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Upper Strut Tie bar Question
Dan,
Very nice mechanic engineering information :) Taken a few classes on this
type of material myself.
I understand the concept here of both tension and compress & bending moment
in the strut bar itself.
But my question is doesn't how it mount also play apart in the design? If
the bar isn't mount right to the strut tower like the Neuspeed bar, but use
and design like the autotech bar doesn't the bolt add a pin connection into
its calculation? Wouldn't that design lower the bending moment on the strut
bar itself?
I hope I am making sense on the subject, I am just curious why the two
designs are that much different from each other..
-Brian
85 Scirocco 8vT 01 Jetta 20vT 84 Rabbit GTI
----- Original Message -----
From: "Dan Bubb" <jdbubb@verizon.net>
To: "Scirocco List" <scirocco-l@scirocco.org>
Sent: Thursday, August 03, 2006 8:18 AM
Subject: Re: Upper Strut Tie bar Question
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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