Carbon seat post recommendation ?

And no, I'm basing it on on the fact that the seatpost does not deflect
enough to cause a difference in "feel", or damping.


Actually, on a compact bike, the seatpost can deflect quite a lot.
Unlike the tubes of a diamond frame (of any material), a long seatpost
sticking up all by its lonseome has no triangulated bracing to prevent
deflection. The more seatpost there is (and the more layback at the
head), the more it deflects.

Whether this is more pronounced with a carbon post, I have no opinion
(although I would imagine it varies with the post manufacturer &
stiffness/wall thickness of the post). Speaking from experience,
however, lighter (thinner walled) aluminum seatposts exhibit far more
deflection/damping than heavier (thicker walled) posts. The one caveat
(more pronounced on sloping TT hardtail MTB than road) is that too much
exposed seatpost may act like a softride beam, launching an
unsuspecting seated rider going over a sufficiently large bump. Ask me
how I know...

SYJ

On Mon, 21 Nov 2005 23:40:10 GMT, "Jay S. Hill"
wrote:

rs wrote:
I've got a low-end road bike, Giant OCR3 and would like to get a little more
damping, thinking a carbon seat post would help. Obviously I don't need the
lightest but is there a brand recommendation for a fairly inexpensive and
decent one for a road bike?

Nyet. They're too easy to crush, they're expensive, and they aren't
going to help with damping. Get slightly larger tires and run them at
slightly lower air pressure.

Have you ever tried crushing a CF seatpost?

I got a deal on an Easton EC70 CF seatpost that was 350mm. It was
much longer than I needed. After cutting it, I tried damaging the
piece that I removed. I couldn't crush it, or even get it to crack. CF
is some strong stuff.

I'm sure under some circumstances they can be crushed, but not easily.

As for damping, the combination of CF bars and post, did cause a
change in the vibrations felt, going over a wooden slat bridge. I do
think CF can do some damping. Now, whether it is of any real value,
that is debatable.

I went with CF to cut weight, personalize my ride, and because I like
the way it looks. For a good deal check out Ebay, but only buy new.
Another way to get some damping, is to go with handlebar padding, I
have used Specialized Bar Phat, but now a number of brands are
available.


Life is Good!
Jeff

On Mon, 21 Nov 2005 01:34:39 -0600, rs wrote:

I've got a low-end road bike, Giant OCR3 and would like to get a little
more damping, thinking a carbon seat post would help. Obviously I don't
need the lightest but is there a brand recommendation for a fairly
inexpensive and decent one for a road bike?

There are plenty of good carbon seatposts, but that's not the way to get
the effect you're looking for.

Try some fatter tires, and then look at saddles. Shock-absorbing saddles
like the Turbomatic and Koobi *do* work. They offer a cm or so flex in
exactly the direction that's helpful, rather than the sideways waggle of a
flexy carbon or Ti seatpost. Until very recently, Turbomatics were still
popular among pros. They damp road buzz like nothing else (except bigger
tires).

Matt O.

On Mon, 21 Nov 2005 21:27:33 -0500, Matt O'Toole
wrote:
Shock-absorbing saddles
like the Turbomatic and Koobi *do* work. They offer a cm or so flex in
exactly the direction that's helpful, rather than the sideways waggle of a
flexy carbon or Ti seatpost. Until very recently, Turbomatics were still
popular among pros.

Stil -- this is 2004 at the earliest, or maybe this year:

http://www.cervelo.com/images/2004/v...ist%20Team.jpg

JT

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StaceyJ wrote:

Actually, on a compact bike, the seatpost can deflect quite a lot.
Unlike the tubes of a diamond frame (of any material), a long seatpost
sticking up all by its lonseome has no triangulated bracing to prevent
deflection. The more seatpost there is (and the more layback at the
head), the more it deflects.


To approximate how much the post bends, the equation for a deflection
of a long tube with a perpendicular load at the end is:
x=WL^3/(3*E*I)
and I= pi/4*(r1^4-r2^4)

For this case I'll use:
L=.3m (300 mm of unsupported length)
W= 220lb =9.81*100kg* cos(65)= 414N perpendicular force with setback
seat
E= 7x10^10 m^4 for aluminum
r1=.0136m (27.2mm post)
r2=.0116m (2mm wall thickness)

And I get x= .0042m or 4.2mm or 0.16". This is the amount that the post
would bend perpendicular to its length with a 220lb vertical load at
the seat... the vertical motion would be less. It doesn't seem like
very much to me (though it is more than I expected), and I've looked at
a fairly extreme case. If I push down on the seat with my palm, I can
see a much greater deflection there with much less force.

The one caveat
(more pronounced on sloping TT hardtail MTB than road) is that too much
exposed seatpost may act like a softride beam, launching an
unsuspecting seated rider going over a sufficiently large bump. Ask me
how I know...

Is it possible that you are experiencing mostly "spring" from the seat
or even the tire, rather than the post?

Ron Ruff wrote:
StaceyJ wrote:

Actually, on a compact bike, the seatpost can deflect quite a lot.
Unlike the tubes of a diamond frame (of any material), a long seatpost
sticking up all by its lonseome has no triangulated bracing to prevent
deflection. The more seatpost there is (and the more layback at the
head), the more it deflects.



To approximate how much the post bends, the equation for a deflection
of a long tube with a perpendicular load at the end is:
x=WL^3/(3*E*I)
and I= pi/4*(r1^4-r2^4)

For this case I'll use:
L=.3m (300 mm of unsupported length)
W= 220lb =9.81*100kg* cos(65)= 414N perpendicular force with setback
seat
E= 7x10^10 m^4 for aluminum
r1=.0136m (27.2mm post)
r2=.0116m (2mm wall thickness)

And I get x= .0042m or 4.2mm or 0.16". This is the amount that the post
would bend perpendicular to its length with a 220lb vertical load at
the seat... the vertical motion would be less. It doesn't seem like
very much to me (though it is more than I expected), and I've looked at
a fairly extreme case. If I push down on the seat with my palm, I can
see a much greater deflection there with much less force.


The one caveat
(more pronounced on sloping TT hardtail MTB than road) is that too much
exposed seatpost may act like a softride beam, launching an
unsuspecting seated rider going over a sufficiently large bump. Ask me
how I know...


Is it possible that you are experiencing mostly "spring" from the seat
or even the tire, rather than the post?

the extended 27.2mm carbon [easton] post on my old mtb would bend nearly
1cm. cartainly a good deal more than the previous alloy one did.

but that's not the point - composite carbon does not transmit vibration
as well as metal. that's fact.

Bill Sornson wrote:
jim beam wrote:

Ron Ruff wrote:

rs wrote:


I've got a low-end road bike, Giant OCR3 and would like to get a
little more damping, thinking a carbon seat post would help. Obviously I
don't need the lightest but is there a brand
recommendation for a fairly inexpensive and decent one for a road
bike?


Despite the marketing hype, I don't think a carbon post will help you
at all.

What is your tire pressure? If it is over 95psi, then reduce it. If
you then get pinch flats, get a wider tire (28mm).

Another option would be a suspension post.


"I don't think... " so you don't have one then? is that a
qualificaton for advice? or is it just opinion based on other r.b.t
blowhards?


I don't think that was helpful or necessary.

liberal.


eg

(Got me to look at the thread, at least. If the OP wants a carbon post for
a "low-end" road bike, then I would recommend a Supergo or Peformance house
brand. I've been using a "Weyless" carbon post on my mountain bike for
quite a while, and it's been fine. I have an FSA post in my road bike that
cost twice as much; it IS prettier )

StaceyJ wrote:

And no, I'm basing it on on the fact that the seatpost does not deflect
enough to cause a difference in "feel", or damping.


Actually, on a compact bike, the seatpost can deflect quite a lot.
Unlike the tubes of a diamond frame (of any material), a long seatpost
sticking up all by its lonseome has no triangulated bracing to prevent
deflection. The more seatpost there is (and the more layback at the
head), the more it deflects.



I had a compact frame w/ Specialized Pave carbon post, and not only
could I feel it flex, you could see it if you were riding behind me.
More than a few of my fellow racers commented on it. Of course, I've
been hitting the scales at around 195# (yea, I'm a little fat
overweight, but hey, I'm a trackie) so probably any post would've
flexed some. The Pave flexed enough that not only could I feel it, I
found it annoying.

---physics deleted (impressive, but it makes my head hurt)---

Is it possible that you are experiencing mostly "spring" from the seat
or even the tire, rather than the post?

Very possible. I was thinking specifically of an older Kona hardtail
with ~330mm of seatpost exposed, wall thickness of ~1mm (1994 Synchros
Aluminum), and with the saddle slammed most of the way back (and, back
in the day, rider weight of ~170). It is entirely possible that the
saddle flex does account for most of the 'springiness' that I have
experienced. It is worth noting that the same saddle on a Schwinn
Fastback road bike with only ~230mm of seatpost exposed, wall thickness
of ~1.5mm (Easton EA50 of unknown vintage), slammed all the way back on
the rails, has far less perceived springiness. Of course, the tires
may account for some of that too...

In looking over your post, I am curious how the equation accounts for
the heightened load of riding (seated) over large bumps. I am no
physics expert (hell, I do contract and real estate law), but it
appears that you are looking at a static load on the seatpost, rather
than any sort of dynamic loading. Perhaps a combination of the two
(dynamic loading and springy saddle) would explain the deflection?

jim beam wrote:
Bill Sornson wrote:
jim beam wrote:
Ron Ruff wrote:

Despite the marketing hype, I don't think a carbon post will help
you at all.

{snip}

"I don't think... " so you don't have one then? is that a
qualificaton for advice? or is it just opinion based on other r.b.t
blowhards?

I don't think that was helpful or necessary. {evil grin was below}

liberal.

Heh. Good one!

(Actually, upon re-reading I see what you meant: "so you {Ron} don't have
one {a carbon seatpost!} then?" I first took that to mean he didn't have a
valid opinion. I wrote "I don't think..." about THAT, not the post. Err,
seatpost! WHAT WAS THE QUESTION?!?)

Hell, maybe I *am* a liberal; I'm not making any sense! :-P

Good night now...