Infinite Stiffness

[newcomersnewbie]

Hi there!

Anyone out there had an idea on what is the best figure to define the infinite stiffness assumptions? Particularly for single seater race car suspension system. Many assumptions were used to simplify the analysis where these will depends on the application. But it is too subjective. Where we might end up designing something which is over engineered/overweight. Any ideas on the figure which will able us to safely ignored the deflection experienced by the suspension system? For example, in the case of anti-roll bar. How to define the torsion bar and lever arm stiffness ratio?

Thanks!

[PlatinumZealot]

Define infinite stiffness please.

[fastback33]

Seems like a pretty straightforward analysis. For the material you choose for your chassis, the stresses applied obviously can't exceed the yield or ultimate stress of the chassis materials. Generally you don't want anything beyond your yield stress as you get into the plastic deformation region. However, its your chassis and you decide what you want. Albeit your question is quite broad.

[PlatinumZealot]

Not really, fastback.

There are times when you have to design for deflection. The deflection is what you want to limit when you chose a stiffer material in a structure. For example you want to limit chassis flex to less than 2mm in say a 2g turn... Usually the material is far from failure during that event, as you may have also designed the chassis for collision/impact for safety reasons, and designing for deflection might actually cause you to use more material or make the material bigger or make it a different shape. So, when it comes to making a structure stiffer, deflection is the metric you design to.

But still, infinite stiffness...

[fastback33]

Not really, fastback.

There are times when you have to design for deflection. The deflection is what you want to limit when you chose a stiffer material in a structure. For example you want to limit chassis flex to less than 2mm in say a 2g turn... Usually the material is far from failure during that event, as you may have also designed the chassis for collision/impact for safety reasons, and designing for deflection might actually cause you to use more material or make the material bigger or make it a different shape. So, when it comes to making a structure stiffer, deflection is the metric you design to.

But still, infinite stiffness...

Umm, duh. Thanks for the paragraph response anyways though.

EDIT: Was working off the basis you start with material failure first and then work your way onto material performance. Got to crawl before you can walk, run, fly...

[MIKEY_!]

This infinite stiffness stuff really bothers me. I think the idea is that the component must not flex at all within it's expected operational loads. However there are often no FIA tests for this kind of thing (the floor for example). If the FIA won't test a components stiffness then how can they be sure it is infinitely stiff?

[Shrieker]

But still, infinite stiffness...

You apply infinite force, and it doesn't flex at all = infinite stiffness ?

[xpensive]

The definition of stiffness is Young's modulus, which is typically given in GPa, billions of Newtons per square meter of cross section area, where steel is typically 200 something and Aluminium about 70, but what it means is really the following:

The Young's modulus is the theoretical stress that results in a 100% elongation of the specimen.

Keep that in mind and you'll do just fine.

And no, there's no such thing as infinite stiffness, steel is actually three times stiffer that a piece of rock.

[rjsa]

I guess what ever stiffness will only allow deformations within your fabrication tolerances would be a starting value for me.

[xpensive]

Stiffness is actually a difficult concept, the numbers I cracked above was for the material alone, much more important for a structure is the surface moment of inertia, which for a circular object goes with the fourth power of the diameter, which means that a 70 mm Aluminium round bar is stiffer than a 50 mm steel such with less the mass.

[rjsa]

What I understood from the OP is "What can I consider of infiite stiffness when designing a suspension?"

That I would understand as how much can a suspenion member be deformed for me to still be able to consider it as unchenged when doing my calculations.

So, yes xpensive, you would claculate how much your suspension member would deform using the young's modulus and the moments of inertia and all that stuff that I gladly forgot already. And it would be assumed of infinite stiffness if it kept withing desired limits with the expected loads. Just to make the rest of the calculations easier.

[xpensive]

But in that case we would have to identify the monocoque's stiffness in terms og MNm per minute or something like that?

[rjsa]

I'm too rusty with the math. But no, he is not looking for a number. He's just trying to define a design parameter, and that relates to how each component behaves. And for each component, he needs too define how little deformation is little enough. For the sake of calculation simplifying.

And from the OP's text he's not into the chassis problem here.

How much can a wishbone bend and still the suspension geometry work as intended?

That's the infinite stiffness close enough for him. It's a subjective concept. Like a safety factor: something subjective you factor in the math to solve a practical engineering problem.

[xpensive]

Just to crack a number so that we have a vague idea of what we're talking about here, a half meter long
aluminum pull-rod with a 20 mm diameter, loaded with 10000 N (1000 kg) will stretch some 0.023 mm.

[DaveW]

Stiffness is actually a difficult concept, the numbers I cracked above was for the material alone, much more important for a structure is the surface moment of inertia, which for a circular object goes with the fourth power of the diameter, which means that a 70 mm Aluminium round bar is stiffer than a 50 mm steel such with less the mass.

Apologies, but I really don't understand.



The above spring has a variable stiffness (force/deflection), which can be calculated with a bit of effort &, probably with less pain, be quantified by measurement. Is your definition of stiffness different from mine?