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A brace will not reduce racking distortion. Racking distortion is how the body defects torsionally. The front brace will reduce deflection of the upper mount area relative to the firewall connection. The two upper mounts are stabalized to each other to the extent that the firewall does not flex.
The front upper mounts see vertical loads mostly. Very little corning, braking or steering reaction forces are impressed on the upper mount. The couple is the distance from the center of the ball joint ball to the wheel bearing axis. The tie rod ends do exert a steering force on the strut which will be seen by the mount.
The rear axle upper shock mounts see only vertcal dynamic loads, no lateral or static loads from the shocks at all. So to add a brace here is simply an attempt to reduce body flex. The weakness is tortional. When the body distorts tortionally, there is very little reason to assume that as it twists, that the distance between the shock mounts will change. So tieing these points together with a tension-compression member will have no effect. A tension member from the firewall to the upper mounts does provide some benefit by keeping the relationship of the steering rack to the strut more stable.
The rear spring upper mounts seem to be to a locally stiff area.
You have to consider that if the body as a whole is twisting, that simply adding a small componet is not going to reduce the overall torsional flex of the body as a whole.
I went though this exersize with LAV armoured vehicle hulls where there was a concern that racking forces were creating excess stress in the corners of the large rear door opening. This open ended box is weak, much as the situation with a hatch back design. Manufacgturing wanted to put a stiffening member across the top of the opening. I resisted as I knew it would do nothing, and I had to do the FEA modeling, which was a great effort with the equipment available at the time. So I told then that it would do nothing then modelled it and showed them that all that they did was subject some more metal to the same stresses and strain and that nothing was accomplished. So the lesson is that if a large structure is flexing, a local stiffener will not change the overall deflection response of the structure. A stiffener can shape the local deflection to a degree. One mode of relative deflection can be minimized but the overall picture does not change. What I did was design corner piece stiffeners with organic shapes to distribute the strain over a larger region so that it did not all pile up in the corners. One needs to look to nature for optimal design. The best example that I ever saw was the study in scientfic american years ago the invesigated dragon fly wings as a design that is optimal.
So for a rear brace, tying the two shock mounts together will do nothing. To reduce torsional deflection caused by the hatch opening, you will need to create a very significant brace or bulkhead which would mess up the utility of the vehicle.
The rear supension points could be looked at. These are to rubber bushings. If deflection is an issue look to the bushings, and the whole rear axle is meant to twist and move anyways. So I can't see the point of worrying about the defection of rear axle mounting points.
The rear axle needs to deflect in the bushings to be steering responsive. So again if the whole rear axle is meant to twist and squirm, then fiddling with the deflection of the steel body structure seems a bit pointless.
Adding a SAS sway bar will probably create new and different body deflection responses. Perhaps with a stock system, hard cornering will create lots of body twist as the front sway bar is much stiffer than the rear end. With the SAS sway bar, the rear end stiffness is probably closer to the front end. So I would expect that with the SAS rear sway bar that the body twisting is reduced VS stock sway bar. So the SAS rear sway bar probably accomplished a significant reduction in body tortional defection..... for given steering demands. But a SAS bar will perhaps lead to harder corning (human factors).... and on it goes.
The SAS bar does reduce body roll, and this reduces body roll induced dynamic and static steering artifacts in the front end steering geometry.
posted by 207.43.195...
, Mon, 13 May 2002 01:13:15
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