Frame Stiffing to reduce Porpoising?
Got a question for you seasoned drivers with older rigs. I have a 1999 F350 Dually Quad Cab Long Bed. I've been driving it for thousands of miles and for the most part, it's been fine. However, while ...
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Actually fatigue isn't typically from (permanent) deformation of the metal. Most metals (and most materials) will suffer loss of strength and stiffness even if cycled well below the yield stress. Steel is almost singular in having a critical stress value (well below yield) that if the max stress in cycling is under that value, you can cycle it indefinitely. Almost no other material has this characteristic, which is why steel is so popular with engineers!
Making a frame stiffer, and then hitting it with the same shock impulse, may increase the peak stress that it experiences. The stiffer frame resists immediately and over a short deflection, the floppier frame may flex over a longer deflection and the stress can be less. It depends on how the stiffness was added. One big difference between 1999 and 2016 is that in 2016 I'm quite sure the frame was subjected to extensive FEA engineering, in 1999 probably not.
Boxing a frame mostly increases the frame's stiffness in torsion, it does not increase the frame's stiffness in bending (other than that due to metal added). In other words if the boxing metal had instead been added to the C section, the bending stiffness would be increased the same amount, but torsional stiffness increased only a fraction. The infamous Chevy commercials of a few years back demonstrated torsional stiffness, and nothing else.
Making a frame stiffer, and then hitting it with the same shock impulse, may increase the peak stress that it experiences. The stiffer frame resists immediately and over a short deflection, the floppier frame may flex over a longer deflection and the stress can be less. It depends on how the stiffness was added. One big difference between 1999 and 2016 is that in 2016 I'm quite sure the frame was subjected to extensive FEA engineering, in 1999 probably not.
Boxing a frame mostly increases the frame's stiffness in torsion, it does not increase the frame's stiffness in bending (other than that due to metal added). In other words if the boxing metal had instead been added to the C section, the bending stiffness would be increased the same amount, but torsional stiffness increased only a fraction. The infamous Chevy commercials of a few years back demonstrated torsional stiffness, and nothing else.
