Was riding along yesterday when the saddle on my 1988 bike suddenly disappeared beneath me. Fortunately, I reflexively stood up on the pedals when the saddle gave way, or I might have found myself in a very uncomfortable position on top of a naked seat pillar. I pulled over without crashing and walked along the road to gather up:
1) one Brooks B17 saddle, scuffed a bit where it bounced down the road (but still in pretty good condition).
2) the upper and lower seat rail clamps of a 1988 Campagnolo aero seat post (first generation Chorus edition)
3) the failed Campagnolo bolt that normally holds the rail clamps together, and attaches the saddle to the top of the seat post. This single bolt had sheared in the middle, causing the spontaneous loss of saddle.
I found this experience particularly unnerving because it was completely unexpected. I've put thousands of miles on this older bike with no problems since purchasing it used last year on Craigslist. I had made no recent saddle adjustments. I was not carrying any unusual weight on the bike, and had not just hit any large bumps in the road. At 6' 2" and 175 pounds I'm substantially heavier than a typical bike racer, but am not at a weight extreme that would normally have me thinking about shearing or crushing bike parts. Granted the failed bolt was 22 years old and has seen a lot of miles. But I still have several practical questions about both repairing and avoiding this problem in the future?
Is this a common mode of failure for Campy (or other) one bolt seat clamp posts?
If I just get a replacement bolt, what is the strongest type that might minimize chance of similar unwanted surprise in the future? Look only for an original Campy bolt? Find another stainless steel bolt of correct length and thread? Note that the Campy seat clamp design comes with a separate concave washer to help match the head of the bolt to the curve of the saddle clamp (see diagram from 1988 Campy dealer parts catalog, part #3). This minimizes the need to find a special bolt head that matches the underside of the seat clamp. I know Campy sometimes uses special metals or forging methods for either strength or weight. I don't care about weight, but I do now care a lot about strength in this mission critical component!
Should I actually replace more than just the sheared bolt? On disassembling and inspecting the seat post I have noticed several signs of long term wear and stress on other parts. For example, the series of fine serrated ridges that extend across the curved mating surface between the top of the seat pillar (shaded in diagram) and bottom of the lower rail clamp (part #2) have worn nearly flat in some locations along the mating surface. These ridges normally help maintain the angle of the saddle adjustment (nose up or down). Because the ridges have worn with age, more shear stress is probably transmitted to the central seat rail bolt than would be true if fresh mating surfaces were doing a better job on their own helping to hold the seat angle.
Finally, I assume that only the upper seat rail clamp is supposed to have a threaded hole for the key bolt? On my parts, the upper rail clamp (part #1) is clearly threaded. Tightening the bolt works by squeezing the upper clamp tight agains a stack of parts that then includes from top to bottom: upper rail clamp (#1): saddle rails , lower rail clamp (#2) , top of seat pillar, concave washer (#3), head of bolt (#4). By inspection, the central hole in the lower rail clamp (#2) is substantially larger than the threaded hole in the middle of the upper rail clamp (#1), and will not hold the threaded bolt on its own. The bolt broke in the middle of this stack of parts, leaving one half still threaded into the upper rail clamp, and a lower half that fell completely out of the larger hole in the bottom rail clamp.
Thanks for your help and I hope not too many other people get to experience this same surprise when out riding your classic and vintage bikes!
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