Ranked by importance, my criteria for optimum cage design is
1. A safer cage is better
2. A stiffer cage is better
3. More driver space is better
4. Easier driver egress is better
5. A lighter cage is better
The following series of charts is from a thread on bimmerforum.com about cage design. One of the members did a series of mathematical simulations of a roll cage designs using ProE to build the model and Mechanica to run the simulations. These simulations estimate what happens to a roll cage when a load is applied to one corner. The goal is to understand the stiffness of each cage design and the trade-offs involved in different designs.
This approach has a few weaknesses, primarily the lack of a similar model of the unibody chassis in order to evaluate how the two structures work in concert.
That said, this is really interesting work and bears thorough review.
This first picture shows a basic 6-point rollcage with “X” style door bars, harness bar, and a rear diagonal.
In this simulation, a 500 Lb. upward force is applied to the driver’s A-pillar leg and the strain on the rest of the cage tubes is represented graphically.
Notice how large the windshield and door openings are in this design – probably not good for stiffness.
Here is a second chart showing what happens if you add an A-pillar down-bar, change the driver’s door-bar to allow easier access, add a bar from driver’s shoulder to rear bulkhead, and small upper windshield gussets. The stiffness of the cage increases by over 60% with the addition of only a couple of strategically placed bars.
Here we have added a bar from the A-pillar straight to the floor and dash bar gussets. This is 76% stiffer and weighs only 23 Lbs more then the original design.
All the models so far have used 1.5” .120 wall DOM tubes. What happens when you go to 1.75 .090 wall DOM? Cage weight goes down 2.5% and stiffness goes up 7%.
This chart shows the difference in stiffness between two different A-pillar configurations. The design on the left is about 10% stiffer than the design on the right. Clearly there are trade offs to be made here, as the new A-pillar bar on the left is pretty close to the driver.
Bryan Watts has this design (with an additional gusset) in his E36 M3 and says it doesn’t interfere with driver’s vision at all.
The door bar configuration shown here – typically called “Porsche style because it is derived from the a Porsche GT-3 Cup car – is about as stiff as the stiffest designs and makes entry/exit easier and because they bow outward they provide more room in the drivers “office”.
This chart shows the stiffest design to date without crazy braces in the windshield area. It has a fairly radical A-pillar bar, small roof and dash bar gussets and a roof diagonal all in1.75x.090 DOM.
At this point I’m planning a cage like the stiffest shown above but with either Porsche-style door-bars or with a standard X. Both of options are pretty close to the ultimate in stiffness. The standard X would allow me to keep the windows in the car. Unless you have an enclosed trailer (not in my plans for a few years) then windows are really practical. The Porsche door bars can be made to just fit while keeping the window glass, but removing the door panels (which are pretty thick on the E36).
I will also extend the cage forward from low on the A-pillar bar to connect to the firewall. This provides some additional protection for the drivers feet.
The final key feature I'll add is a jacking tube integrated into the door bars. This creates a reinforced point from which to lift the car. A short "spike" attaches to a floor jack effectvely preventing the car from slipping off the jack.
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