Mechanics Of Seatpost Clamping
bicycle_disciple
this link : http://bit.ly/7zdQu5.
I was encouraged to highlight the kinds of forces acting when a
cyclist tightens the clamp screw due to seeing another Thomson
seatpost failure. This one was at the site of the clamping so
hopefully my article is relevant to the situation.
Jobst and others here are invited will read my material and critique/
discuss if need be.
Jay Beattie
wrote:
Gads, that post looks like it has a 2mm wall thickness. I would think
that a scratch at the collar combined with any clamping force could
cause that thing to fail -- assuming the wall is actually as thin as
it appears in the picture and that it was a long post run out to the
usual heights we see these days. -- Jay Beattie.
Lou Holtman
2 mm? That is extremely think these days.
Lou
Bill Sornson
> 2 mm? That is extremely think these days.
Thick so?
Bill "sorry, couldn't resist" S.
bicycle_disciple
As we're on the topic of word jokes, I was doing some research on
"screw loads" before writing that article. Unexpectedly, typing the
words in exactly that sequence into my browser yielded some
*interesting* results. :)
Leo Lichtman
"bicycle_disciple" wrote: (clip) I was doing some research on
"screw loads" (clip)
^^^^^^^^^^^^^^^^^^^
Morning-after prayer: "Please, dear Lord, unscrew me."
thirty-six
The overtightening comes as the result of using grease which allows
the post to slip despite greater pressures. Using thin oil lets it be
squeezed out allowing the clamp to grip the post adequately at lower
pressures. You only need to tighten the clamp sufficiently that you
cannot twist the saddle using reasonable force. Using thin oil in the
seat tube and on the seat pin keeps the required clamping force low
and failures unlikely. If you wish to eseal the seat tube from water
ingress, drip molten wax around the top and the pinch area.
bicycle_disciple
Thanks thirty-six. The greasing could be a likely issue while
torquing. Do you feel the a similar kind of situation happens when
using seatpost spacers?
I have noticed from personal experience that given the same amount of
torque, a seatpost with a spacer is still likely to slip which makes
you want to give it a few more extra turns, exceeding torque specs.
thirty-six
wrote:
> On Dec 31, 4:58 pm, thirty-six <thirty-...@live.co.uk> wrote:
>
>
>
> > On 31 Dec, 14:23, bicycle_disciple <1.crazyboy.o...@gmail.com> wrote:
>
> > > I explore the mechanics of seatpost clamping and you can read it at
> > > this link :http://bit.ly/7zdQu5.
>
> > > I was encouraged to highlight the kinds of forces acting when a
> > > cyclist tightens the clamp screw due to seeing another Thomson
> > > seatpost failure. This one was at the site of the clamping so
> > > hopefully my article is relevant to the situation.
>
> > > Jobst and others here are invited will read my material and critique/
> > > discuss if need be.
>
> > The overtightening comes as the result of using grease which allows
> > the post to slip despite greater pressures. Using thin oil lets it be
> > squeezed out allowing the clamp to grip the post adequately at lower
> > pressures. You only need to tighten the clamp sufficiently that you
> > cannot twist the saddle using reasonable force. Using thin oil in the
> > seat tube and on the seat pin keeps the required clamping force low
> > and failures unlikely. If you wish to eseal the seat tube from water
> > ingress, drip molten wax around the top and the pinch area.
>
> Thanks thirty-six. The greasing could be a likely issue while
> torquing. Do you feel the a similar kind of situation happens when
> using seatpost spacers?
HAPPY NEW YEAR!
I guess you mean a filler piece between post and tube. In which case,
yes it will still happen. Again, you dont want lubrication here.
Come to think of it, I have seen seat tubes which were painted inside
after reaming. I think my own race frame is such an example, it has
not corroded or required lubricating. With the pinch bolt released
the seat pin will slide up and down with less than 20lb of force (dont
know precisely, its been some time). I think I have only had the pin
out once to remove a burr at the pinch point. If you look at the
typical hex key size used here, it is a 5mm with a lever around 100mmm
or 4". This is the length I use (or thereabouts) with a regular hex
nut and spanner and using thin oil as a thread lubricant. If you use
a thin oil and appropriate length spanner then you should feel the
thread sticking as it approaches the correct torque. You can stop as
soon as you feel it stick/slip when turning slowly. It's no good
backing it off after you have broken it, that is not acceptable UK
practise. Especially on a public holiday.
bicycle_disciple
Dear Thirty-Six,
About your comment on not using grease, Thomson has a different stand
on the issue. Thomson's instructions for both stems and seatposts call
for grease, not anti-seize, ONLY on the threads of the bolts.
According to them, greasing under the heads of the bolt is not needed
and can greatly change the amount of force generated by the bolt at
our recommended torque, this can lead to premature failure of your
components. With all Thomson products they plead with the user to take
special care to grease only the threads of the bolt and use a torque
wrench.
What thin oil are you suggesting?
BD
thirty-six
They are arguing the same point from a different angle. If , as they
suggest, you do not provide temporary lubrication under the bolt head
it may tear the the surface upon which it seats. Providing a MACHINE
oil on the thread and under the head prevents damage and allows the
bolt to stay secure with the correct torque to provide adequate
clamping force to the dry seatpin.
Ben C
> On 1 Jan, 23:52, bicycle_disciple <1.crazyboy.o...@gmail.com> wrote:
>> About your comment on not using grease, Thomson has a different stand
>> on the issue. Thomson's instructions for both stems and seatposts call
>> for grease, not anti-seize, ONLY on the threads of the bolts.
>> According to them, greasing under the heads of the bolt is not needed
>> and can greatly change the amount of force generated by the bolt at
>> our recommended torque, this can lead to premature failure of your
>> components. With all Thomson products they plead with the user to take
>> special care to grease only the threads of the bolt and use a torque
>> wrench.
>>
>> What thin oil are you suggesting?
>
> They are arguing the same point from a different angle. If , as they
> suggest, you do not provide temporary lubrication under the bolt head
> it may tear the the surface upon which it seats.
Also you can round out the hexagonal slot in the bolt head before it's
tight enough to stop the seat slipping down.
Ron
Is that sarcasm or something valuable to the discussion?
-Ron
Ben C
Intended to be the latter. The point is if you don't grease under the
bolt head, it can feel like you've done the bolt up much tighter than
you have because it's just sticking against the collar.
Then the seat keeps falling down, so you do it up tighter, to the point
where you've rounded out the bolt head.
I'm surprised therefore at Thomson's instructions. Seems to me it's more
sensible to specify all torque settings on the assumption that
everything is greased.
thirty-six
If you use grease rather than oil to grease your parts then something
will get damaged if the post is retained fully during riding. I have
used grease on seatposts and they slip, so out it comes, is cleaned
and put in dry or with a thin oil and there is no further problem.
Handlebar stems are not so fussy, a little give can be usefull to
mitigate injury in the event of a collision. I set handlebars so that
I can force them to a new position without resorting to tools.
Ron
Thomson maintains that no grease is to be used under the bolt heads as
they set torques assuming a dry bolt head.
-Ron
Ron
Interesting. However, Thomson has it that machine oil has a different
co-efficient of friction than the grease they use to set torques with.
Machine oil is not to be used, they maintain. But you seem to have a
history of it working well.
-Ron
thirty-six
It is an odd request. As long as the fastener in moving at a
reasonable speed the reduction of friction is adequate when using
machine oil on a well finished thread. It has the advantage of the
fastener holding without resort to over tensioning the bolt or resort
to a nut retaining device. I seriously don't know the torque on my
seat pin bolts, just that they are enough and probably little more. I
don't carry a torque wrench on a ride and I may need to adjust saddle
height, so I use technique that uses what I can carry. If everything
fits perfectly you may get away with slapping grease on everything as
long as it is not a lightweight pin but otherwise experience shows
that light oil and a judicious use of the spanner works better in more
cases and also works on a ride.
I don't use Thomson grease. A manufacturer or professional mechanic
may wish to use Thomson grease in order to transfer liability to
Thomson. I prefer, considering liability completely rests with me (as
far as I'm concerned) on my equipment, that threads are checked to be
clean running with oil.