Integrated seatposts in steel - wall thickness?

[130_R]

Can you elaborate? I'm left wanting more detail about the intentions and the results........

When non-framebuilders start designing bikes and then seek out frame builders that will bend to their wishes, intentions and results are randomly thrown darts on a board. Some thought the bike would build lighter, some though it would give a more "compliant" ride, some thought it would give them a more connected feel to the road. Most commented that the computer reinforced their assumptions, but the end result was not necessarily what they desired.

[edoz]

As a professional metal fab guy, I can't count the number of times that I've had to go to engineering with a problem, have them show me the drawing or the cad model and tell me it's fine, only to take them out to the shop and show them the problem and have them say "Oh, we missed that". The worst engineers are the ones that never leave their computer.

[Amaro Bikes]

"Oh, we missed that". The worst engineers are the ones that never leave their computer.

Nailed!

[Too Tall]

Pardon the brief interruption. This is wiki-worthy for several reasons not the least is how professional the responses are and for critical thinking without prejudice. Sure we all have "opinions" but a real mark of a mature community is how willing y'all are to stick with the subject and raise the collective intelligence. bravo

[Mike Mcdermid]

its an interesting thread

a seatpost in a tube will fail eventually for similar reasons to a ISP it becomes locally stiffer a tube in a sleeve a weld to a tube for example (and theres a whole load of other stuff also which will contribute to your buckling problem) not just how thin a wall before a tube buckles under a load

i know from a previous working life we have had guys who sit and analyze welds ,its their job they know how to set this shit up in their fancy abaqus ,nastran whatever software and have developed their own proprietary software to do it a bit far removed from what we do for FEA however often they are ticking boxes certifying and validating

VALIDATING is the key word you can get any software to output a pretty pictures but unless you have something to VALIDATE IT against ie real world results real world tests it means nothing, it never will and it doesn't matter how much you technically back it up its never going to cut the mustard and in "proper engineering" you are never going to rely solely on what that processor output as 1s and 0s and displayed them in glorious technicolor.

Doing Buckling in a bit of software is no different to calculating by hand .....actual testing is proving by hand and thats where the rub might lie here

[Mark Kelly]

Regarding your application of Euler's formulae:

This isn't a simply loaded column, the likely failure mode is shell buckling not column buckling as you have supposed.

The critical stress for shell buckling from Koiter's circle is given by t / R * E / sqrt(3(1-ν^2))
Taking ν as equal for steel and aluminium the stress is proportional to t /R * E. As you note E steel / E alu is about 2.9 but in the example you give t / R for alu is 2/27.2 = 0.074 and steel is .6 /28.6 = 0.021, the ratio is thus 3.5 so the steel post will have the lower critical stress. In the case of the tube you originally talked about using this is even worse as its t / R ratio is .38 / 28.6 = 0.013 so it's about half as strong as the alu.

Since this thread is now stickied, I should point out that this is very badly written and capable of misinterpretation, for which I apologise.

For the buckling stresses given by Koiter's circle to make sense they have to be compared with the stresses in the loaded shell. Since the steel shell is thinner it has a smaller X-sectional area so it will be more highly stressed for a given load. The comment "it's about half as strong" is thus wrong.