Push Rod Flex

[Jersey Tom]

Everything flexes, but for the most part suspension components should be stiff enough to have negligible deflection under normal operating conditions (such as going over kerbs). You don't want to have to think about a bunch of springs in series, you want to be able to think of them as infinitely stiff and ignore them. It's not like an airliner's wing, which is supposed to flex visibly, to a certain extent of course.

Certainly there are compliance rates in everything, but that video looks like buckling in a compression element. Bad stuff!

[marcush.]

please explain the term buckling to a dumb person .
I see bending and I see oscilation .

[Lycoming]

clearly the deflections in this situation is negligible.

What is buckling? simply put, its the reason why you can't push on a rope.

[mep]

I think it’s a lack of translation capability. Buckling = Knicken, marcush (and me) probably understands a catastrophic failure under it whereas english speakers are happy to describe the general deformation with that. I also say that the rod bents but has not buckled yet.

[Richard]

Marcush. & mep - Yes, "buckled" = past tense = failed

I think the term "buckling" should mean plastic deformation has started due to lateral deflection under axial load. It has past the yield point but not yet collapsed.

I'd say elastic deflection under axial load is called deflection.

[olefud]

It's enough to make one rethink pull rods

[WilO]

Or simply design the push rod system properly.

[DaveW]

See here for a treatise on buckling. There is a good illustration that is worth expanding part way down. the section titled "Limit point vs bifurcation buckling" shows that buckling can occur without yielding, and the section ** titled "Lateral-torsional buckling" is probably very relevant to this case (I would like to think, so, anyway.)

I haven't looked closely inside the Caterham nose, but another similar looking vehicle had a rocker held in single shear and the push rod was also attached in single shear. The effect was that under load the reaction force applied to the push rod was increasingly offset, bending the push rod. That vehicle had a very poor "installation" stiffness.

** That was a poor quote, since it appears to be intended for open sections. I was after a quote that was directed at combined bending and compression, which can reduce the load-carrying ability of the strut.....a bit like the fixed-free case of the illustration.

[Tommy Cookers]

Marcush. & mep - Yes, "buckled" = past tense = failed

I agree the term "buckling" should mean plastic deformation had started due to lateral deflection under axial load. It has past the yield point but not yet collapsed.

I'd say elastic deflection under axial load is called deflection.

there is a critical axial load beyond which lateral deflection will develop (if that load is maintained) from ZERO to failure
this failure is called buckling
it is due to elastic instability (the critical load being the load at which elastic stability becomes elastic instability)
(true, a lateral load will lower the critical axial load)
so plastic deformation due to maintained axial load doesn't occur

if there is sufficient structural redundancy buckling will be contained (eg squashing a beer can axially)

IIRC yield point is not the start of plastic deformation, it's conventionally eg 2000 ppm strain (a lot)

[Richard]

Thanks Tommy. I should have paid much more attention in those strength of materials classes. I've got too used to limiting stresses to elastic behaviour because people think something is broken when there is plastic deflection!

There is obviously remaining strength in the formation of plastic hinges that is used in calculating the ultimate limit strength. If you get to that point the thing is destined for the bin, one would say it has "failed" even if has not yet reached yield.

I'm not sure about the zero lateral deflection prior to critical axial load, or is that assuming a perfect geometry? In practice, nothing is perfectly straight, and the load is not perfectly applied exactly on the neutral axis, so there will be some lateral deflection as soon as axial load is applied? That's what causes the flexing.

DaveW - I thought lateral torsional buckling primarily applied to open sections? The push rod is a tube so likely to fail according to classic Euler buckling?

[Jersey Tom]

Perhaps this is semantics, but in my education buckling was not synonymous with plastic deformation and failure. You could do an axial compression test on a specimen pinned on both ends and achieve a buckled state / shape without actually failing the part... and upon reduction of load would snap back to its initial shape. I thankfully haven't had to deal with such discussions since junior or senior year... thankfully as that behavior is generally synonymous with "bad things."

In any event could we not agree that what we're generally seeing is too much axial load for the section & stiffness? Which then has a high potential for rapid failure?

IMO they dodged a bullet with that member not shattering.

[DaveW]

DaveW - I thought lateral torsional buckling primarily applied to open sections? The push rod is a tube so likely to fail according to classic Euler buckling?

Thanks Richard, you are right. I have taken the liberty of editing my post.

[strad]

Caterham = Lotus = Colin Chapman = minimally designed cars.

[marcush.]

http://www.gef.es/Congresos/24/PDF/3-13.pdf

to put some science on the table ,Mr Savage (formerly BAR/Honda ) has come up with this.Very informative and readily available here in f1t since a few years....

[strad]

Thanks for that marcush..
originally this was my concern:

As the load increase,
the equilibrium becomes unstable and any slight lateral
load will produce a deflection leading to collapse
through bending