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nocones
nocones UltraDork
12/19/19 11:34 a.m.

In reply to maschinenbau :

I don't know if I need a full cage or not.  I can read into the rules that the roll bar does and does not support a tube frame.  That said the other things the car is being built to do require the full cage design. 

NASA specs require a 1.5" OD .094 wall cage.  SCCA is similar.  The tube size is smaller then NHRA but the design is similar.  I theoretically could build the main hoop to NHRA roll bar specs (1-3/4" vs 1-5/8) and then build the balance to NASA which may save some weight but would require me to buy 2 sets of bender die so I'm not sure it's worth it.  

I also cannot use ERW.  DOM is required by non NHRA.  I'm sure ERW is safe it's just not allowed.  I've estimated the steel for the car based on published retail pricing from metal supermarkets and the whole chassis is estimated at $514. 

I suspect I can get better pricing once I actually order the steel but for budgeting that was safe.  $500 bucks is a lot to spend but the overall expected budget still has some room for overages.

 

 

So overall does my interpretation of the rules seem to meet them for a tube frame car in the 11-13 second range?

 

SVreX
SVreX MegaDork
12/19/19 12:13 p.m.

I’d build to the NASA specs, and not worry about the NHRA. You won’t be running 10’s. 

1/4” difference in outside diameter doesn’t seem like much, but in a car this size it’s cumbersome. 

Stampie
Stampie UltimaDork
12/19/19 1:14 p.m.

In reply to SVreX :

If he builds a cage as pictured then he can use 1 5/8ths tubing. Don't think those door bars will meet nhra. Why not build to nhra tubing if it exceeds minimum spec for NASA?  Otherwise he's limited to 11.5 in the Challenge. 

 

SVreX
SVreX MegaDork
12/19/19 1:27 p.m.

In reply to Stampie :

Because building to NASA or SCCA specs insures a decent life for the car after the Challenge. 

This car will shine in the autocross, not the drags. 100 lbs less roll cage will make it more nimble in the corners. 

The FASTEST quarter mile he is projecting is 11.00. We are ALL optimists, so I’ll bet less. But even if it could go 11.00, the .49 second penalty for not having an NHRA legal cage could easily be made up in the autocross with less weight. 

The cab of a 360 is REALLY small. I’ve sat in Nocones’’ car (Before I sold it to him)

Building one-off Challenge oddballs sucks. This is too big an effort to penalize himself in future competition venues by overbuilding the cage and adding weight he doesn’t need. 

Thats why I would build it to NASA. 

gumby
gumby HalfDork
12/19/19 4:22 p.m.

1.5x0.095 = 1.42lb/ft

1.625x0.083 = 1.36lb/ft

It will be lighter to build the cage to NHRA spec if you use 4130, and will be overbuilt for NASA/SCCA. You can add any extra tubes, beyond the NHRA minimum, at the smaller NASA/SCCA tube sizes and the cage should be certifiable for multiple sanctioning bodies.

SVreX
SVreX MegaDork
12/19/19 5:20 p.m.

In reply to gumby :

It’s been a while since I checked, but I don’t think those tubing sizes are permissible by the NHRA. 

IIRC, ALL  tubing must be 1 3/4” OD. If it’s 4130, the wall thickness can be .083. But if it’s mild steel the wall thickness needs to be .118. 

That makes the mild steel cage really heavy for a car like this, and the 4130 very expensive for a Challenge budget. 

SVreX
SVreX MegaDork
12/19/19 5:25 p.m.

 

1.75x .118 = 2.089lb/ft

1.75x 0.083 = 1.478lb/ft

Stampie
Stampie UltimaDork
12/19/19 5:45 p.m.

In reply to SVreX :

1.75 is for a roll bar.  1.625 is for a cage. That's why I posted above because he's building a cage. 

SVreX
SVreX MegaDork
12/19/19 5:52 p.m.

In reply to Stampie :

Ah , yes. You’re right. 

But the wall thickness is still required to be .118, not .095 or .083. 

gumby
gumby HalfDork
12/19/19 7:47 p.m.

1.5x0.095 is the NASA/SCCA spec, and I was comparing that to the NHRA 4130 cage specs(1.625x.083). I wouldn't go anywhere near a car of this scale with 0.120" wall anything, but that is my chassis builder mentality. I understand the debate here, I just couldn't stomach it myself

WRT using MS at the NHRA spec of 0.120" wall, 1.625 OD for a full cage is 1.93lb/ft. Comparing this to the SCCA/NASA 1.5x0.095:
1.93-1.42 = 0.51 lb/ft
0.51lb x 60ft = 30.6lbs heavier to build the cage with NHRA spec main tubes. While significant, this is no where near the alleged 100lb difference. Going to 1.75x0.120 for only the roll bar tubes and 1.5x0.095 for everything else will likely net the best compromise in the weight discussion but, as mentioned, cockpit intrusion is a real point of consideration.

There is a note in the NHRA diagrams for substituting an X in the door openings. However, the tube they call out(4130) for the substitution is lighter than NASA/SCCA would require. The X as drawn is legal, but would need to meet the heavier of the sanctioning body requirements to make the jump.

Always remember, chassis building advice on the internet is worth exactly what you paid for it, check with your local sanctioning admins, YMMV, etc....

Stampie
Stampie UltimaDork
12/19/19 8:58 p.m.

Just a note, Gumby, being who he is, is using the term 4130 which lay people refer to as chromoly. NHRA allows that to be .083 wall. 

Indy-Guy
Indy-Guy PowerDork
12/19/19 9:11 p.m.
Stampie said:

Just a note, Gumby, being who he is, is using the term 4130 which lay people refer to as chromoly. NHRA allows that to be .083 wall. 

 To elaborate and clarify: Gumby is a professional fabricator who builds race car Chassis for a very respectable company.  He knows his E36 M3.

Plus. He's a cool dude.

Stampie
Stampie UltimaDork
12/19/19 9:28 p.m.
SVreX said:

In reply to gumby :

It’s been a while since I checked ...

That makes the mild steel cage really heavy for a car like this, and the 4130 very expensive for a Challenge budget. 

I just checked onlinemetals.com and 4130 .083 in 20 foot sections is actually cheaper than mild steel DOM .120 in 20 foot sections. 

nocones
nocones UltraDork
12/20/19 12:48 a.m.

I really appreciate the discussions about various cage rules and options.  You are basically all bringing up the internal debate raging in my head.

If I use 4130 are there any special things I need to do to weld it to the rest of the chassis?   I would plan to TIG weld the whole thing.  

Reading again in the NHRA rule book I believe even at my slow ET due to the modified nature of the firewall and floorpan I will have to have the 2" square side rails.

Section 21 pg 26

Unless an OEM framerail is located below and outside of driver’s legs (i.e., ’55 Chevy, ’65 Corvette, etc.) a rocker or sill bar, minimum 1 5/8-inch x .083 CM or .118 MS or 2-inch x 2-inch x .058-inch CM or MS rectangular, is mandatory in any car with a modified floor or rocker box within the roll-cage uprights (excluding 6 square feet of transmission maintenance opening). Rocker bar must be installed below and outside of driver’s legs and must tie into the main hoop, the forward hoop, frame, frame extension, or side diagonal.

 

My local Steel place has 1-5/8 .118 DOM at around 6.45/ft.  4130 1-5/8 .083 is $8.13/ft.  The price difference would be ~$100 to save 30lbs.  I think I would try to make that work assuming there is no issue with welding the 4130 to the rest of the frame made out of MS.  

I will probably go with a more traditional X brace side impact protection vs the bent tube intersection.   The SCCA stands alone in sanctioning that style.  I will likely end up with the X brace or perhaps Nascar style bars.  I can alway run something simple for NHRA compliance for the challenge and add proper W2W protection later.  That type of addition to the chassis is easy.  I want to be sure to future proof the main and front hoops as they will be built into the chassis and basically impossible to change.  

 

Thanks again for all the input. 

SVreX
SVreX MegaDork
12/20/19 6:13 a.m.

...and to further clarify...

I don’t know E36 M3. Anybody that makes technical decisions based on something I say regarding this is a complete idiot. 

This is a learning experience for me, just as it is for Nocones. 

But I HAVE done pretty well at the Challenge, and I HAVE learned the perils of building a car specifically for the Challenge that had limited use later. I will never do it again. 

Carry on...

gumby
gumby HalfDork
12/20/19 6:45 a.m.

In reply to nocones :

You are on the right track. TIG is required for 4130, and that is where the greatest expense delta shows up. While many(all?) sanctioning bodies discourage mixing DOM and 4130 in the cage structure, the whole shebang must still be attached to a production chassis in a stock tub build. The same methods will be fine for you attaching to a MS rail.

I read the 2x2 paragraph the same as you are. A call to your local NHRA division office is probably the safest route to an official interpretation.

I would also recommend making contact with any other sanctioning body you plan compete under. Each tech guy will have his own interpretation when he sees a "bastard" chassis. Example being a main hoop that is larger OD than called out, but thinner wall. A note from main office is typically enough to clear things up. wink

poopshovel again
poopshovel again MegaDork
12/23/19 7:05 p.m.

Is this the part of the movie where we realize svrex and frenchy are the same person???

nocones
nocones UltraDork
12/24/19 9:59 a.m.

I found 4130 for $6/ft semi local.  I will be doing the 4130 cage.  It's lighter and cheaper.  I have a TIG so it's a win win win.  

nocones
nocones UltraDork
12/24/19 11:03 a.m.

Also as a blast from the past my Father in law had pictures of when we picked up the car 10 years ago!

It really shows how tiny the car is.

That's a Harbor Freight 4*8 trailer.  You can also see how rough the car is and what kind of insanity I'm in for.

TurnerX19
TurnerX19 Dork
12/24/19 11:07 a.m.

In reply to nocones :

Never was the term "we picked up the car" more appropriatelaugh

Pete Gossett
Pete Gossett MegaDork
12/24/19 11:16 a.m.

In reply to nocones :

Damn I almost didn’t recognize SVreX with all that dark hair!

SVreX
SVreX MegaDork
12/24/19 11:23 a.m.

In reply to Pete Gossett :

Who’s that??

Bent-Valve
Bent-Valve HalfDork
1/3/20 8:43 p.m.

Stumbled across this and though about your build. I am looking for my next build candidate and a friend at work happens to have a junkyard. I have only seen pictures. I can't help but notice the old iron in the background...

I'll find out what they want for it Monday. If I don't use it for my build and you want me to get it let me know. Oh and my ideas are very different about this, I was thinking a motorcycle engined car. Then last night I saw a video of autocross using a stock 360, hairy and scary sort of describe a 25 horse 2 stroke and swing axles trying to corner.

David S. Wallens
David S. Wallens Editorial Director
1/3/20 8:53 p.m.

Bring it.

loosecannon
loosecannon SuperDork
1/3/20 9:18 p.m.

Cool build, I'll be following along.

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