Tabor

If I am reading the PCA 2002 rule book correctly, I could build a roll cage out of either 1.75" x .095 or 1.50" x .120 mild steel.

Am I reading this correctly? Even the main hoop?

Is it a good idea to only use the minimum size? I am asking because some people seem to think you should use at least 1.75" x 0.125.

Thanks!

Joe@Fabspeed

Hi Tabor......what they state is material thickness and diameter are based on a cars weight.

This ALSO means that you can choose rollbar diameter and material thickness BUT this is based on what type of steel rollbar material you are using for the rollbar or rollcage.

Essentially you have two choices seamless steel tubing DOM or DOM 4130 N steel tubing. If you are using the former it is weaker and has to be thicker and larger in diameter due to its engineered properties. If you use 4130 N(what clubracing is alluding to) you are using steel aircraft quality tubing called "chromemoly steel". This stuff is the lightweight strong material that seriuos racers use as it is the lightest strongest structural tubing you can buy and still work with when bending up a rollcage chassis. By the way when you order chromemoly ask for 4130N which is "condition normalized" and therfore not annealed. When you build a rollbar/cage/airframe with chromemoly it is approximately 20-30% lighter than a regular steel alternative

Tabor

From the rules:

Cars over 2500 lbs.
Mild Steel - 1.75" x .095 or 1.50" x .120
Alloy Steel - 1.50" x .095"

I am just curious if 1.50" x .120 of mild steel is really enough.

Geo

[quote]Originally posted by Tabor Kelly:
<strong>If I am reading the PCA 2002 rule book correctly, I could build a roll cage out of either 1.75" x .095 or 1.50" x .120 mild steel.

Am I reading this correctly? Even the main hoop?

Is it a good idea to only use the minimum size? I am asking because some people seem to think you should use at least 1.75" x 0.125.

Thanks!</strong><hr></blockquote>

One of my pals who is also building an SCCA ITS car now is an engineer. He has determined that the 1.75 x 0.095 is lighter and stronger than the 1.500 x 0.125.

James Achard

All, Tubing stregth and rigidity increases exponetially with diameter( Sorry I don't have the formula here at home). As a result, the wall thickness of a larger tube can be reduced yet retain the same strength as the smaller tube thus giving you a slightly lighter tube with greater stiffness. In regard to different Alloys, Mild steel is DOM( Drawn On Mandrel) and is formed by rolling a flat into a tube shape and then welding it. Once this is done it is then drawn through a mandrel in order to attain the correct diameter and wall thickness. 4130N or Cromoly as it's sometimes called is "seamless" tubing and as a result has no "seam" in the tube as a result of it's forming process. This said, I would go for the 4130 over the mild steel since the tubing is of much higher quality and strength( All 4130 tubing you buy carries a mil. spec # . Mild steel has no such specs per say). The downside to 4130 is that it's hard to fabricate and needs special attention when it is welded. Careful preparation should be done and ideally it should be welded using the GTAW process( Tig welding). GMAW( Mig welding) welding can be used but will not yeild such good results due to the fact that; 1. you are welding a thinner wall tube 2. 4130 is more sensetive to contamination and 3. the correct wire is harder to aquire for a mig machine. So, what does this all mean??? Well, since this is a Safety device I would find someone who is willing to fabricate a cage out of 4130.
I have been an aerospace welder for 14 years and am hoping to have the facilites soon to do this myself. If I can drum up enough interest on this list for cages I would be willing to make several for folks who are in the New England area. I hope this explination helps

Julian Allen

I rise to defend mild steel.
First of all, DOM (drawn over mandrel) is seamless tubing. ERW (electro-resistance welded) tubing is made from a flat rolled around a mandrel and welded. DOM costs 2-2 1/2 times what ERW does.
Cages are supposed to be constructed of DOM mild steel or 4130, not ERW.
For me, the major theoretic benefit to mild steel is it's ductile nature. It can be bent back and forth several times before it fractures. 4130, for all it's benefits with strength and weight, does have the construction difficulty raised by James Achard, and is supposed to be less ductile. That would mean fracturing after fewer back and forth bends.
In a real world crash, the cage doesn't have to absorb just one blow (or bend). Mild steel may be able to sustain more bending, and more significant bending, before it fractures.
When you add the ductile nature along with the relative ease of construction (MIG welding vs TIG), I'd go with the 1 3/4 .095 DOM cage, with a .120 main hoop.
Just my .02 worth.

Tabor

Thanks everyone.