95ONE

I was posting in the racing forum and thought some of you would be interested also.

Here's what I posted. I took some time to reference with hyperlinks and proofs for those that would like to confirm my information. But, sometimes, like now when I get tired, I screw some things up. So, not hard feelings if you correct me with solid references and hyperlinks.





"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. Roll bar vs Cage rules are different, but you are allowed a smaller diameter for alloy in cage construction. And be careful how you interpret the GCR Roll Cage rules. It will trip you up into other class rules that don't necessarily pertain to your specific class.

Here is a cut and paste from the manual.

E. Material:
1. Seamless, or DOM (Drawn Over Mandrel) mild steel tubing
(SAE 1010, 1020, 1025) or equivalent, or alloy steel tubing
(SAE, 4130) shall be used for all roll cage structures. Proof of
use of alloy steel is the responsibility of the entrant.
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. HEREI 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 in an Alloy cage under SCCA rules. I confused them with the NHRA drag rules.
Using those numbers.. $1000 + $107 = $1107 more for a 15lbs savings. Is it worth it? To some. Not to most.

-- Yes, chromoly is brittle. The DOM is more "giving" in an impact, therefore absorbing some energy. But, it is not as large a difference as many on the internet love to say and claim with no actual experience.

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."

eclou

awesome info Bruce

DanR

i did not think some governing bodies allowed you to use smaller diameter tubing? (late so will look into it tomorrow)

white924s

Actually I think Chromoly can be MIG welded with good results. Some say that you have to heat treat the entire structure afterwards, but I'm not 100% sure of that. I have seen an open wheel chassis made from 4130 MIG welded and it held up fine (not in impact, but in auto-x style racing)

95ONE

As referenced. Page 147 of the SCCA General Competition Rules "GCR" Book.
But like I said. It also depends on what specific class you are running in. Spec Miata has Some rules that are a little different. Cars heavier than 3000lbs have different specs. Stock classes only allow a small number of bends in each tube. Etc, etc.

95ONE

Here's the cut and paste from the SCCA manual. I also inserted it above.


E. Material:
1. Seamless, or DOM (Drawn Over Mandrel) mild steel tubing
(SAE 1010, 1020, 1025) or equivalent, or alloy steel tubing
(SAE, 4130) shall be used for all roll cage structures. Proof of
use of alloy steel is the responsibility of the entrant.
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

95ONE

You know. I went to reference the SCCA manual where it states you have to..

YOU DONT!!! you're right. I've done so many NHRA Drag racing cages - where it mandates a TIG weld for chromoly, that I've grouped it here.
I guess this is where internet myths and truths get all mixed up.

To re-cap. TIG welding for SCCA is NOT required. It is only required in NHRA.

blown 944

what's funny to me is WHY one would mig cromo???

All the extra $$ for material........ TIG FTW.

95ONE

Totally.

evil 944t

[QUOTE=95ONE;6334666]Yes, chromoly is brittle. The DOM is more "giving" in an impact, therefore absorbing some energy. But, it is not as large a difference as many on the internet love to say and claim with no actual experience.QUOTE]

This is a can of worms but I'll put in my .2C, I have seen a ton of rolled and crashed drag cars at my friends raceshop. He only uses Chromoly and of course Tigs them. He is a certified welder and his work is like art. He builds full tube frame cars and just bolts the bodies to them. He holds many records in the Mustang/Ford world(which I know nothing about) but long story short, After all the wrecks, the drivers all walked away and the cage always did its job. That being said, I personally like Chromoly.

I have seen him cut out a ton of mig welded DOM cages that were folded like a pretzel. Granted they were not designed as well as his, I would still choose a chromo cage.

95ONE

[QUOTE=evil 944t;6337450]And.... I'm not sure my quoted statement up there was actually succinct. I was trying to say that chromoly is better than just a brittle piece of metal. That sure it is brittle, but definitely a quality option that many don't understand because of the extreme internet rumors about it breaking off into spears and killing people.

Van

Good info here.

evil 944t

[QUOTE=95ONE;6337626]Oh yeah, no worries. I meant no disrespect, Just sharing.

white924s

Chromoly isn't a lot more money when you've got a hookup .

as for why MIG vs. TIG, in the case I mentioned above, it was honestly a cost and time calculation that involved a lot of variables particular to the project. The cost savings of MIG made sense for us (the shop with just MIG was cheaper by the hour than the shop with TIG, MIG goes faster than TIG, and we had had bad experiences with the TIG welding shop), but ymmv.

white924s

that being said, if I can have my pick, and time and money are no object, I'd pick TIG too, if only so I can stare at the beautiful stack of dimes effect