Cage materials?

[horizenjob]

I've been trying to go thru various rule books to understand the requirements at COM, SCCA and NASA. My application is a cage for a Locost.

My first question is an SCCA cage legal in COM, they seem to specify a minimum tube thickness of .083 for 4130 cages on a very light car? ( maybe that's just for sports racers, but I don't know if a Locost can be considered a Seven ).

COM specifies "DOM/seamless/alloy", but the SCCA and NASA rules are less clear. So far as I know all seamless tubing is DOM. Most DOM tubing is made from ERW though. For instance SCCA says something like "DOM or seamless, no ERW". Maybe they mean to say ERW must be DOM?

My basic desire here is build a good cage and then weld the missing parts of a Locost onto it...


Edit: Oh, I see the COM rule is for roll bars on cars with no roof. So I assume SCCA legal cages are OK for us.. If folks know what these folks want in terms of seamless tubing or DOM made from ERW, I would still like to know though.

[jlwhorf]

I agree, it dose seem like the people who write the rule books don't know tubing. DOM is ERW that has had the wall thickness made uniform by drawing through a mandrel. If you look at a raw piece of DOM, you can still see the seam in it. Also I would bet that most any tech inspector could look at a painted ERW cage and not know it was not DOM.

In COM any current SCCA cage would be legal.

[zchris]

Well you guys are kinda of understating the process to take and make DOM tubing. You make it sound like with a simple single process, it goes from ERW to DOM. In reality the tubing has to be drawn from a much thicker tube, 4 to 8 passes of thinning down to the finish size. Lots of heat and pressure later you have a much more bend and dent resistant tubing. And much tougher to bend in my hydraulic bender. Hell, ERW isn't really even round. Now there have in recent years been some American companies making the 4 pass tubing in a cold process and that stuff is crap. I've had to send 2 loads of it back in recent years. You can always tell by how well it forms. I have never seen DOM with a visible seem on the outside. And only have seen it on the inside with cold process stuff.
Chris Howard

[horizenjob]

tube-process.jpg (62.89 KiB) Viewed 2543 times

Chris, who do you recommend to buy tubing from? So my understanding was that DOM was Drawn Over Mandrel, not so much drawn thru. It should then have a more consistent inside diameter and also wall thickness.

Now I'm even more confused because the SCCA ( or FIA, I forget now ) specifically insist on cold worked tubing not hot. So is the good tubing you mention cold drawn? Certainly I'm no expert, but to me the rules didn't seem clear..,

[horizenjob]

It's interesting, to me anyway, that that diagram above mentions RF welding, which to me would seem different then resistance welding, which is what I thought ERW is...

[zchris]

The process chart above is a rough process chart. Which is to say that it is welded and drawn ect.. But the exact process is determined by the manufacturer. As to what type is best. I like the dark mill scale type made by Wheatland and a few others. None of the offshore stuff is any good. All to soft for the bender. I tends to bite into the bender shoe and ruins them. As to who to buy from, depends on where you are.
Chris

[horizenjob]

Thanks fort the help, Chris. I live in Natick. I did talk once to Standard Tube Sales in Maynard, I think. It didn't seem like they would have DOM in the 1 3/8 size.

As you were saying there are many details in the specifications. The tubing can be of several compositions, hot or cold rolled and then annealed or stress-relieved etc. my count after looking at Wheatland and Plymouth tube companies there are something like 30 types offered. I don't think the issue is made offshore or not, probably more like American managers trying to foist off crap. I'm sure the Chinese, for example, are happy to make as a high a quality tube as possible. That's my experience after working with Chinese engineers anyway.

The SCCA spec calls for getting tubing with good elongation, so the softer stuff might be preferred. Is the tubing you buy Normalized or stress-releived?

Are you willing or able to bend up a couple of roll hoops?