Structural integrity & roll cage building (someone who knows statics?)

[STARION]

hello all,

This is just me pondering about the shape of the metal and its strength. I fabricate a good number of items on my car and am thinking about a roll cage. Now, quite obviously, roll cages available for sale are produced from circular metal tubing (most commonly chromoly steel I believe). Generally, when I am fabricating a part for my car and the situation calls for tubing, I tend to favor the use of square tubing. It began, because I was a lousy welder and the flat sides made things simpler to run a bead on, but now I use it because I believe it to be stronger. However, I do not have any proof to claim this as factual. Anyhow, things brings up the question, why not buld a roll cage of square tubing as opposed to circular?

Let's say you have 2" OD circular tubing and 2" OD (diagonal) square tubing, keeping things constant, we'll say they are identical thicknesss (gauge) and identical type (cromo steel in this case), and their weights would be very similar. Now I know everyone is thinking that you use circular because it can be bent, curved, etc... Sure, thats a major concern is mass production I suppose, but if I were building one from square, I would just angle-cut it and re-weld it (keep in mind that a good, well executed weld is as strong, if not STRONGER than the metal itself) heck, you see some japanese tuning firms still using this method on their prize cars if you think I'm crazy.

Now, onto the question, with all things as listed above, and I go ahead and build a roll cage from square over circular, will the car be more structurally rigid than if I had built a cage with circular (this, assuming that I build the 2 cages with the exact came points of contact with the car, frame, same angles, etc..). I believe this field of study is called statics? I'm sure there must be some mathematical formula for this, or somebody who has done tensile strength testing and all that business.

I was looking for some kind of existing evidence to try and find my answer, but there was always a point, counterpoint. I would decide that square was stronger, because I would look at how darn near every production car I saw always had a square/rectangular frame....then I would look at just about every "tube chassis" car I had ever seen and realized that they all used circular, each of these respective parties must be using the most rigid form available. I could go on and on with examples.

anyone?

 

[Defiant]

Square is stronger in two directions than round is, but is terribly difficult to bend into shapes without destroying its strength. Try to imagine a Cobra roll-hoop made of square stock. Bleagh. Despite the weld's greater strength, an angled weldement will be weaker than a mandrel-bent tube.
It only takes a slightly larger or thicker-wall round tube to be stronger than square tubing, and (especially for a monocoque to begin with) it doesn't take much to make a cage for a 3000-pound car. I think it'd be tough to make a good-looking one from square tube, and square is more expensive than round.

If the materials were the "same" as you specify, the square-tubed cage will wind up weaker, uglier, and take _much_ longer to make (more expensive, even using your own time).

I wouldn't have a start with the math, but I suspect it'd bog a Cray.

 

[2-0turbo]

Well, you don't state gages of material, so it is possible that one or the other could be stronger, depending on the gage of each.

Keep in mind, the cage you propose will not be legal for any sanctioning body because they all require the main hoop to be one-piece, no welds. Forgive me if I'm wrong, but I'm assuming from your post that is what you intend. Bending the square tubing into a radius for a main hoop would be difficult; Mitres would be easier, which is what I'm assuming you are planning.

Structurally, they are not going to be terribly different. When you get to the point of utilizing a roll cage for its intending purpose (keeping the structure from crushing too far in a severe accident), it comes down to a matter of energy absorption. The unbraced lengths for round vs. square will be similar and the distribution of mass (or Sx if you desire) are similar (assuming 2X2 X .187 and 2"OD X .154). Not the same, but for rough work, a reasonable estimate. (I just checked AISC and they are almost identical)

Both materials are going to be either 36 KSI or 50 KSI yield. The 50 ksi material is fast becoming the standard and both shapes are available in both strengths.

The only caveat is circular tubing has no "weak" axis. It has the same capacity in all directions. Square has the strongest direction along the diagonals, but only marginally more than across the flats. I think the geometry issues of trying to fit the pieces together would negate any possible moment of inertia advantage.

My suggestion is buy a standard roll cage kit, go enroll in a semester welding class and learn how to properly weld pipe/tube.

 

[C. Black]

Dude, I fabricate for a living (from tube chassis to cages) and I tell you this mos' sincerely.... BUY a 1 3/4" kit or get a local dirt-racer (circle track) shop to build you one. It doesn't have to be mandrel-bent, most low-buck shops use a manual bender anyway (not the little hydraulic press benders, if they get out the hyd. bender RUN AWAY).
As good as you think you can make the welded joints on the square tube cage mitered-cage....it'll fail and you'll die finding out it was a stupid
idea (as one of your mitered joints impales you). ESPECIALLY since you mentioned you can't weld too well.
Guys, quite this ghetto shit, ESPECIALLY WHEN IT COMES TO SAFETY. Next thing you know people will be building their own harnesses.... "You don't need some Simpson Cam-lock, I built mine using 3" webbing from Jo-Anne Fabric......".

 

[NDgsx]

I agree 100% with C. Black.

http://www.artmorrison.com/ Sells premade hoops and what not.

 

[Defiant]

Originally posted by 2-0turbo
Well, you don't state gages of material, so it is possible that one or the other could be stronger, depending on the gage of each.

"Let's say you have 2" OD circular tubing and 2" OD (diagonal) square tubing, keeping things constant, we'll say they are identical thicknesss (gauge) and identical type (cromo steel in this case)"

Tsk, tsk. There goes the comprehension scores.

You don't need some Simpson Cam-lock, I built mine using 3" webbing from Jo-Anne Fabric......".

But Simpson doesn't offer a nice gingham plaid with flower trim.

 

[MNGSX]

How do those kits go in?

Like how long it takes.

I guess when I do one I'll use swing outs and run my tubing as close to the body as possible. Perhaps pull all interior trim and run it really close. Maybe even welding the beefier portions of the unibody to it? I'd like to hide it as much as possible.

 

[GRNDSM]

>Let's say you have 2" OD circular tubing and 2" OD (diagonal) square tubing,

First, if you want to keep things comparable, you would want to use 2" square tubing, as in 2"x2". That is how they make it and sell it. There is no such thing as 2" diagonal tubing. And if there were, it would be weaker than 2" OD tubing of the same wall thickness.

2x2" tubing would be slightly stronger and heavier, but everyone has already brought up a lot of good reasons not use it. But I will add one more, if you get into an accident, what do you think would do more damage to you: a 2" diameter tube or a square tube with 1/8" diameter corners?

I must agree with others, this is NOT a good idea.

Leon
RR