.......... A properly designed SCTA cage should be built to be strong and stiff and prevent any type of failure that could be caused by the cage collapsing on the driver and it should have sufficient room for a Hans device and proper high impact padding to protect the drivers head and a proper seat belt, shoulder belt arm restraint and seat system to retain the driver inside the safe zone of the cage. Rex
Rex - You and others are right as far as you go. But the devil is in the details.
For one thing, by your statement, the transition to 1-5/8 tubing for 2 wheel streamliners would cause no harm other than throwing thousands of hours and dollars of past and ongoing construction effort into the trash heap.
What has generally been left out of this discussion is the character of impact events in a crash. The stiffer you make the anvil and the hammer head the better. But we don't want anvils here. We want the chassis to soften the blow rather than sharpen it. Here the workpiece is the driver, not a piece of steel. But too soft a roll cage will deform either elastically, plastically or both and in the worst case rupture. So a compromise is needed.
You workshop guys: Look at all the hammers in your shop from the softest rubber mallet to the hardest ball pein hammer and think about why it is the way it is. Every time you open a product package or a shipping package take a close look at it and think about the way it's designed. Are you tuned into the idea that the real shock absorbing function an a car chassis is in the springs and pneumatic tires and that the shock absorbers are simply dampeners?
The following is for the engineer or physics trained folks among us.........
You guys who like myself suffered through some formal engineering education, especially the physics and calculus part: Were you as confused and suspicious as I was about the apparent dichotomy between energy and momentum? Or the equivalence of impulse and momentum?
Or the fact that a perfect sharp impulse (time=0) contains every frequency of vibration?
If your work is in the world of dynamics you know this stuff pretty well. Please speak here.
If like the rest of us that kind of math is long forgotten and like me you add an experience based safety factor for impacts to your stress calculations and rely more on what happens on the test stand then understand why I try not to sound like I am any kind of expert.
I remember in the old days many engineering schools had a fun test project for all the mechanical engineers. You formed up in teams and each team designed a box to hold a fresh egg. They built it and dropped it from a high window in the engineering building onto a concrete sidewalk. Build a box got you a "D", "C" if the box didn't break. "B" if the egg inside didn't crack. And "A" if you mathematical analysis of what happened was correct.
Oh yeah, some other thoughts here. Drag out your old metallurgy or materials science book. Look at a stress-strain tensile test diagram for low carbon steel and consider where the most initial energy absorption takes place. (realizing that the elastic energy is given back to work in continuing cycles of vibration until finally dissipated as heat through some kind of dampening).
Look in your strength of materials book (mine was Timoshenko, which dates me) at the subject of beam bending and especially column buckling under compression. Consider the effect of gussets on individual members of a roll cage
Note on the Mars Rover the balloons were full of gas. Gases are incapable of sustaining plastic deformation. It's entirely elastic. Not also that a balloon flattens at the impact point slowing the time of impulse. This is slowed and dampened even further if the soft surface of the ground is reshaped. The dynamics of a "pencil rolling" streamliner have much in common.
Ed Weldon
