Roll Cages: Chromoly VS. Mild Steel

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StrokerValiant

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Hey everyone,

On another forum, the question of cost, and weight came up over a Chromoly rollcage vs. a Mild steel roll cage. As for the price, it depends on who build the cage for you. The guy that caged my Aspen told me the MS cage would cost me at least the same as far as fabrication would go, but he prefered chromoly, because he prefers to TIG weld, over MIG welding. SO, I ordered an Art Morrison Chromoly cage kit for the Aspen, which fit AWESOME, and came in a timely fashion.


So, We all know the Chromo cage will be lighter, and numbers are always thrown out as to what the weight savings is. However I have never seen any actually calculations on it. Since today is Sunday, and I am bored, I figured I would nerd out, and do some math for fun.

I went out the the garage and pulled the tarp back on my 1967 Plymouth Valiant that is currently playing the role of my spare shelf holding up old clothes, empty boxes, etc. I roughly measured out the tubing that would be needed to make a 12 point cage in the car. I figure I would need about 740" of tubing.

I am going to abbreviate some of these down.

A1= Area of tubing using the outside diameter (OD) of the tubing.

A2=Area of tubing using the inside Diameter (ID) of the tubing.

AT =Area of total profile of tubing.

PI = 3.1416. PI is a mathematical constant whose value is the ratio of any circle's circumference to its diameter

R = Radius of a circle

Density of Mild Steel = .2833 lbs./in³

Density of Chromoly = .2833 lbs./in³

V = Volume

W= Weight

MILD STEEL ROLL CAGE TUBING

Roll cage tubing in MS is 1.625" in diameter, and has a wall thickness of .134".

OD = 1.625"

ID = 1.625-(2*.134")
= 1.357"

1. A1 = PI*R²
=(3.1416)(1.625/2)²
=(3.1416)(.8125)²
=(3.1416)(.660015625)
= 2.074in²

2. A2 = PI*R²
=(3.1416)(1.357/2)²
=(3.1416)(.6785)²
=(3.1416)(0.46036225)
= 1.446in²

3. AT =A1-A2
=2.074-1.446
= .628in²

4. V = AT * Length of tubing.
= .628in² * 1.00in
= .628in³

5. W = V * Density
= (.628in³)(.2833lbs./in³)
= .178lbs.

So, the weight of Mild Steel Roll Cage tubing is .178lbs. for every inch of tubing.

CHROMOLY ROLL CAGE TUBING

Roll cage tubing in chromo is 1.625" in diameter, and has a wall thickness of .083".

OD = 1.625"

ID = 1.625-(2*.083")
= 1.459"

1. A1 = PI*R²
=(3.1416)(1.625/2)²
=(3.1416)(.8125)²
=(3.1416)(.660015625)
=2.074in²

2. A2 = PI*R²
=(3.1416)(1.459/2)²
=(3.1416)(.7295)²
=(3.1416)(0.5321705)
=1.672in²

3. AT =A1-A2
=2.074-1.672
=.402in²

4. V = AT * Length of tubing.
= .402in² * 1.00in
= .402in³

5. W = V * Density
= (.402in³)(.2833lbs./in³)
= .114lbs.

So, the weight of Chromoly Roll Cage tubing is .114lbs. for every inch of tubing.

THIS WORKS OUT TO A 36% WEIGHT SAVINGS

SO, now that we know the weight per inch, how much is the weight difference between a MS roll cage, and a Chromoly Roll cage in the same car?

Let's Assume we will be using 740" of tubing in this example, based off of my Valiant.

Total weight using a mild steel cage = 740" * .178lbs.
This gives us 132lbs.

Total Weight using a Chromoly cage = 740" * .114lbs.

This gives us 84lbs.

Now, The 740" is a very rough measurement. I suggest you go out to your car, and do some measuring. Figure out how many inches your cage will be, then multiply that by .114lbs. for chromoly, and .178lbs. for Mild steel, and see what you come up with.

Please feel free to check my math too, I am human.

-Kenny
 
What?!? You're a human?!?


Seriously to me the cost difference and the welding hassle (neither of us owned a tig at the time, now we both have them. lol) weren't worth the small weight savings to me as my car is obviously not an all out effort heads up car. Many heads up classes also have minimum weights so I don't know the real advantage except that if you aren't close to the minimum you can put the weight where ever you want. Other than that, I dunno.

Oh and pi(e) in my book is a delicious variously crumbly or flakey crust treat with a sweet creamy or fruit filled center. Most excellent with cream either iced or whipped as an added topping.
 
Will be putting a 6pt. in my car, nice to see the difference in weight between the two metals. Thanks.
 
Guitar Jones said:
What?!? You're a human?!?


Seriously to me the cost difference and the welding hassle (neither of us owned a tig at the time, now we both have them. lol) weren't worth the small weight savings to me as my car is obviously not an all out effort heads up car. Many heads up classes also have minimum weights so I don't know the real advantage except that if you aren't close to the minimum you can put the weight where ever you want. Other than that, I dunno.

Oh and pi(e) in my book is a delicious variously crumbly or flakey crust treat with a sweet creamy or fruit filled center. Most excellent with cream either iced or whipped as an added topping.
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I KNOW RIGHT!? I am mostly human. haha. I have a little bit of titianium in one shoulder, but the rest is all human!

The welding cost really can vary weather your MIGing or TIGing. I can't weld, so, I used a chassis shop, and he would have TIG'd the cage either way, because he doesn't like MIGing. So, the cost difference wasn't much, if any.

I think I will go get some pie now.
 
VERY interesting!
However, to me there are some variables that come into play with this sort of debate that can't simply be rationalized away.
MS is easier to weld, less costly, more plentiful and to a point, more forgiving than CM. Around here fab shops make a distinction when welding CM vs MS and the cost is considerable. ow if you have a competent welding shop nearby, good for you, but many guys start their projects only to have it stalled for a variety of reasons which may lead them to take their car to another shop-and that's where the problems start.
Shop "A" may prefer TIG welding, as in the OPs case, but doesn't like the MIG welds already done on the car. Then what? Shop "B" may prefer MIG welding, is good at it & knows his customers aren't building max-effort type machines, so why use CM? If the customer insists on CM but can't afford the extra labor costs does he go with MS, svae his pennies for the extra labor, down-grade, his cage or what?
While I prefer CM my current project is MS only because of cost. My altered is CM but it's a faster, higher HP car than my Dart so I feel the "need" justifys the expense.
IMO, there's too much emphasis on this debate, forcing enthusiaists to feel they MUST have CM stuff or somehow their car is inferior. Somehow that just seems wrong to me..............
 
remember, mild steel is better on the street because moly cant withstand a lot of vibration and it can crack so after some big potholes or bumps you can have problems, where as mild steel is softer and can absorb shock alot better than moly.
 
I'm not saying one way is better than the other really, I went chromo because it was lighter, and the cost was $235.00 more. That's not a much more in my case with the chassis shop I used.

If the welds are good, I doubt you will see any cracking, my car is a street car, and it is just fine. If mine ever cracks, I will let ya know!

-Kenny.
 
84 pounds is a lot, especially when you consider the weight you're adding to the car with the roll bar, plus I like to add braces over and above what is required.

Also, narrow diameter tubing will be more "out of the way" for regular usage. The car will be more rigid with the moly, and A bodies need all the rigidity they can get.

I like tig welding better, even for mild steel.

If you're not running a heads up class, lighter is better.

If you are running a heads up class, lighter is still better so you can put the weight where you want it.

Lots of street driving a car with a roll bar is a bad idea. Hitting your head on a bar in even a minor collision will crush your skull like a hard boiled egg.
 
hw612 said:
remember, mild steel is better on the street because moly cant withstand a lot of vibration and it can crack so after some big potholes or bumps you can have problems, where as mild steel is softer and can absorb shock alot better than moly.
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I don't buy that one. All Off Road Race Cars and most Dune Buggy's frames are made out of 100% Chromolly. They see more bumps etc than any street car could ever imagine without cracking or seeing any fatigue.
 
Mad Dart said:
I don't buy that one. All Off Road Race Cars and most Dune Buggy's frames are made out of 100% Chromolly. They see more bumps etc than any street car could ever imagine without cracking or seeing any fatigue.
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I was thinking the same thing. Been in a full chromolly preerunner buggy doing 80 through big whoops and 120 through small ones and flying 100 foot jumps. Think a street car is going to be just fine.
 
Yes, CM does get more brittle with time. But I have yet to see a properly welded and designed cage or bar made of it split. I think that's an "overthought" detail... In terms of costs, on a car that is double duty, CM is expensive for a savings in weight that just doesnt make sense in a car full of compromises. But on a race car, when you're paying for light brakes, glass, and various other savings, and you need more tubing... You should pay for that savings. Ialways used 1/3 for weight savings.
 
Google AED metal they sell mild steel and chromemoly and have a weight per foot chart and you can ordrer additional chromemloy tubing alot cheaper from them, i installed a c/m 14pnt cage in mine and yes i migd it it can be done and its a street car only so i dnt wana hear nhra says this and that about mig welding c/m. i called professional circle trak chassi shops who build c/m chassis and they mig as well and they take alot more abuse than a drag or street car will ever see
 
CHEATHAM said:
yes i migd it it can be done and its a street car only so i dnt wana hear nhra says this and that about mig welding c/m. i called professional circle trak chassi shops who build c/m chassis and they mig as well and they take alot more abuse than a drag or street car will ever see
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Exactly. I think in terms of the NHRA's stance it is more the fact that there are many fewer home TiG welders than MiG and by specifying TiG you remove the possiblity of poor welding. Penetration is the critical factor. Not the source of heat or material. I've seen some frightening Mig welding before. Certainly not anything I'd place my life on... And that's really what we're talking about with safety stuff.
 
Moparpwrstude said:
Will be putting a 6pt. in my car, nice to see the difference in weight between the two metals. Thanks.
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Just a suggestion, but if you are going through the trouble of installing a 6-point, for a very little more weight you should consider going to an 8-point with 2 bars running diagonally behind the seats from the main hoop/cross bar junction to the frame connectors (which you should also have). It will help stiffen the entire chassis. "Triangulation" is the key.
 
Also, concerning mild steel roll bar material, there appears to be 2 different types - DOM and EWS according to the S&W site. http://www.swracecars.com/
(Not trying to promote them. Just a convenient reference.)

EWS is cheaper but @ .134"
DOM is @ .120"

I believe someone told me that EWS is cheaper because its inconsistent in thickness, so @ .134" it will still pass minimum tech specs.

DOM has better quality control and consistent thickness throughout its length, so it can be thinner without going under spec. (It may also be seamless.) Lighter and possibly stronger.

Somebody correct me if I'm wrong on any points.
 
Use DOM tubing the tell everyone its CM. Unless its a all out race car it is overkill. Then if CM is welded too hot it will need to be heat treated to bring it back to its designed strength and to stress relive the metal. Now how many shops check for overheated welds? None. The NASCAR builders have their welders set up by the welder suppliers for a set size and type of metal. Regardless check your welds for cracks.
 
Locomotion said:
Just a suggestion, but if you are going through the trouble of installing a 6-point, for a very little more weight you should consider going to an 8-point with 2 bars running diagonally behind the seats from the main hoop/cross bar junction to the frame connectors (which you should also have). It will help stiffen the entire chassis. "Triangulation" is the key.
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I'm building a Studebaker gasser, the body is very similar to a 67-69 Dart. The car has a full frame, which I just upgraded to 11 gauge steel (V-8 frame from California)-car had a rusty 14 gauge(6 cylinder frame) under it. I was thinking a 4pt halo with a crossbar behind the seats to mount belts to, and two bars off the halo into the trunk(no rear seat being used). I'm 6'5, so I want to avoid crossbars that are in the doorways if possible.
 
Awesome. My buddy races a studebaker tube car with a turbo'd stovebolt six. He was in the mid 9s. Last I knew he was working with the new straight 6 that comes in the GMCs.
 
If you cam do cm I would over ms because once you go faster than 7.50 you will need the whole chassis done in cm. So why not save yourself some money later and just go cm now. That way when you need to upgrade to an sfi cage later you don't have to remove what's already there just add more to it. Now on the flip side if you never plan on going that fast I say it's up to personal choice. My cage is a 10 pt currently and we are going to make it 12 pt next weekend. It's all ms and willstay that way. This car will never see a 7.49 do I'm not worried about it. If I go that fast I'll be changing everything so this car would be sold or given to my daughter to race. (she's only 7 now so there is no plan on getting another car anytime soon)
 
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