Mild steel, formed by the Electric Resistance Welding (ERW) or Drawn Over Metal (DOM) process, and Chrome-Moly are the most popular materials used for cage fabrication. DOM mild steel is weaker than Chrome-Moly while ERW is the weakest. ERW starts as a flat sheet of mild steel and it is simply rolled and then welded. It is also generally not allowed for cage construction anymore. Though existing cars with older ERW cages may be grandfathered in, be sure to check with the sanctioning body you plan to compete with first. DOM tubing is superior to ERW because the sheet of mild steel is drawn over a mandrel cold. Simultaneously cold forming the outside and inside gives the tubing a more uniform wall thickness. This process eliminates ridges and other stress risers where cracks can originate. Cold working the steel also refines the grain structure, improving the mechanical properties of the metal.
Chrome-Moly is an alloy of steel with chromium and molybdenum added to it. A common misconception is it’s much lighter than mild steel, but it actually weighs about the same. Its alloying elements give it about a 50% higher tensile strength over mild steel allowing the use of thinner wall tubing, thus reducing the total weight of the cage. Again, check if your sanctioning body allows the use of Chrome-Moly tubing. Many, including NASA, do not allow it due to its difficulty to weld and susceptibility to failure if not welded properly. It’s strength can be greatly reduced by welding heat. Proper construction requires TIG welding, fine electrodes and very tight mitering. It is also less malleable and more brittle when overstressed or overheated.
|NASA’s Performance Touring class is now the new home of former SR Cup pilots.
Generally speaking, DOM mild steel is a better cage material for a unibody car than Chrome-Moly as it can bend and conform but not fracture as easily. It can absorb impacts, reduce G-loads and is less likely to break apart in a serious accident. And it’s cheaper, CHA-CHING! When you are out looking around for tubing you will be looking at two numbers; the diameter and the wall thickness of the tubing, both should be expressed in inches.
NASA’s Club Codes and Regulations (CCR) require us to have a roll cage made of DOM steel, ERW is not allowed. For cars weighing between 1501 lbs and 2200 lbs, 1.50″ x 0.095″ DOM tubing is required. If your car weighs between 2201 lbs and 3000 lbs, you will need either 1.50″ x 0.120″ or 1.75″ x 0.095″ DOM tubing. Since you may be sharing the track with much heavier cars, it is recommended you go with 1.75″ x 0.120″, but not necessary. We went with 1.75″ x 0.095″ tubing on our cage. It is hard to determine if our car will weigh under the 2200 lb limit for 1.50″ x 0.095″ tubing, so we went with the thicker tubing.
|Uhh, good luck giving your friends a ride in the back seat now.