Upper Strut Tie bar Question

[fahrvergnugen at cox.net (David Utley)]

Dan,
  No worries, Mr. Bubb.  If I need to be schooled, please do so as you 
have already done...  :-)

The only thing I can say in my defense is that perhaps the difference in 
the way these bars mount explains why the autotech one works so much 
better?  Perhaps the ring at either end more evenly puts pressure back 
into the body (as opposed to just the tow strut bolts)?  For me, it was 
by far the best money I have spent on the suspension. 

Again, thanks for sharing the long and the short of it.  I would go out 
there and measure the dimensions of the tubing (it -is- pretty thick), 
but it is late and I am overdue for bed.  More later...

Cheers,
  David


Dan Bubb 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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