[DeTomaso] Engine Bay Brace question

[mikeldrew at aol.com]

In a message dated 12/20/09 10 55 23, jderyke at aol.com writes:


> But since the actual a-arm attach point is the 'horse-shoe'
> weldment surrounding the areas where the supposed flex is occurring, and 
> those
> parts don't fatigue-crack or break their spot-welds even on dedicated 
> track
> cars, I doubt if a lower bar does much of anything except add weight and
> complication,
> 

>>>The sole function of the lower bar is to triangulate the upper bar.   If 
the upper and lower bars aren't connected to each other, the lower bar 
really isn't doing anything substantive.   But by connecting them, it creates a 
rigid structure, which prevents the top bar (and thus the chassis) from 
moving side-to-side relative to the bottom bar under heavy cornering loads.   
Having the bars that join the upper and lower pieces angled at a 45 degree 
angle (or thereabouts) is no accident.   

Envision the car as viewed directly from the rear.   Imagine a cornering 
force on the right rear suspension, which would otherwise force the top rear 
chassis on the right side to deflect inwards.   By installing a simple bar 
across the top the way De Tomaso did, that force is transmitted to the other 
side of the car, greatly increasing its strength.   Now, the right side 
deflects inwards, and the left side deflects outwards, although the total 
movement is considerably less.

With a proper rear chassis kit installed, the force applied at the top 
right, is transmitted not only to the top left, but also to the bottom left in 
compression, and the bottom right in tension, through the use of the two 
45-degree angled tubes.   That results in a radical increase in strength, in 
what is arguably the weakest single area of the chassis.   While the other 
three braces in the system likely do something, the one atop the transaxle 
undoubtedly does the most.

Note that as Jack said, this system only works when there is no freedom of 
movement between the brace and the car.   Securing it only with bolts will 
lessen its effect, most likely, although perhaps not significantly.   
However, the structure itself must be perfectly rigid in all planes in order to 
have any measurable function.   Systems which use heim joints fail that 
critical philosophical test, and are thus borderline useless.

Imaging having a roll cage with heim joints at each corner.   How effective 
do you think that would be?   The chassis stiffening kits are simply 
miniature roll cages designed to keep small portions of the car perfectly rigid.

One thing that the manufacturers haven't caught onto is the fact that by 
having the upper and lower mounts on each side parallel to one another, they 
are giving up a little bit of rigidity.   Even with the 45 degree tubes 
mounted, you can see how one side could move up and down slightly relative to the 
other side, as both the upper and lower mounts pivoted on the mounting 
bolts on each side.   If the bottom mounts were rotated 90 degrees, so that the 
upper bolts faced fore-aft and the bottom ones faced up-down, that would 
probably impart greater strength (although whether that would really make a 
difference from behind the wheel is rather doubtful).

Mike

Mike