Upper Strut Tie bar Question

[scirocco16vr32 at gmail.com (Chris Bennett)]

Holy Shit I got a nosebleed reading that.
Chris


On 8/3/06, Dan Bubb <jdbubb@verizon.net> wrote:
>
> David,
> I'm not trying to bust your chops here, but aluminum is not more rigid
> than steel.
> Ignoring any cross sectional or geometric differences and just considering
> the material itself, aluminum is 1/3rd as stiff as steel.
> Aluminum's modulus of elasticity or Young's modulus is 10,000,000
> lbs/in^2. What this means is if you take a 1" square rod that is 10" long
> and put a 10,000 lb weight on it, it's height will reduce by .010". Steel's
> modulus of elasticity is 30,000,000 lb/in^2, so in a comparable load case
> the steel rod will shorten by only .0033". So, for a comparable geometry
> aluminum will deflect 3 times as much as steel.
> As it turns out steel is virtually 3 times the weight of aluminum. .286
> lb/in^3 vs .10 lb/in^3. So, you can have almost 3 times the cross section
> area of aluminum for the same weight as steel. Sort of like having 3 1"x1"x
> 10" rods, so the load is reduced to 3333 lb/rod and the deflection is
> reduces by 1/3 and virtually matches that of steel. So, on a weight basis
> aluminum is as rigid as steel.
> Both of the above circumstances or just for straight compression or
> tension.
> Where things get interesting (or not!) is in bending and this is the case
> that is applicable to the tie bar.
> I know you're thinking that the tie bar just transfers a straight load
> from strut tower to strut tower and then it would be a case of straight
> compression or tension. But, I've never seen a perfectly straight tie bar
> and the fact that it has bends in it allows much more deflection end-to-end
> for a given load that a perfectly straight bar. As an extreme example
> imagine a piston ring. In that form it's easy to deflect and increase or
> decrease the end gap, but if the same cross section was in a straight rod it
> would be much harder to increase or decrease it's length. The one case being
> pure bending, the other pure compression or tension.
> Anyway, the biggest determinant in bending deflection is the moment of
> inertia of the cross section or the geometric properties of the cross
> section. As an example a hollow tube with outside diameter of 1.414" and
> an inside diameter of 1.000" is 3 times stiffer in bending than a 1.000"
> diameter bar. The weight is the same in both cases but the tube is 3 time
> stiffer in bending. You can, of course, take this to extremes and keep
> making the tube outside diameter bigger and bigger and the wall thickness
> less and less and keep getting a stiffer bending member (sexual innuendo
> thrown in just for the ladies and to see if anybody has actually read this
> far ;^)
> The limit here is the wall thickness (disregarding things such as fitting
> a 3" tube across your engine compartment). At some point the wall thickness
> will be so thin that it can be easily damaged or will suffer localized
> wrinkling under load. This is where aluminum starts to look much better than
> steel. For any given diameter it will have 3 times the wall thickness for
> the same weight. So, it's easy to have a stiffer bending member in aluminum
> if you can take advantage of a bigger member diameter.
>
> Bringing all this back to reality, the Autotech bar doesn't look like it's
> outside diameter is much bigger than any of the steel bars I've seen. If
> that is the case then it will have no geometric advantage and it's down to
> straight material properties and the amount of material in the unit. Without
> knowing the outside diameter and the wall thickness it could be either more
> or less rigid than a steel item, but in this case probably not by much.
>
> So, when you say aluminum is more rigid than steel, the material itself is
> not, it's 1/3 rd as rigid. For an equal weight it's a match for steel. In a
> bending situation, depending almost entirely on the cross sectional
> geometry, it may or may not be.
>
> Anyway, thought I'd "clarify" that little situation. ;^)
> Dan
>
>
> From: "David Utley" <fahrvergnugen@cox.net>>
> Sent: Wednesday, August 02, 2006 7:41 PM
> Subject: Re: Upper Strut Tie bar Question
>
>
> > Autotech upper bar is by far the best I have used...
> >
> > http://www.autotech.com/prod_susp_stress.htm#front
> >
> > Go halfway down (I hope the link works).  The bar puts more even
> pressure
> > all the way around the strut mount.  That, and since it is aluminum, it
> is
> > more rigid.
> >
> > I just got the rear bar and also the hollow sway bars.  I hope to have
> > them installed in the next month.
> >
> > David
> >
> >
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>
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