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The "feet" are flat steel and there are uprights that are used to clear a big hump in the floor, but the overall structure is still more than 2" lower than before. There will be additional structure added under the car to reinforce the seat mounting points, which will tie into the cage.
Now that picture above shows Ryan in the driver's seat and he's around 6'4" tall - yet his head has massive clearance to the roof and upper cage bars. We kind of went to extreme lengths on this install to gain a massive amount of headroom, and shorter driver's might get a thicker seat pad, but for street use without a helmet its finally deemed safe. Of course we will add SFI padding to all bars around both occupants.
This is the final seat mounting with both Circuit II seats. The driver's side is on a slider but the passenger side is fixed. Both seats are 2+ inches lower than the initial fitment - you can scroll up and see the first mounts that kept the 3" tall seat braces and see how much higher the seats are relative to the roof and upper cage bars.
ROLL CAGE FABRICATION
We talked at length about cages vs roll bars in
our last update to our "daily driven track car" project build thread, which is the E46 330 we built and I race in NASA TTD Time Trial class. That car's purpose was to show how far we could go on track in a "street car", and for that build a roll cage wasn't appropriate. It has 245mm Hoosiers and less than 220 whp.
The Optima series does you no favors when it comes to safety allowances, but this V8 powered M3 will have more than TRIPLE the horsepower of our TTD classed 330. It also has giant 345mm Hoosiers for NASA use, big downforce, and a lot more potential for speed and carnage. So on this M3 a cage is not only a good idea, we consider it a requirement.
Obviously some of the cage progress was shown in the seat mounting section above, but that's because the cage and seat mounting go hand-in-hand - another "concurrent" set of tasks that I am splitting apart for clarity. We also showed "considerable interior structure removal" from the back seat bulkhead in the very first project thread update.
Jon, Jason and I all had input with Ryan so that we were all on the same page with respect to class rules and series requirements for the cage material and layout. We chose 1.75" dia x .095" DOM 1018 seamless steel tubing for this cage. The design emphasis was on getting the tubes as far away from the driver as possible - while still keeping roll up door windows for Optima. This would
not be a "wheel to wheel" cage (which has more robust door bars and sometimes additional tubes) but a Time Trial car with very limited street use - I go more into the series and classes this car will run at the bottom of this post. We built the cage with those aspects in mind.
After the main hoop was bent up and set in the car the rear downbars were landed in virtually the same spots as the 4-point welded roll bar we added to our shop 330 last month. This M3 cage happened first and I liked the layout so much I asked Ryan to duplicate the back half for my 330 (minus one of the diagonals in the main hoop; the M3 got two).
Many BMW roll cages you see on the interwebs have these rear down bars landing on the shock towers (not doing coilover rear springs, so that's not useful to us here), or into some elaborate jungle gym of triple-triangulated tubes tying into rear subframe mounts (we've already beefed up the subframe mounts) and everything else. With respect to those cage builders, we just don't feel that any of that is necessary on a time trial car. It's just throwing steel at a perceived problem with minimal return. This is a phenomenon we call:
Steelitus.
I already know how these "roll cage design" discussions go, and I'm not here to argue with the internet cage experts. I just know what works well for what we have built in various cars and different racing classes. No, we don't have a chassis jig to measure torsional stiffness / frequency (almost nobody on the planet has these types of tools), and we didn't do FEA on the structure, but this cage will be appropriate for the final weight (2600) and type of racing (Time Trial) this car will do.
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