I have gone over this subject many times, but the weight of the vehicle is supported at the spring perches at all 4 corners. On the rear, the springs are mounted on perches at the frame rail under the hatch floor. The rear of the vehicle exerts force on the spring and the spring pushes forces back into the perch. It equally applies force on the rear suspension, which has the opposite perch. The springs sit vertically and the forces are vertical. Under cornering, a small horizontal force is induced into the spring as the rear beam rotates around the rear roll center. Given the short height of the springs, this horizontal moment is pretty small. Any horizontal force is transmitted into the spring perch and the frame rail. That force is distributed into the unibody. The minimal horizontal force puts the hatch floor into compression. The geometry of the rear floor includes a number of surfaces that affect the MOI, increasing the stiffness. Ok, so that covers the rear springs. There are no braces for the rear spring perches.
Oh wait, there are braces for the rear shocks. Ok, lets break down the rear shock area. The shocks are mounted to the twist beam and to the chassis inside a box section that is welded to the hatch floor and the inner quarter panel. The shocks are mounted vertically. The compression force in the shock exerts an equal and opposite force on the shock mount. The force direction is vertical. There is a bushing of some compliant material at both ends of the shock. This allows for some articulation as the suspension rolls. The bushing acts like a compression spring. When the vehicle is in a roll state, the vertical forces receive a horizontal component. The bushing allows the shock to deflect and provides a spring force on the shock. The horizontal force exerted on the chassis at the shock mount is transmitted into the box section of the mount and into the unibody. In order to deflect the box section of the rear shock mount, the inner quarter panel must also deflect, which is welded to the entire unibody, meaning the horizontal shock force needs to be great enough to a large portion of the unibody in order to require bracing to prevent deflection. But wait, none of the vehicle load is passed through the shock! It is passed through the spring. The spring carries the load and shock dampens the spring forces. But there is a horizontal spring force during roll! Yes, but given the low vertical force of the shock compared to the spring, the horizontal component is pretty much nothing. Any deflection caused by the horizontal component is taken up by the compliant bushings. In order for the rear trunk brace to counteract any horizontal forces caused by the shock, the shock's horizontal component must be great enough to fully compress the bushing creating a direct load transfer to the shock mount (the one on the shock assembly) and then transferred to the chassis shock mount along with applying a shear load on the fasteners. Now, if you have ever taken the rear shocks off a Fiesta, you will see how incredibly small the upper shock screws are. Meaning they aren't designed for a great deal of load. Since shear is a weak strength condition for a bolt, if the engineers saw a high shear load, they would need to increase the fastener size. Bolts and screws do much better in compression and tension loading. The small fasteners is a clear indication the loads at the chassis mount are vertical, placing the fasteners into a compression and tension load condition, where they are much happier. Take a look at the shock mount at the twist beam, it is way bigger! Why, well, the shock puts the bolt into shear, so a bigger fastener is needed.
For the rear trunk brace to do anything in a roll condition, there needs to be enough horizontal load exerted into the chassis to deflect a large section of the unibody. Given the HSLA steel that was used in the construction of the Fiesta (if you know your Fiesta history, you will know that Ford had to design new blades for the jaws of life in order to cut through the unibody since it had a far higher grade of steel than what was being used at the time for car construction), the small size of the upper shock fasteners, the minimal horizontal force exerted on the chassis from the shocks, and the fact that the vehicle load is transmitted through the spring perch under the frame rail, it is highly unlikely a trunk brace offers any significant strength in the chassis due to cornering forces. Wait, how are cornering forces generated? Tires. A typical summer tire won't produce enough traction to create enough cornering force to deflect the unibody construction. The cornering force must be transmitted through the tire, which has some absorption into the tire carcass, then into the wheel, which deflects and absorbs some of the force, into the wheel bearings, into the twist beam, then through the soft rubber bushings, and then finally into the chassis. But racecars uses braces. Yes, they also use tires that produce far more cornering force and use solid bushings that transmit forces much greater than soft rubber. They are also designed for a greater level of chassis rigidity to take advantage of the stiffer suspension, bushings, cornering forces, increased safety due to the far higher speeds and forces exerted in a crash, and are driven by drivers who can actually feel the difference in suspension and chassis tuning.
But deflection is bad! Yes, chassis deflection is bad. Since none of the rear suspension pick-up points are located in the trunk, the forces aren't transmitted through the trunk area in order to cause enough deflection to alter the suspension geometry. There is far more bushing deflection that alters the suspension geometry. The stiffness of the rubber bushing is far less than a steel section with lots of geometry to create a higher MOI.
So, tell me how a trunk brace is going to decrease the deflection on the unibody when it requires a high amount of horizontal forces exerted on it to place it in compression or tension (because that is where a single beam is the strongest), the forces are transmitted into the frame rail under the hatch floor, and there are 6 rubber bushings with a high degree of compliance deflecting under load?
As for my paper on the wall, I am sure you don't listen to doctors since they have a paper on their wall. You don't think pilots should fly you around the sky with their paper on the wall. Let a lawyer defend you in court with their paper on the wall. You don't know the amount of work I had to put into that fucking piece of paper, so don't tell me it means nothing.
