Matt Romanowski

kcpaz - that is a very nice looking cage. You spent some time on it and take pride in your work and it shows!

kcpaz

I saw all you guys at NASA Nats. but was busy working on that Yellow Radical and never got a chance to come by and say hi. You all represented the class very well

Thank you very much.

J richard

It seems like voodoo but its pretty simple to look at a cage and tell quickly if it is an efficient design. (and not critiquing the cage above)

1. Triangulation: Each panel in the cage will be strongest if it is a pure triangle, triangulation puts each member in a pure axial tension or compression, the tubes are strongest in this axis and relatively weak in bending. Any bend of the tube that is a side of the triangle creates a weak point that puts the tube into bending, the straighter the better, but this is always a compromise of getting things to fit.

2. Load Path: if a cage is well designed each tube will work most efficiently in pure tension or compression, any tube taking a bending moment will be the weak point. So as kent pointed out you want the loads to be resolved back to the chassis. Easiest way to see this is anywhere there is a "T" that is unsupported. This is most bolt in cage kits. Even small offsets put torsion and bending into the mix. Looking at a cage just follow the tubes, forces flow through from one element to another a well done cage will take all these forces in simple and fluid lines.

All of this is compromised by squeezing it into a chassis and you can overcome some of these issues with heavier or more tubes but that just adds weight vs a more efficient use of material...that's the art of the engineering...and then there's the prescriptive issues that sanctioning bodies add to the mix that compounds the issue...

multi21

Thank you for the explaination. After reading your post and examining all the photos I see what you mean.

Rceron

You would think you could take a 911 tub and have a standard template constructed, that met this criteria, and just have welders copy. Maybe getting agreement on a properly designed template is a none starter. Would the alternative be to just have a qualified welder copy the cage that is in a factory cup car?

Excellent thread in any case.

333pg333

? Do you mean transplant a Cup cage into a Boxter? Square peg - Round hole.

jdistefa

i.e. (no affiliation)

http://rothsport-racing.com/Products...l-Cage-Kit.htm

J richard

This was the approach I started with with my spec box, a cage identical to my cup in the boxster, but it quickly became apparent that they are two different animals altogether. After takiing through all the details and nuances my specbox sports one of Kent's cages like the blue car, it is a really nice build and the craftsmanship is top notch.

fatbillybob

Kcpaz,

What is the best gauge of steel to use for the "A" pillar gusset?

jdistefa

Still wondering how the seat mounts fared...?

mglobe

Thanks for the informative post. I love it when I can learn from the folks here.

Nader Fotouhi

Very sobering thread. I just cut the existing cage out of a 993 in order to replace it. The plan is to replicate a Cup car cage because I do not want to gut the doors. I have looked at PCA, NASA, and SCCA rule books and given that minimum weight for car+driver in PCA is 3170lbs, it looks like 1.5"x0.120' tubing would be adequate. I was thinking about 1.75"x0.12", but given my age and shape, I am concerned about getting out quickly in an emergency.

Any thoughts on cup car cage design and the tubing size? NASA is the only group that requires 1.75"x0.12" for cars weighing 3001-4000 lbs, but that is without driver and fuel.

sundog

Does anybody have a structural analysis of their cages performed by a professional? We do crash analyses for many types of vehicles, and find that the "intuitive" structural solution is rarely the optimum or the safest.

J richard

This is really the root of the problem IMHO.

Most of the cages built these days are based on prescriptive requirements from sanctioning bodies (tube placement, wall thickness and diameter, hoop and uprights number of bends, etc..) that are best practices based on experience. Accidents like this happen and then changes are or aren't made to the requirements, and for the most part everyone is safe.

The issue is that a better cage could be made by engineering a cage in an analytical method, it would be stronger and lighter, a factory cage, P&M, NASCAR are all examples but they cost big$$ to do the engineering and the you end up with a cage that doesn't meet the specific prescriptive requirements of the sanctioning bodies. It took a FIA cert and wagenpass to get an early cup approved through NASA. And it still had to be drilled :-(...

I also has a safety devices cage in my 964 that was an engineered cage but did not meet prescriptive requirements of PCA/NASA without changes...

So if you are so lucky as to have an engineered cage that can be duplicated its a great place to start but you may have to make it heavier due to prescriptive requirements, or get a shop that is run by an engineer (shameless plug for Kent) or take a swing at an intuitive prescriptive example of what others have done and threads like this...

niche

Never really had an appreciation for cages until my friend sent me a video of that crash. Subscribed.