I found this picture on bimmerforum.com. It inspired me to begin to document some roll-cage design considerations.
Lets start with the basics. Note how the main hoop is built on boxed foundation blocks called Plinths – the front legs are on similar blocks. This allows the cage to be tack-welded in place, then lowered from the blocks and fully welded, then raised back to the final location. The footers cannot exceed 100SqIn per SCCA regulations.
The rear upper bars attach at the rear bulkhead. This is an area of contention regarding the E36 chassis. One camp advocates connecting the main hoop to the bulkhead claiming that picking up the chassis as high as possible yields the greatest increase in torsional stiffness. The other camp argues that connecting the main hoop with the shock-mounts and therefore the suspension is best. I fall in the former camp. The cage pictured manages to do both. The main hoop is connected to the rear bulkhead and the rear shock mounts are tied into the rear subframe. Nice.
All 8 rear suspension mounting points (4 sub-frame mounts, 2 trailing arm mounts, and 2 rear shock mounts) are all connected. This increases the stiffness of the rear of the chassis. The smaller size of the tubing connecting the rear suspension shows this is not a full tube-frame car. If it were the tubing would be the same size as the main cage.
This cage also uses the construction method known as “super-nodes”. This is when tube intersections (called Nodes) are designed so the all the tubes intersect precisely and the location of intersections is carefully planned so that the maximum number of tubes joins at each intersection.
In the upper foreground a pair of diagonal windshield can be seen. The windshield opening is one of the larges openings on a roll-cage. These diagonal provide a significant increase in stiffness.
This cage has one a really cool feature. It’s a “jack-tube” welded to the door bars at fore/aft balance point and descending through the chassis. It provides a perfect location to lift the car from below. A short spike-like attachment on a floor-jack goes into the tube providing a totally secure method of lifting the car.
Note also how the entire chassis has been seam-welded. This is a technique to stiffen the chassis by adding additional welds to the body and chassis panels. The factory only welds a chassis at a few locations.
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