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Old 01-12-22, 03:49 PM
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Originally Posted by Chombi1 View Post
Safest approach, based on what most structural designers are typically comfortable with, is 1/3 of the load bearing structural member to be anchored to structure and 2/3 cantilevered, max..... but it looks like seatpost designers did not really follow this rule.....
My degree is in physics and math, so I've always looked at engineers as a curious species. They have a strange mix of intuition and pragmatism. Having been trained in physics and math, and now working as a programmer, my mind always goes straight to setting up equations (albeit with a generous helping of simplifying assumptions). So, I'm curious, from a structural engineering perspective, does the fact that the force typically comes in at an angle modify the 1/3 2/3 rule? Granted there are cases where the impact may be perpendicular, and I guess structural engineering is all about those worst case scenarios....

Also, having started imagining force diagrams and equations in my head, I wasn't happy with @79pmooney's simplification of the force being concentrated at the end of the seat post. I wanted some integral computation with incrementally increasing force dispersed along the length of the section of post below the pivot (and some above if you've got a Cannondale!) -- which led me to the picture above with several kids sitting along the length of a see saw. But when I thought about why he'd make that assumption I decided that it must be something along the lines of "if you make a calculation based on this simplified assumption, it won't fail and the math is easier."

At some point, there's a trade off to be made. For a race bike, you might want the shortest seatpost that won't definitely cause a failure. For a touring bike, you probably want one long enough to be confident that it will never cause a failure. Many of us here are buying bikes and components that were designed for racing, but we really want to get touring-like reliability out of them.

Originally Posted by bikingshearer View Post
Think of it another way: How much pain from and damage to your teeth (failed stem or bars) or nuts (failed seat post) are you willing to endure?
That was the first thing that popped into my head, even before the hypothetical force equations. I've watched multiple cyclocross races where someone crossed the finish line standing to pedal with part of a carbon seatpost sticking out of their frame. That's an unlikely result with a metal seatpost, I guess, but I could imagine the seat cluster coming unclustered and having a similar effect.
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