Originally Posted by Andy_K (Post 22371058)

I'm not convinced the Cannondale high collar really helps much. The pivot point is still the top tube. I think the high collar just offers a false sense of security. Or takes away the manufacturers "room for error" if you prefer.

I'm picturing the forces kind of like this:

And the collar above the top tube would be like having another kid on the other side of the see saw trying to hold it up from the bottom.

Originally Posted by oneclick (Post 22370752)

No, it'll be even less than that because what you add to the bottom you take away from the top; the pivot remaining as the top of the seat lug.

As to the question I'm with the get-a-longer-one camp.

Originally Posted by 79pmooney (Post 22371456)

I consider seat height to be sacred. So seat top to top of seat lug doesn't change. You could go to a seat with deeper rails; allowing you to push the post down. The lever distance hasn't changed because the force location (you) hasn't changed.

Originally Posted by jdawginsc (Post 22371481)

But isn't the lateral force distributed better the deeper the post is? The force opposite/against your force would be greater within the tube since the post is deeper (further) away from the pivot?

Originally Posted by Chombi1 (Post 22371302)

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 likeseatpost designers did not really follow this rule.....

Originally Posted by jdawginsc (Post 22371419)

Shhh...I am trying to sucker someone into buying a really cool Cannondale!

Originally Posted by Andy_K (Post 22371593)

Well, I wouldn't think selling a cool Cannondale would require much suckering. Education at most. ;)

I bought a Cannondale recently on a whim, and I've been really surprised by the enthusiasm of the Cult of Cannondale folks here and on other forums. I knew they existed, but I had never really paid much attention before. There's a lot of love for these bikes.

Originally Posted by Chombi1 (Post 22371302)

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.....

Originally Posted by bikingshearer (Post 22371533)

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?

Originally Posted by Andy_K (Post 22371633)

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 [MENTION=392125]79pmooney[/MENTION]'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.




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.

Originally Posted by 79pmooney (Post 22371864)

Andy, you are looking for an exact solution. No issues. Just model up the seat tube, lug, TT & seat stays plus post, seat and rider in a FE program, add the load and solve. You now have spent many hours and have one solution to a problem that may never play out just like that. Huge waste of time for any engineer tasked with getting plans and products that work out the door. My simplified approach is, yes, very crude but if you can run across one or two bikes with bulged seat tubes and many without, it probably won't be hard to see that a seatpost sticking down this far and a seat tube this thin bulges but all these more conservative bikes and seatposts are fine. And what is the weight cost of a seatpost that is inserted a cm or two more than needed? 10?, 20? grams. (For an aluminum post built like a tank.)

It comes down to: what is the cost to the ride of too safe a system; in weight, race results, etc. vs the risk and consequences of a post snapping or a seat tube being bulged (metal) or cracked (CF). The answer to that won't come from theory.

Originally Posted by jdawginsc (Post 22371612)

You could have had a beautiful red one!

I will just build it up and sell locally but I thought it would get more traction on here.

Went off like a Lead Balloon...not a whole lotta love for it. I was a bit dazed...and confused by the lack of interest for a for sale thread I put all of my love into. How many more times I bump the thread is the question. I thought it was going to California with one member, but...

Originally Posted by ThermionicScott (Post 22372245)

There must have been a communication breakdown... ;)

Originally Posted by Andy_K (Post 22371633)

My degree is in physics and math, so I've always looked at engineers as a curious species.

Originally Posted by jdawginsc (Post 22372253)

I was worried I would ramble on, but thank you for noticing.

Originally Posted by Andy_K (Post 22372349)

Don't worry. Your time is gonna come.

Originally Posted by Classtime (Post 22372880)

I doubt any old 27.2 will fit in that Ironman. 27.0 in any I've had.

Originally Posted by Chombi1 (Post 22371302)

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 likeseatpost designers did not really follow this rule.....

Originally Posted by jdawginsc (Post 22372834)

Glad to see more participants in the Led Zeppelin pun-a-thon...