Race Chassis - SF, CA - Exocet

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Hi All, 

Last night I finally bit the bullet and ordered my race chassis! 

A bit about my goals.

I'm Andy, and I love cars. Cheap/simple cars are best cars. I've been obsessed with track days/hpde since I did my first one at Sonoma. I got a really ghetto honda civic with a cage and drove it around to Buttonwillow and Sonoma. 



I wanted some rear wheel drive that was easily registered and maintained in CA so I picked up a Miata.





I had an urge for a safer miata after nearly being taken out by an Nissan GT-R with a bad driver (running in high-intermediate...) in turn-11 at Sonoma. He found out about terminal understeer. Ate the wall at ~100mph after passing me and nearly clipping the front of my car (With his wife in the car). That was the moment my tiny miata felt very insubstantial, I ran the rest of the day very conservatively, it took a way a lot of my fun factor. I started looking into fully caging the car or getting a more substantial car; all the options made little sense in terms of money (~3k for a good cage + weight penalty) or the increase in consumables (I'm frugal, brakes/fuel/tires all go up on heavier cars). 

Enter the ghetto miata: I found a supercharger/torsen diff/several good parts for sale that came with a "free" miata. My original plan was to just sell the miata and throw the parts on my track miata. Then it dawned on my that I would feel no guilt about stripping this miata ... and thus we have arrived. Exocet!



My wife found a love for ripping apart cars; very therapeutic.









My plan is simple, I loved my miata on the track so let's keep it more or less the same. Strip the donor, check the consumables and plop the frame on as is. Get it registered. Iterate. I'm more or less channeling Rick's in Las Vegas' build. I have an m45 supercharger on hand if I want to go a bit faster, they're not good for too much power over stock, but a healthy increase. 

[jon]

Welcome!

[steadfast]

Welcome, looks like we have 10 or so people in the Bay either building or have finished Exocets.

[einy]

CLEAN looking engine! Welcome!

[lhintze]

Good luck! It's a great project and will be worth it in the end

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Thanks for the welcome; I'll probably need to quiz some of you registered CA guys after I sift through the sb100 docs.

[BTD]

Welcome! Still wish I could have picked up your ghetto civic on the GRM forums (I think you're the same guy?) - Exocet is too many $$'s to stuff into a wall on a track day for me, but it's a great car and a fun build!

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Nov 8, 2015 4:14:15 GMT BTD said:

Welcome! Still wish I could have picked up your ghetto civic on the GRM forums (I think you're the same guy?) - Exocet is too many $$'s to stuff into a wall on a track day for me, but it's a great car and a fun build!

That's me!

I was sad to see it go, but it didn't make much sense to keep around. I'll have another EG sometime.

I would much rather put a well built caged car into a wall then anything else, cost be damned (I'm putting all my faith in you Mr exocet welder dude). Feeling safe in my car goes a long way for my personal enjoyment. My favorite track is Sonoma and its nothing but walls. I don't care much for cruising or canyon roads; track time is it for me!

[Trenon]

Do not think of the exocet as a fully caged car. It's not. I've had fully caged rally cars and the strength is not the same.

Only the roll bar is made of thick DOM tubing required for protection. The rest is made from thin walled ERW tubing that would, in my opinion, crush quite easily in a collision. Even Exomotive suggests thinking of it more like a really safe motorcycle than a caged car.

There are some obvious weak points in the chassis from a glance. The floor is not structurally sound from side to side, the tunnel has no bracing to stop the chassis from pinching the center in where the occupants are. The seat mounting steel is way smaller than any governing body would allow, its also braced to steel that is arguably undersized. All of the 1x1 square steel would need to be a fair bit larger to stop it from buckling in a crash which is why typical sizes are 1.5"+, its much more stable when it comes to buckling. The floor is not triangulated (unless you have welded floors, the steel floor would brace it). The front bulkhead is not made from large enough steel to stop the engine from entering the cockpit, the riveted aluminum would not really stop anything. The rear roll bar does not go to the bottom of the car, it stops at the mid tube and is continued on bu my smaller 1x1" steel that goes to the floor.

I'm not saying the car was poorly designed. It is designed to be light and fast, and safer than a motorcycle. It does all of these things very well. But If I had to choose hitting the wall in a stock miata with a roll bar, or an exocet I would choose the miata. Considering it a valid replacement for a fully caged car is not right.

Some of the good things are the tires should rip off nice and cleanly, (hopefully missing the occupants). The rear and should also eject in a serious collision. Reducing the mass of the car staying with the occupants and dispersing some energy. 

A side collision or 45 degree frontal collision would probably be the worst case for this chassis. I think a straight on collision would drive the engine down missing the occupants. 

Source: Mechanical engineer that runs a stress analysis company. I have not backed any of this up with analysis.

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Maybe Warren can chime in; he has actually run the numbers. This is in regards to the race chassis which comes with welded floors, etc.

grassrootsmotorsports.com/forum/grm/grm-exclusive-reveal-exocet-spec-a/57663/page1/

This also says the main hoop and downtubes are made of the same diameter and wall thickness.

Another big issue for me is head clearance - which is tight with floor mounted seats on the stock miata. From the diagrams online from Warren I get a bit more head room with the same mounting in the exocet. Arm restraints/HANS/etc are already part of my arsenal!

We'll see; I'll ping he nice people at exomotive and have them weigh in! I'm very excited, lots of lead time to get me components in order as well.

[Trenon]

I'm not sure what you are calling the downtubes. I have the sport chasis and the only DOM tubing is:

Roll bar
Harness Bar
Diagonals
Curved front sub frame tie ins behind the front tires.

Everything else that is round is ERW and thinner. The main structure of the car is thinner ERW tubing. Also Warren talks about rigidity and stiffness. Those are both great for handling, I'm talking about strength and buckling which is for safety.

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"Downtube" is what they call them as well. These are the pictures Warren put up on GRM. 





Which seems to be the same diameter and wall thickness as the main hoop/crossbars according to Warren.

[Warren van Nus]

If it helps, the colors in those images actually denote the trim differences, not materials used. The rear downtubes are definitely DOM. There are actually three different 1.75" round profiles in the frame: 0.095" DOM, 0.083" ERW, and 0.049" ERW (for less-structural things such as the dash tube). All the cage tubes are 0.095" DOM except for the itty bitty gusset tubes. The main sweeps are 0.083". For comparison, I measured one of our original UK frames to be on the thin side of 0.049" (18ga).

The renders in most of the press are from early in the design phase (notice the rear subframe mount tubes sit in slots); no US cars were actually built to that spec. The new "3D" tunnel is much stronger than those old renders. The seat mounts tubes are eighth-wall; there are again quite a few different wall thicknesses in the 1x1 square tubes.

That said, it definitely should not be treated like a rally cage. For serious wheel-to-wheel action, owners should order their frames bare and consult with a cagebuilder to build the cars up to their desired level.

Side impact bars would probably be where I would start for additional safety. Dual-stage SFI foam should cover the side tubes, and I'd want to slap in a full containment seat with arm restraints and a HANS. The "3D" tunnel update really helped space between the driver and side tubes, which is something that can always be improved. An underfloor "butterfly brace" would yield stiffness and strength improvements, too, but that's really hard to make as a product, especially with all the crazy exhaust implementations out there. A petty bar would be great, as well as center and side nets. Tubes to tie the steel tunnel into the firewall would go a long way, especially if they were skinned with welded-in steel. A driveshaft loop would be nice, and thankfully the clutch assembly is completely in front of the driver's feet. The race cage could use a view-blocking tube to triangulate the windscreen (or support the center), and the rear roll hoop support downtubes could be braced as well. The "Race" was designed to be a great baseline that would appeal to majority of owners, but I won't pretend it's safer than a caged Miata that weighs 1000 lbs more.

Could be worse, though.

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Thanks for the input on all fronts Trenton/Warren! A lot of food for thought and solid advice. I was hoping to avoid painting it myself, but I might be changing my order to naked steel 

SFI padding/arm-restraings(I've seen too many videos of track rollovers)/etc are already part of my kit and will remain so, I try to control the little things that I can.

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I don't have better pictures, but the car is most of the way apart.

Picked up a MS PNP2 tonight! Came with a BMW VTPS, and rx7 550cc injectors.



Now to finish reading this documentation. I'm going a bit crosseyed looking through it. Very excited to get rid of the airflow meter, that thing has always inexplicably annoyed me.