamped at the bottom you'll still get some rocking which is part of the reason why threadless stems were invented and are used on so many MTBs today especially those designed for hard downhill use. You'll make your modified seatopost and either snap it off or you'll be back here complaining that the seatpost is rocking. Since you know exactly what height you need the saddle at why don't you make a steel seatpost or if your frame is aluminium get whatever seatpost you have welded to the frame? Better yet, get a dirt bike and convert it to pedal power and all your troubles will magically disappear; after all weiight's not object to you.

Cheers

[DATAKOLL MARINE RESEARCH]

There are shims for under clamp.

Is the post wong sized as installed ? or is the post another ibreakeverythingtoughguypost ?

If the post moves laterally from ovalization try sand shims n consulate a Yellow Jersey level LBS ...ura not curing ovalization with obsolete tech

[DATAKOLL MARINE RESEARCH]

More length is more contact suck area, same as ionic transfer area tween steel/Al.

Where linseed/AL antiseize/sand grip filler stops slippage ...from the linseed/Al..

But not for ovalized rocking.

I doahn see where post rocks n not slips.

Are mtb posts wearing even with maintenence n install procedures ?

wuch I yield ...

[Doug Landau]

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?

[Doug Landau]

On Tuesday, December 6, 2016 at 1:57:30 AM UTC-8, John B. wrote:
> On Mon, 5 Dec 2016 16:16:30 -0800 (PST), Doug Landau
>

I said it correctly. Remove the pinch bolt, wiggle the seatpost, and watch it expand the top of the seat tube. If the pinch bolt is in place and tight that can't happen; the ears can't move farther apart, so the diameter cannot become greater in any horizontal direction.

[Doug Landau]

On Friday, December 2, 2016 at 7:07:42 PM UTC-8, AMuzi wrote:
> On 12/2/2016 5:58 PM, John B. wrote:
> > On Fri, 02 Dec 2016 16:36:45 -0600, AMuzi <a...@yellowjersey.org> wrote:
> >
> >> On 12/2/2016 3:04 PM, Gregory Sutter wrote:

> >>> On 2016-12-02, Joerg <ne...@analogconsultants.com> 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/photosfromthepast/wiltaxi.jpg
>
> Interestingly cracks these thin hard steels seem to progress
> perpendicular to the weld bead.
>

Regarding the pic in the upper left: Is that from seattube bending, arching forward?

[DATAKOLL MARINE RESEARCH]

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

[AMuzi]

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.

[AMuzi]

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.

[Frank Krygowski]

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

[DATAKOLL MARINE RESEARCH]

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

[John B.]

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