Here’s an interesting thing. (Well, I find it interesting!) Since becoming involved in Autograss, I have at times been shocked by some of the roll cages I have seen. So I decided to do a bit of research into the subject.

The NASA rule book stipulates that roll cages must be made from one of the following;

• Square steel tube; 1"x1" 0.102"
• Round steel tube; 38mm diameter x 2.5 mm
• Round steel tube; 42mm diameter x 2.5 mm
• Round steel tube; 50mm diameter x 2 mm

So, which is best to use? Which tubing gives the greatest strength for the lightest weight?

The purpose of a cage is to absorb energy in a collision. If a cage gets bent in a collision, - that's a good thing, because the energy which it took to bend that bit of metal was energy which did not have to be absorbed by any other part of the structure. Including the nut behind the wheel. Well, the first thing to point out, is that a major factor in the strength of any engineering structure is the quality of the material being used. If you're using Grade SH1T steel, it doesn't matter what size the tubing is, the strength of the cage is going to be compromised. NASA stipulates material strength of 350 N/mm² but how could they ever test that without access to a full engineering test laboratory? If this was in industry, there would be a material certificate issued with the tubing, which would trace the material back to the rolling mill, and would guarantee the specification of the tube. Now, be honest, when did you ever see anybody at an Autograss meeting brandishing a Certificate of Conformity for the roll cage material? I thought so! NASA also stipulates seamless tube, and again, the Certificate of Conformity would confirm this. Or in many cases, it will be printed onto the tube, - but with a good coat of paint...!

But assuming that all the sizes of tubing are of the same strength material, which one is going to give your head the greatest protection? Well, if I was really really clever, like my good friend Sam (who has helped with this article), I could work it out doing good old fashioned engineering calculations. But I’m going to take the easy route and let a Computer Aided Design programme do all the hard sums for me.



So with each of the sections drawn on a CAD programme, we can ask the computer to analyse the Mass Properties. The results are;

1" x 1" x 1.102 wall square tube

Ø 38mm x 2.5 wall

Ø 42mm x 2.5 wall

Ø 50mm x 2.0 wall

Blimey! That’s an awful lot of data!

If we look at the cross sectional area first of all, the 50mm tube is the greatest area, and therefore the heaviest. Being a competition car, we want to keep the weight to a minimum, because weight affects the performance. Low weight means faster, and therefore a increased chance of a free ride in a blood wagon. Higher weight means that the nut behind the wheel will need to drive like a lunatic to keep up.

But the overall difference in vehicle weight between a car with a cage made of 1” square tubing, or 50mm round tubing is going to be negligible. So how do these materials compare in terms of strength.

The figure which gives the shape its strength, is the Principal Moment, and since we were also interested in the greatest strength for the lightest weight, to make things a bit clearer, we will extract those figures into a table;

Section	Principal Moment	Area
1" x 1" x 0.102"	20710	236
Ø 38mm x 2.5 mm	44140	278
Ø 42mm x 2.5 mm	60747	310
Ø 50mm x 2 mm	87009	301

This shows that the Ø 50 round tube, is more than four times stronger than the 1" x 1" square tubing, but is only 25% heavier.

Then we come to the fabrication of the cage. If care is not taken, the round tube might become oval during bending, and even a small deformation here, will significantly reduce the strength. If you’re good at fabricating, use the 50mm. If not, it might be safer to go with the 42mm

Then we come to the welding...! But that’s another story for another day.

And you thought building a cage was straightforward !!!!

Ian McRae
Thanks to "Doctor Sam's Metal Surgery" for help with this article.