Friday, June 27, 2008

E36 Rollcage Design Considerations

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.

6 comments:

mark said...

nice topic. i'm on my phone, so i'll have to come back later and get a better look at the photos. i'm at fernley this weekend. (believe it or not, but you can get decent maguro in fernley.)

i think i'm finally driving the car hard enough that the suspension and tires aren't masking errors. can you say 3rd gear tto?

mark said...

what i meant to say was 3rd gear throttle on oversteer. yeah i played with tto, but that's pretty familiar. getting loose under power in 3rd was hard to do in the e30 m3. in the e36 m3, not so much. rfr has lots of off camber stuff, providing plenty of opportunity to over do it.

Ren said...

RFR looks to be a really cool track. When i was at Miller I heard a couple of drivers waxing poetic about it. It's on my short-list now.

True the E30 M3s were a bit down on power, but I sure miss that rev-happy little motor. The S50 seems lethargic by comparison.

mark said...

RFR has some really bumpy sections that were harsh with my suspension. However, some of the more important parts are pretty smooth, so I don't know that I'd soften up a lot for lap time.

Turn 12 just needs to be re-paved. They've taken a grinder to the bumps, so now it's chewed-up bumpy pavement. Coming around the bend in 12 is pretty bumpy too, but the tires seemed to stick okay.

After I sort through the track video, I'll see if I have anything to post. My camera produces files that will just play in Quicktime. But ... 720p60 in AVC is a challenge for most CPUs to keep up. I think I should transcode to something more CPU friendly.

Anyway, I had a lot of fun with the elevation change 6-7-8 run up a hill to 9, then it's a shoot down through 10-10a to 11. That whole section is a lot of fun once you know where the track goes. At first it's a little disorienting when 9 is blind from 8, and 10-10a-11 are blind from 9.

mark said...

RFR in a V8 M3. He doesn't appear to be particularly fast on the main straight. His absolutely sucktacular line through 23 may have something to do with this. I got a ride in a Z06 that was in triple digits at the exit of 23, and was somewhere in the 130+ mph range at S/F. (I couldn't see that part of the speedo very well.)

mark said...

Let's try that again ...

M3 V8 at RFR