Project Professional Awesome Time Attack Evo: Part 1 – Roll Cage and Chassis in Detail

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Once you get the materials figured out, how to build your jungle gym is next on the list. Steel is strongest in tension and weakest in bending, this has to be kept in mind when making the cage. A roll bar without support can easily bend and the protection you thought you had is non-existent. Make yourself familiar with the term dead node a.k.a.  T-Junction. This is when one tube dead ends into another tube without any support, creating a T. These are quite common and should be avoided wherever possible as the tube that dead ends into the other can easily cause the other tube to bend and fail. Plan for how the loads will spread across the cage and minimize bends. The more bends a bar has, the more likely it is to fail when you need it most.

This accident was widely circulated a few years back. This Mustang had a bolt-in cage without proper floor plates or boxes installed. Noticed the bottom left picture, in the circle the cage punched straight through the sheetmetal floor of the car. The roof completely collapsed, but luckily both people in the car walked away. Photos copyright: ThienNam Ninh Photography

When choosing tubing size, you’ll have to reference the rule book(s) for the series you’ll compete in. In our case, the NASA CCR (Club Codes and Regulations) dictate that we use a tubing size of either 1.75” x .095” or 1.5” x .120” where the larger number dictates diameter and smaller is wall thickness. Using their fancy computers, All Speed Innovations calculated a 5% weight savings by going with the 1.5” x .120” tubing. Not insignificant when considering the total weight of the cage is in the neighborhood of 200 lbs. We also went with 1.5” x .095” tubing for bars that were considered additional reinforcement above and beyond what was required by the NASA CCR. This resulted in an additional ~10.46 lb weight savings as well.

With our cage design we went above and beyond just meeting the basic ruleset to account for additional safety as well as reinforcing suspension pickup points. The goal was to reduce flex from the shock tower area, this helps improve suspension performance in a few ways. First, by reducing flex at the pickup points, the alignment stays more inline with how you’ve designed it, potentially lessening bump steer or other funky alignment problems. Second, by reducing flex at the damper/spring mounts, the chassis doesn’t act as an undamped spring, which can make spring and damper changes more repeatable and easier to tune.

I couldn't find the best picture showing the shifted seat in the accident of our V1.0 Time Attack Evo, but hopefully this photo will emphasize what happened to the floor during the crash. Instead of using the stock seat mounting locations, All Speed recommended integrating the seat mounting into the cage. The stock mounts shifted and nearly failed in our accident.

Other areas to keep in mind are aero loads, pedal placement, steering column support and seat placement. We utilized the existing frame rails for rear wing placement, but reinforced them to reduce chances of failure. We also had our pedal box and seat with the seat rails chosen in advance and given to All Speed to ensure they were taken into account along with the complete roll cage design. The seat rails are actually integrated into the roll cage and chassis both to reduce chance of a mounting failure. When our V1.0 Time Attack Evo crashed at Road Atlanta, the floor buckled in the accident causing the seat to shift. The seat rails didn’t fail, but the floor they were attached to did, something to keep in mind when mounting a seat directly to the stock locations on sheet metal!

Once you have the materials chosen and the design… designed…, it comes down to execution. The importance of a good welder can’t be overstated. Reducing chances of cracks at joints comes down to someone that can properly do their job and that relates back to having strong references and good examples of previous work. When constructing the cage, tight clearances are required where tubes are notched and joined. This is where my skill points let me down. I’m 0 points in the welder category, 13 points in fabrication with metal, and a solid 82 in likelihood of removing an appendage when working with sharp tools. Defer to experienced welders and fabricators for this aspect of a cage build please.

Here's the completed cage pre painting. Notice unique features such as installing the door bars through the B-pillar. This creates more clearance for the driver than going on the inside of B-pillar and reduces the number of bends involved if we went around the outside of it.

The stock dash bar was retained to simplify installation of the dash, steering rack and associated components required by Global Time Attack's regulations. Notice three supporting bars going to the front strut towers and the floor plating that wraps the frame rails for maximum safety in the event of a roll over.