Let me also bring up this point.
CURRENTLY the rule calls for 1 5/8" diameter tubes at these speeds.
Most cars of this type take years to get to the point where they're ready to race, and in that time frame, RULES CHANGE.
Next year, the car may be done, but if the rules change, it might not be compliant.
We almost lost a MAJORITY of motorcycle streamliners due to a proposed tubing size rule change a number of years back. I believe it was only because of the popularity of the class that the change was re-examined. I doubt small-bore special construction classes would garner the type of internal institutional support that the more popular, high-visibility MC streamliners did, if a change in basic construction were to be re-examined.
Nobody's got a crystal ball on this stuff, but while the rules permit 1 5/8" for the speeds I run, when I put the cage in the Midget, I went to 1 3/4"
Overbuilding now can go a long way toward not rebuilding later.
Structurally speaking . . . . . allow me to add this:
Structural strength in tubing is raised significantly by increases in diameter, approximately 1/8" to 1/4" increase in diameter.
Structural strength is only raised modestly by small increases in wall thickness, say from .095' to .120"/.125".
Check any strength of materials table from any tubing manufacturer. Note also that there are differences in strength in tubing manufacturing, ie: ERW Vs Seamless Vs DOM; as well as strength differences in material specification.
And all of this does not consider quality of fabrication or welding or any post welding heat treatment/stress relieving . . . . .
Going toward the "cheap end" on any safety equipment, not just roll cages/structures, might not be the "insightful" thing to do, certainly from a long term planning view, as MM points out.
Also, you might want to have a dialog with a competitor who has survived a "ferocious crash" in a vehicle with an "overbuilt" safety structure. Finding a surviving competitor from a crash of an "underbuilt" safety structure might be "more challenging" . . . . . and is the reason why safety structure requirements are "upgraded".
And finally, if you do not possess the ability to calculate the amount of energy that would need to be "dissipated" in an "unfortunate incident", at record speed in the vehicle you propose to build, and the ability to calculate the structure required to "protect" the driver/occupant", you really need to stick to the "overbuilt side" of the rule book.
Once you are "crashing", you are no longer a driver. You are just a bb in a pail, hoping your pail is strong enough to withstand the "beating".
Fordboy