[ledfoot2]

I talked to them and they have a 10 point DOM cage that would be NASA legal for 499 plus 160 for shipping. It is not complete. IT does not have petty bars for the door or some smaller bracing that I would add, but everything is notched and ready to go.

Any thoughts?

[GlennCMC70]

they are called inspection holes for a reason. they insert a length of stiff wire or rod and look/feel for the seam on the inside.

[StanIROCZ]

they are called inspection holes for a reason. they insert a length of stiff wire or rod and look/feel for the seam on the inside.

But like I was saying, some of the ERW that I've seen doesn't have a visible seam from the inside or outside. The weld gets scuffed off and you can't see or feel anything but a black line. If thats the case no one would ever know.

This post has been edited by StanIROCZ: May 2 2008, 08:51 PM

[Blainefab]

they are called inspection holes for a reason. they insert a length of stiff wire or rod and look/feel for the seam on the inside.

But like I was saying, some of the ERW that I've seen doesn't have a visible seam from the inside or outside. The weld gets scuffed off and you can't see or feel anything but a black line. If thats the case no one would ever know.

Ya, you could try to get your supplier to select tubing that doesn't have a ridge on the inside, and buff it smooth on the outside, and still get nailed by an ultrasonic thickness tester that exposes the inconsistent wall thickness that is characteristic of ERW.

So maybe you get by the tech inspector, race the car for a while and then sell it. Buyer wrecks, gets hurt, and the cage is cut to get him out. Cage is revealed as illegal material, what is your liability exposure?

Or, just build it with DOM like the rules require.

[StanIROCZ]

they are called inspection holes for a reason. they insert a length of stiff wire or rod and look/feel for the seam on the inside.

But like I was saying, some of the ERW that I've seen doesn't have a visible seam from the inside or outside. The weld gets scuffed off and you can't see or feel anything but a black line. If thats the case no one would ever know.

Ya, you could try to get your supplier to select tubing that doesn't have a ridge on the inside, and buff it smooth on the outside, and still get nailed by an ultrasonic thickness tester that exposes the inconsistent wall thickness that is characteristic of ERW.

So maybe you get by the tech inspector, race the car for a while and then sell it. Buyer wrecks, gets hurt, and the cage is cut to get him out. Cage is revealed as illegal material, what is your liability exposure?

Or, just build it with DOM like the rules require.

I'm not recommending that anybody build a cage with ERW although it may have come across that way. If you forget about the mechanics the rule books ask for DOM so there is no reason not to do it. The extra cost of DOM vs ERW is nothing compared to your total investment.

Someone mentioned that 00 Trans Am probably has a ERW tube roll cage. I was saying that I wouldn't worry about it too much, esp when you consider that he is at a 0.143 wall.

http://www.metalmartusa.com/item.asp?id=84

ELECTRICAL RESISTANCE WELDED is produced from low carbon steel. Tubing with walls heavier than18ga. is produced from hot rolled steel; tubing with walls 18 ga. and lighter is produced from cold rolled steel. Furnished in as welded condition.OD flash removed on allsizes.
For round tubing1 OD and larger ID flash control led to .010 to .015 (depending on O.D.) Sizes under 1 OD and all square and rectangles are flash-in. ERW is the lowest cost of all mechanical tubing. Manufactured to OD and wall dimensions. Meet ASTM A513 Type1 (HR)
or Type 2 (CR). Stocked in 20ft. random lengths.

DOM (Drawn-over-mandrel) is a cold drawn electric resistance welded tube with allflash removed. Each tube is tested for soundness of weld. Preferred over seamless tubing for its excellent OD& ID concentricity. Normalizing and cold drawing over a mandrel makes DOM a uniform and precision product. Made from 1020 (UNS G10200) steel in walls 10 ga. and lighter; from1026 (UNS G10260) steel in walls heavier than10 ga. Furnished in as drawn condition. Manufactured to OD and ID dimensions except as noted. Meets ASTM A513 Type 5. Stocked in lengths of 8 to 24 ft.

I'll try and dig up some more concrete info on the mechanical properties. I used to have access to ASTM A513 but I don't anymore. The strength advantage that comes from the DOM is through cold working. Cold working raises the yield strength but reduces the ductility (% elongation). The yield strength of the roll cage isn't important. What is important is the cages ability to absorb energy. The amount of energy that a material can absorb is proportional to the area under the curve (stress strain curve). I'll confirm this if I am able to find some more data, but I remember ERW and hot roll steels have a higher percent elongation which means that they can bend more before fracture. Net effect is similar ability to absorb energy (its not impossibly that ERW can absorb more). Giving this a second thought the yield could be important where the tubes are close to the driver since it will absorb more energy in the smaller strain area under the curve.

Maybe someone with more experience than I in motorsports can confirm this, but my hunch is that ERW got a bad rep a long time ago from seam failures. I think that is a thing of the past. I worked for a propshaft mfg that shipped over a million ERW props a year. In my 3.5 years we had ZERO seam fracture returns through warranty and one (1) seam fracture that we found in fatigue testing and that was due to a poor quality weld that was supposed to be caught by 100% eddy current inspection.

Having said that our suppliers were held to standards that might be greater than other ERW mfgs in the world. DOM might be a way to filter out poor quality to some degree.

This is obviously my personal opinion based some experience so take it FWIW. I don't think ERW is dangerously inferior to DOM, but I won't be saving $100 or whatever to put it in my car. I just wonder if anyone from a these sactioning bodys really looked at the details or is just doing a cut-and-paste.