This could actually lead to some very interesting discussion. There are certainly trade-offs. Without doing the math, the biggest bang for the buck comes from more depth. The reason being stems from the equation for the area moment. On a rectangular section, this equation is (base*height^3)/12. As you can see, having the height cubed adds the greatest stiffness. To obtain the section modulus, you then have to divide the area moment by the distance from the center of mass to the point in question. Of course, for irregular sections there is some additional math required, but additional height is the greatest contributor to stiffness. In the case of the Isuzu frame, a plate is probably more than adequate. I suggested the extra depth, only because of the amount of strength you can gain from it. Not that the plate on the outside will not add stiffness, it will add a great deal. But, when it comes to things like frames on off-road rigs, I am a big fan of overkill. If I get some time to day, I will do a few calculations and post the results, I think it will be good info for anybody on the board considering an SAS. Does anyone happen to know the dimensions the frame at the point of the engine mounts, including the thickness of the frame? If no one posts dimensions, I will base the calculations on 2x3x1/8" tube.


Okay, I just ran a few quick calculations. Using the equation for section modulus from the Machinery's Handbook, 25th edition. Z=(bd^3-hk^3)/6d. For a standard 2x3x1/8 tube the section modulus is .978in^3. If I add 1/4" to top and bottom, increasing the height by 1/2" the section modulus increases to 2.35in^3. If I add 1/2" to the sides (basically a 1/4" inch plate on each side of the tube), the section modulus is 1.73in^3. You can see that a 1/4" strap, top and bottom is 1.4 times stiffer than a 1/4" plate on each side. Either way, you have increased the stiffness of the tube, but additional material added away from the horizontal neutral axis of the material gives you more bang for the buck. Notice, with the side plates you have almost doubled the stiffness of the tube, but with straps top and bottom, you get more than double the stiffness.