Chombi1 

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.....

jdawginsc 

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

But joking aside, I am thinking that you are correct, though there seems like there should be a little bit more stability, since the forces would be distributed to more metal....but I guess you are right... the pivot point staying the same.

genejockey 

Between this thread and the "Is this cracked stem safe?" thread, I feel like there's an increase in people asking questions they already know the answer to.

79pmooney

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.

jdawginsc 

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?

79pmooney

Oh yes! I was addressing the answer to my first post, that dropping the seatpost both bettered the resisting force and lowered the driving force (the rider on the seat). More insertion is always good for resisting. The amount of post above matters little. All that counts there is the distance seat top (or rider butt when planted on said seat) to seat lug.

easyupbug 

I was a design engineer in my early career and we were often pressured to put out a cost estimate in early stages of a design prior to completing the design and so had a few rules of thumb to apply for rough estimates, but never never would that see a finished product, ever. Variables like loads, deflection, conditions, etc. are all analyzed and used in the computations for final design. Nothing would/should see the light of day cavalierly done by a rule of thumb.

bikingshearer 

In pretty much all questions like this, I start with the assumption that if you have to ask if it's safe, it's not safe. That can change if you can get solid opinions from reliable sources that it's okay, I'm not hearing that here. I'd say don't do it..

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?

Andy_K 

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.

jdawginsc 

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...

Andy_K 

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.


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.

79pmooney

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.

Andy_K 

Yeah, I get that. It's just not the way my brain works. I tend not to care whether finding the pure answer is useful or not.

ThermionicScott 

There must have been a communication breakdown...

jdawginsc 

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

DiabloScott

I'm an engineer and I analyze stuff that broke; usually lawyers and insurance companies are involved.
If someone got hurt on a bike that broke near the lug, and I saw the seat post with less than the minimum insertion... I'd expect my report would conclude "user error".
You see enough cases of user error causing damage and injuries, and you get a different perspective on pushing the limits.

Andy_K 

Don't worry. Your time is gonna come.

jdawginsc 

It was nobody’s fault but mine....but someone might still bring it on home. That’s the way it goes sometimes...

SurferRosa

I'm sick again this happened. How many more times? But, hey, hey, what can I do? I should give no quarter. In the light, we'll have a celebration day.

bikingshearer 

Not inserting the seat post enough. It's what is and what should never be.

Oooh, and the newest hit: "When the Seat Post Breaks."

First verse starts: "If it keep on raisin'/ The seat post gonna break . . . ."

jdawginsc 

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

Classtime 

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

shoota 

RobbieTunes says all of his have taken 27.2.

ThermionicScott 

I'm not a physical engineer of any kind so bear with me, but is that rule of thumb for the integrity of the cantilevered member, or what it's mounted into? Seems like seatposts are generally pretty strong, and it's the frame that we're usually concerned about.

ThermionicScott 

Thank You for starting it.