NASA Spec3 Build

[matt325]

After autocrossing for about 10 years, it's time to move on to road racing.

I have been rallying in NASA's RallySport program as a codriver for about 2.5 years, but really want to get into the driver's seat. I really like NASA and their programs, so, Spec3 looks like a good place to jump in.

While the final rules are still being hashed out, I am turning my 94 325is into a Spec3-in-waiting-HPDE car.

Cage goes in starting next week.

[blkaplan]

Very cool! good to see another Spec project being started.

[matt325]

Carpet is out and about half the sound deadening.

An old R/C airplane heat gun, a flexible paint scraper, and a drywall mud applicator work pretty well to get the sound deadening off.

[BETO]

Or dry ice and chisel (love the air chisel!)

[Greg S]

Or dry ice and chisel (love the air chisel!)

A heat gun and plastic scraper(then goof off) works MUCH better than the dry ice method. Yes I've tried both...

[speedform]

Good job Matt! I picked up my Spec3 donor car a couple weeks ago and have been going through it fixing little leaks here and there before I start bolting on Spec parts. Keep us up to speed on your progress.

[///M3Augy]

Cool, looking forward to seeing more progress.

[matt325]

After reading a lot of threads here on bf.c regarding cage design, I tried to pull together what I thought were the best features.

This is just a rough sketch, and by no means to scale or even in correct proportions.

-1.5 x 0.120 DOM
-Rear down tubes will meet up with the frame rails, and not connect directly to the shock towers.
-Doors will be bent to shape X, with gussets
-Triangulated foot protection
-Dash bar up high, and straight across

[jonmacs22]

nice sketch and great job with the research.

two small recommendations:
1. the upper diagonal that attaches for your foot protection should be attached to the a-pillar bar where the dash bar and door bar connect. so basically raise that bar up about six inches or so on your sketch.

2. strongly consider attaching your down tubes to the rear bulkhead instead of the spring pick ups. that was all of your rear bars can tie into the same place, ie both your down tubes and x-brace.

3. (sorry found a third thing), if you do go to the bulkhead, have another bar go from that attachment point to the base of your main hoop. It should attach wherever your door bar does.

I hope these recommendations make sense.

Thanks,
jon

[osborni]

Another few:

a) Tie the main-hoop/harness-bar/upper-X node back to the rear bulkhead with a tube.

b) Run a tube from the main-hoop/harness-bar/upper-X node to about 12-18" forward of the main-hoop/a-pillar node. Basically by your head. This goes a long way to stiffen the structure.

c) think about gussets around the window and a-pillars

[matt325]

nice sketch and great job with the research.

Thanks

two small recommendations:
1. the upper diagonal that attaches for your foot protection should be attached to the a-pillar bar where the dash bar and door bar connect. so basically raise that bar up about six inches or so on your sketch.

Makes sense, and, after reviewing my library of images, this is indeed how it should be done.

2. strongly consider attaching your down tubes to the rear bulkhead instead of the spring pick ups. that was all of your rear bars can tie into the same place, ie both your down tubes and x-brace.

I struggled with the rear bulkhead vs. rear "frame" rails. I wouldn't be picking up the springs, as (I believe) that involves punching through the unibody and getting to the subframe. I don't want to/can't do that. But going to the frame rails is the next best thing.

I honestly don't know which is more structurally sound, tying the cage into the rear bulkhead or into the "floor" of the rear of the car.

3. (sorry found a third thing), if you do go to the bulkhead, have another bar go from that attachment point to the base of your main hoop. It should attach wherever your door bar does.

Another few:

a) Tie the main-hoop/harness-bar/upper-X node back to the rear bulkhead with a tube.

b) Run a tube from the main-hoop/harness-bar/upper-X node to about 12-18" forward of the main-hoop/a-pillar node. Basically by your head. This goes a long way to stiffen the structure.

c) think about gussets around the window and a-pillars

Combining the recommendations made above, I get something like this.

[jonmacs22]

Now that is a sketch that I think makes a ton more sense.

Good luck getting the horizontal bar portion of the door bars in and fully welded, but if you can, congrats. If not, consider using sheet metal reinforcements/gussets in the door bars as so many photos around here show.

REgardless, nice sketch and plan.
Enjoy.

[matt325]

Settled on this, for now, for the final configuration.



Used the tape measure to get some rough ideas on length


and, finally, a calculation of the weight differences with various tubing sizes...

[Greg S]

I would do away with the red bar in the back, the bulkhead already serves that purpose. I would also do away with either the green or blue bars in the back that go to the bulkhead, using both is kind of redundant. I would also suggest doing a tube gusset in the windshield area between the a pillar and upper windshield bar. Everything else looks good.

[blkaplan]

Settled on this, for now, for the final configuration.

I may be wrong but how I interpreted the rules meant you could only have two straight bars going from the top of hoop to the rear chassis points, you are allowed to brace the bars with an X but it would exclude the lower 4 points that you have connecting at the main hoop going backwards.

Is the approach you are showing legal for Spec class? I was lead to believe it wasn't.

[jonmacs22]

the cage above is the exact design of my SpecE30 car and was legal. Unless rules have changed, that is likely the way the Spec3 car I have will be modified to as well.

Jon

[matt325]

Nothing is ever final...

I had some comments regarding the usefulness of the "strut bar", as well as the redundant main hoop to bulkhead bars.

I will be discussing this design also with my cage builder.



Oh, turns out I was using R=1.5 instead of D=1.5, kept wondering why the cage looked more like a balloon!

[Spec3Racer]

Are NASCAR bars allowed? I have them in my current racecar (non-BMW) and like how it fits.

[B.Watts]

I would also do away with either the green or blue bars in the back that go to the bulkhead, using both is kind of redundant.

It's about picking up the loads from the side X-braces, rather than triangulation.

I had some comments regarding the usefulness of the "strut bar", as well as the redundant main hoop to bulkhead bars.

I would put back the other bar from the upper portion of the side X-bars and get rid of the purple bar if you can only have 2 bars going to the rear bulkhead. I wouldn't want the upper bar of the X-brace terminating in the middle of the B-pillar like what you have shown. The goal would be to transfer that load to other portions of the cage in the case of an impact. You've got it covered in the front, but not in the back. The lower portion of the X terminates at or near the chassis, so it's needed less IMO.

[jonmacs22]

The only other bar to consider would from the base of the main hoop on the passenger side to the top of the main hoop above the driver's head (at the turn dow of the main hoop on the driver's side).

Thanks,
Jon

[matt325]

It's about picking up the loads from the side X-braces, rather than triangulation.



I would put back the other bar from the upper portion of the side X-bars and get rid of the purple bar if you can only have 2 bars going to the rear bulkhead. I wouldn't want the upper bar of the X-brace terminating in the middle of the B-pillar like what you have shown. The goal would be to transfer that load to other portions of the cage in the case of an impact. You've got it covered in the front, but not in the back. The lower portion of the X terminates at or near the chassis, so it's needed less IMO.

Like this?


I don't interpret the NASA CCR cage rules as limiting the number of bars that terminate at a given location. See 15.6.13 and 15.6.14.B.

There are no rules yet to define if a spec cage will be any more restrictive than this (Ref. 15.6.17).

[B.Watts]

Like this?

Yes...if you're only doing 2 bars, that would be my choice.

[Steve J.]

Wow, no Teh Jaffscage jokes yet...is everyone too hungover from the 3day weekend or something?

[Greg S]

Like this?

Before you weld the roof X in place make sure that it won't interfere with your helmet. Oh yeah, add these bars:

[matt325]

Some pics from this past weekend's cage prep work.