So here's the question - does the tubing need to be DOM, or will welded tubing be OK?
This topic is and can be very subjective. I have dealt with this very question numerous times in several different sanctioning bodies of racing and have seen and hear just about everything. I did find the following however, straight out of the ECTA as quoted by a chassis builder:
Mild Steel vs. 4130 Material
There is a myth that a chassis constructed of 4130-chrome molly is “stronger” than mild steel one. While 4130 is a stronger material, the rules allow us to use a thinner wall thickness (.083 8 .065). Therefore, the 4130 structure will be 25 to 30% lighter than mild steel. This may or may not be a consideration in your planning. Other factors to consider are the cost (about $900 more for 4130) and ease of construction. A mild steel chassis can be MIG welded (Metal Inert Gas) while the 4130 tubing requires the TIG method (Tungsten Inert ~Gas) and must be done by a professional.
Mild Steel Wall Thickness
Although the rulebook only requires .120” wall tubing thickness in a mild steel application, it would be wise to use .134 (11 ga.). You must be aware that .120 wall tubing can vary by production runs and can be within acceptable industry mill spec standards yet being under the minimum .120 nominal wall thickness! (Since NHRA has instituted the .118 minimum, I have had to repair many “new” cars as thin as .113!)
When ordering tubing from a steel supplier, specify ASTM A51 3 Type 5, DOM (Drawn-Over-Mandrel). This material is a cold drawn electric resistance welded tube with all flash removed. Each tube is tested for soundness of weld. It is made from 1020 steel in walls up to 10 gauge and 1026 steel in walls heavier than 10 gauge. Normalizing and cold drawing over a mandrel makes DOM a uniform and precision Product. It is preferred over seamless tubing for it’s excellent O.D. & I.D. concentricity.
Cold Drawn Seamless Tubing (ASTM A519) is another good choice but availability in .134 wall thickness is limited. It is made from 1026 steel, produced to OD & ID dimensions; and it is furnished in “As Drawn” condition. Cold Drawn Seamless offers good surface quality and increased mechanical properties over hot finished seamless tubing.
Obviously, it is going to up to the end user (the sanctioning body and the class in which you build your chassis to) to what your roll cage requirements will be. We can talk all day long about material... but it doesnt mean diddley if doesnt meet the requirements for your chosen class, vehicle weight, speed and in the respective sanctioning body.
IMHO, to further the material thought process, unless your entire chassis (frame/unibody etc) is made up of ChroMo... you will be inspecting it for cracks 10 times more than all other materials combined. Not to mention that you'll need someone that knows ChroMo and the requirements to properly weld it. Embrittlement is the biggest issue with ChroMo.
Food for thought in the outside world of racing, any new chassis requiring a 'stamp' or certification will be required to be at a minimum of DOM to meet upcoming rules and regulations. SCTA-BNI, USFRA requirements are different... as are that of the American Iron Series etc etc. It can be of some insight to study the 'other' sanctioning bodies to where their rules on materials are going because it is a good representation of where technology and the science of safety is headed. What is worth your safety and your life is solely up to you. The fact remains that there is nothing wrong with Mild Steel. In fact, if you wad up your car... MS and DOM are going to be the ones who win the war... unless your entire chassis is made up of ChroMo. Ask the builders of SCCA, FIA and NASCAR chassis. But in my opinion... I would choose DOM. The few extra bucks is worth it... just like it is in consideration to your driving suit. Do you feel safe in a SFI 5 or an SFI 20 ?? That can only be answered by you.
Some people believe that 4130 chromemoly cars are stronger than mild-steel cars. I dont believe this to be necessarily true. While 4130 tubing is a stronger material, because it’s made out of an alloy steel, the rules let us use thinner material (.083- and .065-inch wall). Thus, a stronger material that has a thinner wall is about as strong as a thicker-wall mild steel. The 4130 chassis is going to be 20- to 25-percent lighter because it’s made out of thinner material; there’s simply less steel in the car. Basically, what you’re paying for when you buy a 4130 car is weight reduction. In a basic cage, you pay about $XXX extra to save 70 to 80 pounds.
Now, some guys would pay a fortune for 70 to 80 pounds. But, you have to ask yourself: “Could I spend $XXX somewhere else, and be better off?” If you have cast-iron cylinder head on your engine, and you want a 4130 chassis, you’d be better off buying an aluminum head (that is if the rules would allow this). If you’re going to go fast you probably should consider 4130, but remember the difficulties and challenges involved with using it. The only real disadvantage to 4130 is that it must be TIG-welded
for any serious effort. That means every single accessory, bracket and tab should be TIG-welded. For the first-time, build-it-yourself type of guy, this is not the way to go. In fact, he shouldn’t even consider it. Chromemoly is for a higher-skilled, more capable fabricator. Mild steel, on the other hand, is extremely forgiving.
Just a few thoughts... and an opinion. When you study rule books from different sanctioning bodies long enough... and talk to those doing the actual 'Tech' inspection... you learn a little more than hear-say.
