American Classic Seatpost Failure

On Nov 26, 3:25*pm, John Forrest Tomlinson
wrote:
On Wed, 26 Nov 2008 13:20:06 -0600, A Muzi
wrote:

the AC is known for snapping its skinny bolts. IMHO that may be
related to clamp deformation *as much as the bolt itself.

Interesting about clamp deformation. I've had a few of the posts and
they've changed the design over time, perhaps trying to deal with
that.

If anyone is still interested in this:

http://www.astro.umd.edu/~bjw/misc/r...SCN2067_25.JPG
http://www.astro.umd.edu/~bjw/misc/r...SCN2068_25.JPG
http://www.astro.umd.edu/~bjw/misc/r...SCN2069_25.JPG

Both are American Classic posts, the black one on
the left is pretty old, the silver one on the right is
newer (appears to have a date code "99").

These show the different types of lower clamps. The
older lower clamp, which may be made from an
extrusion (it has the same cross-section for nearly
all its length), is fairly thin under the seat rails. The newer
one, which may be a forging, is thicker and has more
vertical structure, which should make it significantly more
resistant to bending under the load from the seat rails.

The older post has a single 6mm clamp bolt. The newer
post has a significantly beefier 8mm clamp bolt, still
taking a 5mm hex wrench.
However, I have at least one AC post with the new
clamp and a 6mm bolt; the 8mm bolt came after the
clamp redesign. Both designs have a 6mm setscrew
for tilt adjustment.

Both old and new clamp bolts have a domed
shoulder on the bolt head, where it bears against
the seatpost surface. This is because as you change
the tilt of the clamp, the angle of the bolt to the post
changes. If the dome were not there, the bolt
would not contact the seatpost surface squarely.
This would put a bending moment on the bolt, and
that's really bad for a highly stressed bolt.

For this reason I strongly believe that you should
not replace an AC seatpost bolt with a standard
square-shouldered hex cap bolt, even if the
standard bolt is very high grade. Get the real thing
from AC or add a domed washer somehow.

(By the way, most or all one-bolt posts have to come
up with some way of addressing this problem of angle
between clamp and bolt, by using a curved surface
somewhere - Kalloy-type posts have a nut with a
curved bottom, there are Shimano posts with a
cylindrically curved washer, and so on.)

Ben

On Sun, 30 Nov 2008 20:32:44 -0800, wrote:

On Nov 26, 3:25Â*pm, John Forrest Tomlinson
wrote:
On Wed, 26 Nov 2008 13:20:06 -0600, A Muzi
wrote:

the AC is known for snapping its skinny bolts. IMHO that may be
related to clamp deformation Â*as much as the bolt itself.

Interesting about clamp deformation. I've had a few of the posts and
they've changed the design over time, perhaps trying to deal with that.

If anyone is still interested in this:

http://www.astro.umd.edu/~bjw/misc/r...SCN2067_25.JPG
http://www.astro.umd.edu/~bjw/misc/r...SCN2068_25.JPG
http://www.astro.umd.edu/~bjw/misc/r...SCN2069_25.JPG

Both are American Classic posts, the black one on the left is pretty
old, the silver one on the right is newer (appears to have a date code
"99").

These show the different types of lower clamps. The older lower clamp,
which may be made from an extrusion (it has the same cross-section for
nearly all its length), is fairly thin under the seat rails. The newer
one, which may be a forging, is thicker and has more vertical structure,
which should make it significantly more resistant to bending under the
load from the seat rails.

The older post has a single 6mm clamp bolt. The newer post has a
significantly beefier 8mm clamp bolt, still taking a 5mm hex wrench.
However, I have at least one AC post with the new clamp and a 6mm bolt;
the 8mm bolt came after the clamp redesign. Both designs have a 6mm
setscrew for tilt adjustment.

Both old and new clamp bolts have a domed shoulder on the bolt head,
where it bears against the seatpost surface. This is because as you
change the tilt of the clamp, the angle of the bolt to the post changes.
If the dome were not there, the bolt would not contact the seatpost
surface squarely. This would put a bending moment on the bolt, and
that's really bad for a highly stressed bolt.

For this reason I strongly believe that you should not replace an AC
seatpost bolt with a standard square-shouldered hex cap bolt, even if
the standard bolt is very high grade. Get the real thing from AC or add
a domed washer somehow.

(By the way, most or all one-bolt posts have to come up with some way of
addressing this problem of angle between clamp and bolt, by using a
curved surface somewhere - Kalloy-type posts have a nut with a curved
bottom, there are Shimano posts with a cylindrically curved washer, and
so on.)

Ben


while the latter post is a minor improvement, it's still putting lipstick
on a pig. the tilt mechanism is ridiculous.

imo, the only modern seat posts worth anything for general use are campy,
easton and some of the ritcheys - ac clearly don't know what they're about
with their convoluted load paths and mis-applied materials.

On Nov 26, 3:13*pm, jim beam wrote:
On Wed, 26 Nov 2008 08:11:31 -0800, wrote:
On Nov 24, 10:35*pm, wrote:
The American Classic seatpost hardware failed on my road bike. *More
specifically, the single clamp bolt broke, causing my saddle to
disconnect from the seatpost.

Sorry to hear about your accident and injuries.

I had a similar failure on a Campy Chorus seatpost some years ago. Ended
up in the blackberry bushes. On another occasion, I snapped another
Chorus seatpost off about 2 inches above the seat tube following an
offroad adventure prompted by severe shimmy on a descent. Remained
upright on that one. Maybe I should call Campagnolo, but I tossed the
parts. Things do fail, and do so under normal circumstances as well as
severe.

and what was /your/ bolt torque???

In the mid-80's as far as I know there was no "bolt torque" for a
Campy Chorus seatpost. I snugged it with a metric hex key.

As for the Chorus post that snapped, no torque was involved other than
that applied by my behind to the back of the saddle as I crossed a
shallow ditch and skidded to a halt just shy of a cable across the
drive to a wooded lot at the bottom of the mountain.

Cheers,

MD