Roll bars, roll cages, and tube chassis are all made from various types of round tubing. Often times you’ll see an option for Mild Steel or Chromoly. ERW (Electric Resistance Welded) and DOM (Drawn Over Mandrel) are both Mild Steel tubing and both are used in lots of applications. For this exercise we’ll be discussing just ERW and DOM. While both are “mild steel”, ERW is most often made from a 1010 alloy while DOM is most often 1026. DOM is typically more expensive and is considered a premium product compared to ERW by some fabricators. In a typical roll bar or roll cage you likely won’t find much difference in strength as both have nearly identical strength characteristics. What you will find is that some fabricators worry about using DOM for NHRA roll cages as the process for creating DOM tubing may impact wall thickness and could lead to slightly thinner walls than NHRA allows. Will it be caught during an NHRA tech inspection? Not unless the car requires a chassis certification at which point it just might be.
With both tubing choices being nearly identical in strength, what is the real difference. Well the truth is they start out exactly the same, that is to say as ERW tubing. Seam welding is commonly used during the manufacturing of round, square and rectangular steel tubing. The steel strip is unwound from coils and side-trimmed to control width and condition the edges for welding. The strip then passes through a series of contoured rollers which cold-form the material into a circular (square or rectangular) shape. The edges are forced together under pressure as a butt joint and then welded by heating the material to temperatures above 2000° F. The flash weld that has formed is now removed from the outside diameter of the tube. Once the weld has been tested the tube then passes through a series of sizing rolls to attain its precise finished size, after which the tube is then straightened and cut to length.
But while ERW is then loaded on trucks and sent out all over the world, DOM has to go through another process. That process draws the tubing over a mandrel to create an even more uniformly dimensional tubing that no longer has a strip of flashing on the inside where the weld was created. In addition that weld seam is arguably stronger and far less apparent on DOM tubing. So which one should you get? That’s a question for your chassis components company, like Chris Alston’s Chassisworks.
We’ve built cages and tube chassis out of both for a variety of applications and can tell you that they all work. Of course certain performance levels will require Chromoly based on sanctioning body rules, but with regards to Mild Steel both will work just fine. But is one stronger than the other despite what the numbers say? Well sorta.
In the video below you’ll see a fairly scientific garage experiment done to see which one bends more. It works and the example is very easy to understand. But there is always a but.
With rare exception, most roll bars and roll cages have very few tubes that are unsupported for 48 inches or more. In this test it takes far more than 48 inches of leverage for a difference in strength to become apparent. In any of the vehicles I have owned and built this example wouldn’t matter as the tubes are not that long. I suppose as much as anything this video reinforces the importance of triangulation in any chassis.
But it still is entertaining and interesting! Watch.