Roll Cages: Chromoly VS. Mild Steel

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