Roll Bar 101

[decoupe]

Can anyone provide the material type (alu, mild steel, stainless) diameter and wall thickness used to fabricate a roll bar appropriate for a street e9? Design considerations?

This comes from a comment Paul Cain made to a posting about chassis stiffness and the best bang for the buck to achieve it. This would be in addition to seam welding. I believe he recommended a full width hoop behind the front seats with braces to the rear coil spring mounts under the rear seats. This would be in addition to seam welding which starts this winter.

The start of this question came from my wife asking what holds the roof up if I rolled. I couldn't give a very credible answer.

 

[gazzol]

Most cages are made from 38mm (1.5 inch) OD tubing. The wall thickness is normally 2.5 mm (just under 1/10 inch) for F.I.A approved roll cages but can be thinner for a road car. When I made my cage it was to do exactly what you intend (to stiffen the shell) which it has achieved however, I also wanted to keep the car as light as possible so I used 1.5 mm tubing (about 1/16 i think??).
Mild steel is all you need use, aluminium has been banned by the F.I.A. for racing. If you want to go the whole hog then extruded (seamless) is the way to go but it is very expensive compared to rolled tube and only needed if you are going racing.

 

[decoupe]

Thanks G. Have you posted any photos showing the installation?

This has to do with making the choice of hitting bambi and company (deer, elk, moose) or going into the ditch. We have hundreds of deer, lots of elk and a few moose hiding along the local highways. They are most active at dawn and dusk and even less preditable than a latte swilling, cell phone chatting soccer mom.
If I hit a 200-600 lb deer, the car is a right-off but I'm fine.
If I hit a 400-1000 lb elk the car is destroyed and I'm in hospital.
If I hit a 700-1600lb moose it lands on the roof (long legs) and I'm dead.

I'll probably choose the ditch but the car might roll - hence the roll bar.

 

[Mario L.]

Roll cages . . .
Roll Bars .

Here in the USA , I do't know where you are on this small planet, roll cage and roll bar specs can follow several different standards.
SCCA GCR
BMW CR Rules
NASA
to name a few . .

That said I'd recommend following the guidelines from the SCCA which incorporate the final weight of the car, All cages should be built using DOM tube (Drawn Over Mandrel) steel tube. DO NOT use ERW which is the seam welded tube.

For a CS Coupe the OD should be 1.750" OD x .095 wall
or 1.50" OD X .125" wall

The 1.750 OD is stiffer and will give you a far stronger cage for the weight.
But most importantly you need to seriously look at the forces on the chassis to determine the mounting points and the base plate size.
Base plates should be 3/16" thick and fully welded to the chassis, or if bolted done with a backing plate under the car of equal size.
All tube fitment joints need to be precisely fit, no gaps at the joints. Welding has to be done by someone experienced, especially the seam welding you are talking about!

A CS Coupe is very flexible . . and can easily be stressed by improper welding .

For a street application consider the roll bar only, anything around your head like a cage is instant death when you're not wearing helmet.
The main hoop should allow for clear access to stock seat belts . . but also provide a cross bar for track harness belts. Consider a removable X-Brace in the center. See my website for a production piece for an E30 M3.

I could ramble on, if you need more help, please post me directly, or ask here.

Good luck!

 

[Malc]

Mounting points

1... Sills at the base of the A-pillar, tubes through to the strut tops at the front an option
2... Bsse of the b-pillar, which is only 1/2 there on a coupe :roll:
3... Rear onto the rear wheel arches
4... Door beams from the A to the B pillars

To be effective it needs a cross piece at the top of the windscreen and at a minimum a diagonal from the hoop at the B pillars to one of the rear wheel arch mounts. "X" bracing is even better.
Now you can go stage rallying!

Minimum is a hoop at the B-pillars with legs to the rear wheel arches and a diagonal brace otherwise it will just fold up if you roll the car.

 

[gazzol]

Here are a couple of photos of my cage. The first is the completed cage prior to being painted satin (semi flat) black, the cage is uside down and the nearest section is at the front of the car.

The cage is mounted to the car in the following manner, the two plates at the front in the above photo mount to the front bulkhead (firewall) via re-enforcing plates welded to the other side of the bulkhead. Then via welded in mounting points to the A post, B post, rear subframe mounting, and rear bulkhead. inside the boot (trunk) bars come down to the rear suspension towers and from there to the differential mounting (3rd photo)

Here is what the cage looks like in the rear of the car, I doubt you will want anything as extensive as this as it nessesitate a stripped interior!

And here is the continuation of the cage into the boot.

 

[chicane]

Does ANY company or anybody MAKE the E9 roll cage kits or is these one off designs that people are building?

 

[shanon]

To my knowledge, no one makes a kit, and my brothers and I have looked!
My brother's racer coupe has a custom bolt-in 4pt rool bar with cross braces (Martini bars) for the harness's. He is now considering making it a 6pt cage with door bars because he is going much faster now with his racing motor.

If your considering a roll bar or cage, it might as well be vintage racing /track day legal for the effort and the car's resale value. Road or race, its the same amount of fab work, it might as well be legal and do what its supposed to do.

Check out Autopower cages (Google) for desgn ideas or as mentioned, check with local track/car clubs for cage requirements.

...sounds like a 'batch of roll bars' should be made somewhere.... :roll:

-shanon

 

[Malc]

The UK MSA "blue book" is worth a look re roll cages to UK and FIA requirements
Malc

 

[Philip Slate]

Gazzol, I really like your solution for putting the cage mounts 'through' the rear bulk head. Very clever indeed.