We running two nets on our Champ/Chump Car. We also have roof nets under our bolted on roof panel. Better safe than sorry. They don't weight much and we don't have to run arm restraints.

 

I have been looking at rules for FIA roll cages. It appears that they spec 1.75" x .095 tubing for the main components of the cage and allow for 1.50" x .095 for the support bars such as a the backstays, diagonals and doorbars.
Am I reading that correctly? I haven't found any that specify tubing size by the weight of the car like some organizations do.

 

NASA is pretty specific... "Required" tubes have a min spec. Optional tubes are optional, with no min spec.
Up to 1500 lbs 1.375” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) 1.500” x 0.080” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) RACING AND COMPETITION 47
1501 - 2500 lbs 1.500” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) 1.500” x 0.120” ERW* (No issuance of log books for cars with ERW cages) *Note- Specifications listed only for reference for inspection of grandfathered vehicles.
2501 - 3000 lbs 1.500” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) 1.750” x 0.095” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) 1.750” x 0.120” ERW* (No issuance of log books for cars with ERW cages) *Note- Specifications listed only for reference for inspection of grandfathered vehicles.
3001 - 4000 lbs 1.750” x .120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) No ERW allowed. Over 4000 lbs 2.000” x 0.120” Seamless Alloy (4130), Seamless mild steel (CDS Mechanical), DOM, or Docol R8 (only) No ERW allowed.

I'm sure SCCA and other commonly-referenced rulebooks are similar.


The references above to "FIA" spec are mainly the use of IMSA style pyramid or 3D X doorbars and the A pillar reinforcement tubes.

 

Make sure that your local tech will accept the cage prior to building true FIA spec cage.
There are a few major changes in the line of thought between FIA and all US organizations.

 

 

 

Pyramid door bars are stronger, since they are triangulated and lack unsupported bends. NASCAR bars are easy to package and execute, and make it simple to have easy ingress/egress (larger / older / less flexible people). That's why totally optimized pyramid bars are mostly featured on more "high-effort" cages.

 

To further that point, make sure they will accept an FIA cage that hasn't been homologated if you plan on taking any creative liberties with it!

 

I built Morpheus with pyramid X door bars and additional front reinforcement.
Keep in mind that FIA bars take a massive amount of room from the door opening and that might not work for many drivers for quick exit purposes.
To avoid that issue in Morpheus, I replaced the tubes with a 3D curved gusset which is a great way to spend 20+ hours fabricating one piece of sheet metal, even if you're well versed at this -

 

 

Go go to the build thread for Vegas. But ultimately it is simple, you are looking for a straight path from the strongest part of the door hoop and dash bar interface to the shock tower. The dash bar is generally a bit too high to the line up with the top of the shock tower so you end up taking it from a point a little bit lower. So thinking ahead means you build your door bars to intersect at roughly the point where you take your extension through the firewall.

Additionally is important to remember that the Miata is not a strut car so the actual shock tower doesn't have that much load going through it. The highest forces are going into the subframe mounts which are those two big studs that poke out the bottom of the frame rail. Those forces are trying to twist the front of the car and deflect the frame rails out of the same plane. So that's really what you are trying to reinforce, box or triangulate. Triangulation preferred as it uses a lot less material than a box, for a given stiffness.

From a safety standpoint it is intrusion. Like most small cars keeping the front wheel and suspension out of the driver's footwell in a major frontal impact is a challenge. A simple triangulated brace anchored at the door bar foot and a little higher up and extending forward to the stiffest part of the firewall helps hugely in this regard.