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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 |
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#53
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American Classic Seatpost Failure
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 |
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