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Author Topic: Roll Cage Safety - Mild Steel vs. Chrome-Moly
Buford T Justice
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I want to talk about roll cage safety and being safe....... I found an article in my search for what are the best roll cages....I know the article talks about NHRA rules but what are the rules for Chrome-Moly and Mild-steel for SCCA and NASA. What are the benefits and disadvantages of one vs. the other?

Does "A chrome moly cage only last about 4+- years before it starts getting brittle, where a mild steel cage may last 25 years"?


I found two articles but they are for NHRA:

4130 Chromoly or ( Chromium Molybdenum ) has a 0.30% carbon content... thus you will have a higher tentancy for stress cracking or metal fatigue... Chromoly still is considered technically a "low carbon alloy steel." But 1020 steel ( Mild Steel ) is also a "low carbon steel" but it has a content that is 0.20%. The carbon makes the metal easier to weld, but the more carbon content makes it more brittle and thus lower the tensile strength of the metal. However, these metals both have a very high tensile strength.
There are several other factors that cause cracking and twisting and metal fatigue but that is a whole other post...
Every national recognized sactioning body recommends GTAW ( Gas Tungsten Arc Welding or better known as TIG ) on a Chromoly ( CM ) cage...
I.E. The NHRA rule book states in Section 16 (General Regulations), Sub-section 4:11 (Roll Cage), second sentence, "All 4130 chromoly tube welding must be done by approved TIG process; mild steel tube welding must be approved MIG wire feed or TIG process".
If you plan on actually using the cage... i.e. reducing torque/flex on the frame and body of the vehicle for a long period of time and don't plan to replace or fix it, go with MS (mild steel) the only way you actually save weight with CM is that you can use it in a thinner gauge. It actually weighs the same if it was the same thickness as the MS. TIG welding process has to be used with CM and . But it does make a slightly better weld. Due to the ability to have a greater tensile strength rod. However, that being said, welding with TIG requires greater skill and knowledge of metals. Which is why it normally cost more to get done properly. ( THIS IS AN AREA YOU DO NOT WANT TO CUT CORNERS ON ) And why would you want to... if it is done improperly you will not be happy with yourself. I know I would not want to trust my potential life on saving a few bucks...
MS can be welded a few ways, but mainly it can be welded in GMAW ( Gas Metal Arc Welding or better known as MIG ) which is cheaper.



Second article:

Ah, function. We had big choices there, chiefly related to whether we ever wanted the Bomb to run 7s at the dragstrip. The problem was that NHRA rules for cars quicker than 8.50 in the quarter now require Funny Car-type rollcages that enclose the driver, and we feel that type of 'cage virtually eliminates extended street driving. We compromised by using the 4130 chrome-moly tubing that's required for the quicker 'cage and designing it such that we could add the Funny Car pod and a few other details to bring the Bomb to 7-second specs if so desired in the future. With an estimated 100 feet of tubing in the car, the chrome-moly saved about 20 pounds compared with the thicker wall required with mild-steel tubing. However, while the NHRA only requires 15/8-inch-diameter, 0.083-inch-wall-thickness chrome-moly tubing, the open-road-racing events demand 13/4-inch, 0.090-wall tubing, so we went with the larger size. The open-road-racing and land-speed rules recommend against chrome-moly tube, as it can be brittle near the welds if the area is not flame-annealed to normalize it after welding, but they do not outlaw it.

Casey Z Verified Driver Made Donation to Website
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GCR 9.4.1

F. Material:
1. Seamless, or DOM (Drawn Over Mandrel) mild steel tubing (SAE 1010, 1020, 1025) or equivalent, or alloy steel tubing (SAE, 4130) shall be used for all roll cage structures. Proof of use of alloy steel is the responsibility of the entrant.

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Z Brothers Racing / East Street Auto

Casey Z - 1.6 Kettle
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Rye
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Chrome moly is stronger, but as stated is more brittle and subject to fatique cracking. The only advantage is lighter weight IF you were allowed to use thinner wall tubing like NHRA. But I believe that in our world the same wall thickness is required with mild steel or chrome-moly, so there really isn't any benefit and possibly some downside.

I used to work on tube frame aircraft (4130 chrome-moly), and part of the DAILY inspection was closely checking around all the welds for signs of cracking. And I found a few over the years. Personally, I would would never use chrome-moly for a club level race car rollcage.

Richard

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Alloy tubing was allowed to be thinner wall and smaller diameters in all cars except Showroom stock, Improved Touring and Touring until this year. Now only Formula and Sports racers can use smaller alloy tubing. There was never a weight advantage in SM or IT and now there is none in GT or Production either.

Dennis

Buford T Justice
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Thanks for you input..I'm going to go with mild steel.

Willie the Tard Verified Driver
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also 4130 should be tig welded if over a very very thin gauge

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William Keeling a.k.a. Willie the Tard

   

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