Project Update - May 18th, 2017: This round we will cover a number of fabrication tasks, including the roll cage and seat mounting. We also did a good bit of work to both the front and rear BMW M3 subframes, modifying and reinforcing them. The rear subframe was also modified to accept a different differential housing (for 8.8" IRS) and the reasons for doing that are explained in detail.
A pair of custom "low profile" inner door panels were built to allow for operational door window glass, which is one of the stock features retained to compete with this car in the Optima Ultimate Street Car series. We talk about concessions needed for Optima as well as the NASA class we are building for at the end. Let's get started.
RACING SEAT CHOICE, MOCK-UP, & MOUNTING
Before we started building the roll cage we needed to pick the seat(s) for this car. This is a critical decision in any race car build, as it needs to fit the driver(s), be comfortable and supportive, and provide the proper level of safety for the class/power/driver. Then it needs to be mounted correctly, safely, and with the right tilt/height/fore-aft placement.
Like with all seat installs we do, we started in our lobby with the customer - test sitting in about a dozen different seat models. We had a number of race cars in the shop with most of these seats also, so he got to sit in them on stands and in cars. We're looking for head placement relative to the halo, shoulder height relative to the harness slots, and width sizing in the shoulder and hips. In the end the car's owner and I both fit the best in the Sparco Circuit II seat (above right). Wait... why does it matter if
I fit? Well, I might drive this CSL clone in a few events at his behest. The sacrifices I have to make... :p
Some marketing speak from Sparco:
The Circuit LF is an lightweight competition seat specifically designed for road course (ie: circuit) use. Made from aramadic fiber-reinforced fiberglass, the Circuit has an ultra-light shell (LF is for lightweight fiberglass). The integrated head bolsters offer added neck protection and the driver’s legs are also supported well with high side bolsters. The shell is wrapped in an impact-absorbing and fire retardant fabric that helps hold the body in place, with everything meeting FIA approval.
The Circuit "II" version is a more appropriate width for "adult humans" - almost nobody over 100 pounds can fit in the "regular" sized Sparco or Cobra seats (which is why we rarely have them in stock). And the weights published for all seats are always low - I don't think they include the weight of fabric and cushions. Of course we weigh everything, and this halo style racing seat is 24.10 pounds.
Initially we had hoped to use an E46 seat bracket design that was identical to one we fabricated for another E46 race car we are building, shown above right. This beefy bracket is made from tubular and angled steel, is very rigid, and has integral anti-sub-mounts. This "chassis bracket" is made for use with a slider on the driver's side, and is more substantial than the seat brackets used in
my TTD classed E46 330, but sit a hair higher. I like this design and might go back and have the 330 brackets replaced with some like this.
At this point the factory lateral seat braces were still tack welded in the floor, and the driver's Sparco racing seat was mounted. We grabbed a similarly sized passenger seat (MOMO) and fixed bracket from another E46 race car and began the roll cage construction around them. The car owner came by for a test fitting, and it was apparent that the seats were going to be too high relative to the roll cage for safe use
without a helmet (ie: for the Optima "road rally" event), so we had to go back and re-examine the seat mounting.
We had little choice but to cut out these factory floor seat risers. This was a tedious task - cutting dozens of spot welds - but it gained the room we needed to make a cage that was safe enough for limited street use without a helmet. Lowering the seat like this normally isn't needed for racing use on an E46, but was deemed necessary for the HUGE compromise of running with "a cage on the street" - which is never an ideal situation.
Once those risers were removed there was even room gained by removing one layer of sheet metal from the transmission tunnel - which had two layers with an air gap between then. This extra layer was angled and took up 2" inboard. Removing it centered the seat to the steering column better at the new lower height.
Ryan built a new driver's side bracket that fit the new contours of the remaining E46 floor pan, which were still anything but flat. The two sizes of tubing used were to make the inboard side fit better next to the tunnel.
continued below