Not true. I'm not sure what property you mean when saying "stiffness" but in tensile/ compressive strengths (which is what one would have in a strut bar for the most part) there is a vast difference in specs depending upon the alloy used. Aluminum alloys can range in yield strength from roughly 100 MPa in 1000 series aluminums to over 150 MPa in 3000 series. 300 series stainless steels (which is only one type of many SS's, which again is only one of many many different types of steels) can range from a tensile strength of 700 MPa to over 1550 MPa with just some cold rolling.

I'm not saying everyone should go out and buy the most expensive strut bar they can find, but materials are still something that one might want to consider before making their purchase.

Does anybody make a flat bar style for fc/t2's? Im not a big fan of tubular style like the Corksport and Cusco

n/m, after going back and looking, the cusco is flat bar and the cork is oval it looks like. dont know why i thought both of them were tubular

no not really, just because i live in Holland... shipping would be killing! specially for the steel version, it weighs a LOT! that's why im going over to aluminium ;)

Yield strength is the measure of the amount of stress (force over area) that a material can take before yielding (permanently deforming). That varies a great deal, and has nothing to do with how stiff it is (how much it'll deflect before yielding).

Young's modulous (E) is a measure of how much a material will deflect for a given stress. That is nearly constant for aluminums and steels (steel being rougly 3x stiffer, but also 3x heavier). That is what I was referring to as stiffness, because for any steel or aluminum bar of a given weight under the same loadings, the bars will all deflect about the same amount.

Stiffness is practically constant for any alloy made of a given material.

Indeed the young's modulous is roughly the same for these materials, but wouldn't you treat a strut bar as a 2 force member (obviously triangular ones are different)? In that case, the strength of the material would be important. Or should I be looking at this problem more from a buckling column standpoint? I'm still a student, so if you could help me work through this I'd appriciated it.

Buckling should be considered also, but the strength of the material is really only improtant for failure, either from excessive force, or from fatigue. But since it's a long, slender column buckling will occur long before yielding will, so it's not really that important what the ultimate or yield strengths are. In terms of chassis rigidity the relevant thing to look at is the young modulous, because you're trying to reduce flexing, and a stiffer bar does that, a stronger bar doesn't necessarily do that.

In terms of how you'd model it, it's a truss, it accepts only axial forces, because it's pinned at either end, so it can accept no moments or non-axial forces.

I'm also a student, I'm in my last half of third year of my Bachelor of Mechanical Engineering degree.

You are about the same place I am in school lol.

I'm just not seeing how the Young's Modulous is as big of a deal. I'm basically approximating the bar as a 2 force member (in a truss), since in can only really effectively transmit force transversely along the car. Following that assumption, lets take a look at this. I'm not seeing quite what you mean by flexing... there will really be only one kind of deformation going on, the axial deformation, or compression from the car during a turn, of which the buckling would be the ultimate failure mode.

black91- i'm sure you've seen this stuff but I'm gonna try to explain it for everyone, any input/corrections are fine by me

I'm gonna use the Johnson curve to model the buckling failure. This curve is a modification to the Euler Curve which is modeled by the following

P/A = Cpi^2E / (l/k)^2

where P is the load
A is the x-sectional area
C is the end condition factor (basically = 1 here)
l is the lenght
k is the radius of gyration
E is Young's Modulous

The Johnson curve takes the Euler one a step further and fits a different curve that eventually intersects with the Euler Curve. The Johnson equation is the following:

P/A = Sy - (Sy*(l/k)^2) / 2*(T)^2

T is the intersection point of the 2 curves at Sy/2 = Cpi^2*E/T^2
Sy is the yield strength

Anyway, you would set these equations up and solve for the x-sectional area.

I'm saying the strength matters based on the 1st part of the Johnson equation, which would be the ultimate failure and the manner in which the part would flex before completely buckling. I don't think you would see much purely axial deformation unless the brace had a amazingly designed mount.

That said, I doubt the car can load the brace up enough to even approach the critical load of steel, which is why aluminum would be used, and even then its probably nowhere near the critical load for that either, which allows the hollow braces and small x-sections seen in higher end bars *cough* cusco.

Anyone feel free to add to this, I'm learning alot.

I've basically started out unconciously making the assumption that the loads will be small relative to the yield strenth and that it'll resist buckling, in which case the Young's modulous is important and the yield strenth doesn't really matter because the column's not going to yield or buckle. Even assuming a load of say 250lbs, which is more than I'd think exists in the bar you'll find that using the curves gives you a very small cross sectional area, and radius of gyration, so much so that any bar ever made is sufficiently rigid to avoid buckling, in which case the strenth is irrelevant. This is backed up by experiance, even the flimsy eBay ones don't buckle or deflect much at all, backing up the low load assumption.

In any case, it won't take a big area and radius of gyration to push the design of the bar into the Euler area of the curve, meaning the strength of material is irrelevant.