ERW vs DOM

receipts for the tubing that was purchased-- and/or sand some paint off of the tube and I believe the ERW will have a darkened line on it where it was welded together.

FWIW: DOM is also welded tubing that is afterwards drawn over a mandrel to even out wall thickness. CDS is seamless and difficult to acquire in needed dimensions for mere peasants. FIA requirements call for "seamless" or CDS and US organizations frequently allow for the substitution of DOM.

I went round and round with DOM vs CDS tubing for our Lemons mini with Rally America and NASA and both would allow 1.5 x .095 for the main 4 legs if the tubing were CDS but they wanted 1.75 x .095 if it were DOM. One organization would have allowed 1.5 x .120 DOM but can't remember which one. (Why bother with FIA/NASA/RA regs? Well, new Lemons reqs call for "professionally built" cage so I figured if a home built cage passed tech for those organizations then it should suffice for Lemons.)

We went with DOM because we knew it would be accepted anywhere, but I still don't see how the ERW tubing is really that much weaker.  Either way, the tubing is rolled from flat stock and welded together.  The difference (as stated previously) is the DOM has a more consistent wall thickness after being drawn over a mandrel. 

I guess I'd like to see the data (or a real world example) where a cage failed because ERW was used.  If the cage were to fail it would be much more likely to be caused by poor welding at the joints than by using ERW over DOM.

DOM is stronger, but not in a roll cage application...the heat affected zone of the cage's welds are a far more likely place for a failure than a tube seam...if the tube is pressurized internally, like in a hydraulic system, it's different. I'd rather race in an ERW cage with good welds and extensive gusseting than a DOM cage without. Insurers won't see it that way, of course, because they just look at the specs and say "stronger". Jay is doing the right thing by using thickness, diameter, design and welding as the criteria for tech failure, not tube specs.

DOM is almost twice as strong in terms of tensile and yield strength over ERW.

ERW = 45K psi tensile, 32K psi yield
DOM = 80K psi tensile, 70K psi yield

http://www.emjmetals.com/esl/

Ever seen a cage bend?  I have...  It doesn't take all that much pressure to bend a cage especially if that pressure is appied in the "right" spot - a few tons pressure at the center top of the windshield will pretty much collapse a cage.

If it's only $1/foot more for a DOM vs. ERW, I'll take DOM everytime.

"Strong" isn't the point....if the cage is properly designed, triangulated and gusseted it will protect the driver...by that argument you could use chrome-moly....since these are largely amateur-built cages, I think safest is the material that can be welded safely with home equipment and non-pro skills...ERW or DOM. Unless you're trying to save weight by down gauging the wall thickness, ERW should be fine. If you can get DOM cheaply, go for it, but it won't be the cold-working of the tubes that saves your life in an accident...The difference in price could add 100-150 bucks to the material cost....about the price of a shielding gas kit for a home MIG welder...or an upgrade to a better welder. If I have to choose, I'll stake my life on strong welds and softer tubing.

Are there any tech inspectors here that have had to check for ERW vs DOM?  I just need to know how to check, I don't care which is stronger (stop threadjacking).  It seems the answer is to sand some paint off to check, but is this confirmed? 

TEAMDFL, the wall thickness is helpful, but are you saying you cannot get .134 DOM?

BTW, poopsmith FTW!

Based on how the tubing is made, this makes sense..  Basically it seems like DOM would be a harder requirment to enforce. Anyway, I already planned on using DOM for the Lemons car, I just wanted to know how to find out what is in my ARCA car.  Thanks for the info!

:dunno:

I found that picture with only a couple of minutes searching trying to find a rollcage crushed in the way that I was describing (answering the question about wanting the tubing to be twice as strong).  As you can see - this was 100% tube failure, not welds breaking or bad design.  If there was someone buckled in that seat correctly, they have a broken neck.