[DeTomaso] Engine Bay Brace question

[Thomas Borcich]

Mike that makes total sense looking at the rear of the car...it's basically a cardboard box with no top or bottom laying on it's side. Putting the braces at 45's locks it so there is no swaying of the two towers and gives you a rigid structure to hang the suspension on.  I wasn't sure if my 351 and 285 tires would generate enough force to make it a worth while mod....but this has confirmed my initial thoughts. Thanks

Best regards,



Tom Borcich




From: mikeldrew at aol.com
Date: Sun, 20 Dec 2009 15:18:34 -0500
Subject: Re: [DeTomaso] Engine Bay Brace question
To: jderyke at aol.com; tborcich at msn.com; detomaso at realbig.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