Bill Lehman

Regarding tkerrmd's last post quoting rule 11.4.7, using a 2500# car, they require 1.75" dia x .120" wall for DOM or ERW (Drawn Over Mandrel or Electric Resistance Welded) and 1.625" dia x .095" wall for C-M. In this case, the C-M would be lighter because of the reduced diameter and wall thickness. Otherwise weight per cubic inch would be the same.

PedroNole

Tom-

Relative to NASA and PCA, why don't you get a good idea of where you will be relative to the classes relative to your weight to hp. Odds are that you will likely lie at less than the optimum end of the range. You will then want to ADD a bit of weight to be at the very tippy top of the next class (PCA allows you to add weight to move up only one class). I ended up having to add about 140lbs to my car to be at the top of GT4s in PCA and a bit less for GTS4 in NASA (I think that's the way it worked out but I don't have the info with me).

I say all of this to say that I think the weight concern you have between the two might be a moot point. One counter point to this is that WHERE you add the ballast matters relative to handling/balance.

Bryan Watts

No, but I've talked to plenty of experts who have.

Like I said, if you are allowed to use thinner/smaller tubing if you go with Chro-mo, then yes, you can save some weight. However, most amatuer road racing orgs that I'm aware of now require the same size tubing whether you use mild steel or chro-mo.

Chro-mo is plenty strong. Stronger than mild steel, but more "brittle". That needs to be considered somewhat in your design.

Like I said in my first post and this post, the weight savings comes from using smaller tube. NASA doesn't not allow you to use smaller chro-mo than mild steel, so there's no weight savings to be had.

Bryan Watts

Where do those rules come from? Those like like rollbar, rather than roll CAGE rules.

The NASA roll CAGE rules require the same size tube whether you use mild steel or chro-mo. You're allowed to use chro-mo, you just won't see any weight savings from using it because you won't be able to use a smaller tube in the cage rules the way you can in the roll bar rules quoted above.

DWalker

1> nasa "safety" rules are not on the same level as SCCA, so I recommend building to SCCA regs which are accepted by most if not all other organizations

2> 4130 is ok if you are using a very professional builder, otherwise stick to DOM

3> NEVER use ERW tubing in a roll cage- period.

95ONE

I'm currently building / Welding one. I've searched through SCCA, NASA, and PCA. SCCA gets very specific for sub classes and most other racing groups just point generally in SCCA's way of things. The rules for the cages are very close if compared to similar class between sanctioning bodies. If the classes are different, the rules are different. I don't know any other way to explain that to you.

Chromolly and Seamless DOM tubes weigh exactly the same. 1.5" .095 wall chromoly is exactly the same weight as 1.5" .095 seamless DOM. They both weigh in at 1.426 lbs per foot.

The weight savings is strictly because you are allowed to use a smaller diameter tube. Quoted from the SCCA GCR manual Page 147. Proving Post #19 Sorta right but wrong. Roll bar vs Cage rules are different, but you are allowed a smaller diameter for alloy in cage construction.

Here is a cut and paste from SCCA GCR. It applies to following classes.

"2. Minimum tubing sizes (all Formula, Sports Racing, GT, and
Production Category automobiles, and all automobiles registered
prior to June 1, 1994) for all required roll cage elements
(All dimensions in inches):
Vehicle Weight Material
Without Driver Mild Steel Alloy Steel
Up to 1500 lbs. 1.375 x .095 1.375 x .080
1500-2500 lbs. 1.50 x .095 1.375 x .095
Over 2500 lbs. 1.50 x .120 1.50 x .095
1.625 x .120
1.75 x .095

Chromolly weight here
Seamless Mild Steel weight here

That being said. Lets compare the weight savings of going with chromoly tubing.

Estimate about 120' needed for a full cage. Less or more, but lets just use 120' for now.
Car is under 2500lbs and is allowed a 1.5" tubing for Seamless DOM, and 1.375" Tubing 4130 both with .095 wall

Seamless DOM labeled as "Mild Steel" by SCCA gives a cage weight of (120' X 1.426lbs/ft) = 171.12 lbs
4130 Chromoly labeled as "Alloy Steel" by SCCA gives a cage weight of (120'X 1.299lbs/ft) = 155.88 lbs

So you roughly save 15.24 lbs by going with chromoly. Seriously. That's it. NOT anywhere near 40%.

HERE'S THE KICKER

Whats the Cost difference?
Price per Foot is clearly different.

4130 1.375" OD and .095 wall is expensive. The cheapest I found it was for $6.03 /ft. here I keep getting quoted more than $7.50 / ft. locally.
DOM Seamless 1.5" OD and .095 can be had around $5.13/FT Here

SO...

120' Alloy = $723
120' DOM = $616

$107 more in material.

BUT Chromoly MUST be TIG welded. I TIG. I would charge at a minimum, an additional $1000 for this. - (This is where, numbers start to very greatly from one experience to another though.)
EDIT: You do NOT have to TIG weld an alloy cage under SCCA rules. I confused them with the regulations in NHRA drag racing that I did for many years.
Using those numbers.. $1000 + $107 = $1107 more for a 15lbs savings. Is it worth it? To some. Not to most.

-- Yes, chromoly is brittle. But not as bad as a lot of stories on the internet claim. The DOM is more "giving" in an impact, therefore absorbing some energy. But I'd rather use Chromoly
Even still, there are many who are much more knowledgable on this topic, and I still have to continually pick up the phone to ask questions on this topic
I Hope this helped and answered all of your questions on a cage.
I start welding my personal one up in the next few weeks.

EDIT: Here are some classes that aren't allowed to use smaller tubing sizes if using alloy.

Showroom Stock, Touring and
Improved Touring Category auto-mobiles registered after June
1, 1994)

tkerrmd

thanks all again for the info and discussion. 95ONE thanks for all the info!! awesome.....

Peter good point will check into it and then you need to give me some driving lessons!!

tom

stownsen914

People keep saying that chromoly tube is brittle. I don't think that is the case. It's only brittle if you weld it incorrectly (some will tell you that stress relieving in an oven is necessary after welding to avoid brittleness). But chromoly is not inherently brittle ...

Scott

MJHanna

If you go with Chrome Moly preserve the paper work that it came with it, in that you are using a smaller dia it leads to inspectors questioning what was used. On one of my cars the cage is chrome moly, the “cert” for the tubing that was wrapped around the tubing when bought has been preserved and covered in clear tape and left on the tubing to further reduce questions…..

DWalker

It really doesnt matter much what we "believe", 4130 is more prone to "snapping" than DOM, which is bad. This is not a matter of internet myth, its a simple matter of fact that has been evident since the 70's. It is also stiffer than 4130, which is good until you crash, then it transfers much more energy from the crash into the driver(you), which is also bad.
Its my very brutally honest opinion that 4130 cages built by most "performance" shops are not properly constructed nor properly welded and while likely do thier job in the event of a crash, I for one do not want to be the one that proved otherwise. There is simply far too little margin of error with 4130- too hot and its brittle and the weld or surrounding metal WILL FAIL, too cold and the weld WILL FAIL, and only the welder and the X-Ray will know for sure.

VZ935

you need to have someone who really knows how to weld and does not over heat the chrome moly.... or the cage needs to be brought up to a temperature in its entirety and allowed to cool to take out all flash points. hard to do unless you have a tube frame car. Other wise... if you get into a situation the chrome moly could splinter and you end up with a nice spear.... My shop is at Infineon and we saw this happen with a drag car a few years back . The chrome moly cage broke and the driver took what amounts to a spear through the chest.

Is any wieght savings all that important? I did one of my cars with 120 wall on the interior and .90 fore and aft . I even went with crash bars on the pasenger side and put some extra tubing in the car to make it very safe. I still went out and kicked but and won a lot of races so even though lighter is nice I dont think you will give up that much using mild steel . Other wise find out who the best fab shop in your area is or call up Dave Klym at Fabcar and ask him.

stownsen914

Agreed on the above. Probably many builders don't take the right steps to properly construct with chromoly and wind up building something that at best is no better than a mild steel cage, and at worst dangerous.

Scott

Bryan Watts

BS. NASA's cage rules are nearly exactly in-line with SCCA except that NASA doesn't allow you to use a smaller diameter chro-mo tube like SCCA does for their GT/Production/Formula/Sports Racer classes. That's a + for NASA. Their driver equipment is quite a step up, requiring either a head support seat or a right side net and requiring a H&N Restraint Device. Another couple of +'s for NASA. A cage built for a SCCA Production class car built with the smaller chro-mo wouldn't be allowed in NASA or BMW CCA club racing classes because they both have stricter cage requirements regarding tubing size for chro-mo cages. BMW CCA has made an allowance to let such a car run under it's appropriate SCCA class as long as it has an active SCCA logbook, but the car wouldn't be able to meet the safety requirements for a BMW CCA classed vehicle running for BMW CCA class points...and the driver would still need to have a H&N Restraint Device.

If you want to see good club racing safety rules, look at BMW CCA:

- Right side nets required, whether you have a head restraint seat or not
- H&N Device Required
- Fire system required (no hand held extinguishers allowed...what a worthless excuse for a "safety" device)
- Bolt-in and bolt together cages are NOT allowed...cages must be welded to the car and at all joints
- Harnesses must be 6 or 7 point, 5 point are no longer allowed after they expire

Bryan Watts

In most cases, a modern Porsche is going to fall more into the Showroom Stock, Touring, or IT rules when considering safety rules than it is the GT/Production classes that are more designed around the idea of "tube frame" type cars. ESPECIALLY when you consider that you may want to race the car in a different series like NASA (or possibly PCA?) where the smaller diameter Chro-mo isn't allowed.

mark kibort

ARE you an expert in the rules of cage building too?

Hey, for a quick data point. SCCA doesnt allow the smaller diameter tubes in any of the IT classes. (LIKE PORSCHE RACING CLUB, POC and PCA DOES ALLOW). SCCA wont allow the cro-molly 1.5" diameter x .095 tubes as the porsche clubs allow for cars under 2500lbs. (WC was like this for awhile as well) . SCCA doesnt care about the material of the cages between cro-molly and DOM, as long as its 1.75" x .095"min.

Building to SCCA club or BMW club rules is not a bad Idea, since they seem to be a little tougher than the porsche club variants.

How I know this, is that we are trying to get two 928s from so-Cal to run with us in races up north with SCCA ITE and SP. they wont let them run with the small primary hoops, where in NASA, PRC, PCA and POC, they are fine.

Mk