About six months ago, I brought up the age-old question of when and how to stress-relieve 4130 in a posting to this newsgroup. The ensuing thread, titled "4130 debate," was interesting, but at the end of the day, nothing was settled.
Some very respectable people said go ahead and TIG 4130 to build car chassis and airplanes without any additional heat treatment; others contended that to do that was to invite disaster and that mild steel was the way to go.
That discussion mainly involved theory. At this point, I would be interested in empirical evidence.
This is my question: we know that NASCAR Nextel Cup (formerly Winston Cup) racing cars are built with integral steel roll cages, and that these cages survive tremendous impacts without "shattering" or otherwise failing catastro- phically without first undergoing significant plastic deformation. How are they made? Specifically, what material and what welding process is used in their construction? What post-weld procedure is followed?
Thank you, MGD
P.S. this being the internet, it's difficult to believe claims based mainly on a poster's say-so. If anyone out there knows how these cages are fabricated, it would be helpful if you could cite your source.
(formerly Winston Cup) racing cars are built with integral steel roll cages, and that these cages survive tremendous impacts without "shattering" or otherwise failing catastro- phically without first undergoing significant plastic deformation. How are they made? Specifically, what material and what welding process is used in their construction? What post-weld procedure is followed?
I build second-tier, short-track race cars for a living, and talk with two Cup engineers on a regular basis in order to keep up with what is happening one level above me in an effort to introduce innovation at my level.
Thanks for the info. I presume that NASCAR mandates the use of mild steel. If so, the as-welded-4130 advocates might argue that this is just another rule intended to keep down costs and to keep fabricators from screwing up a critical safety system, or perhaps a nod to NASCAR's low-tech tradition.
Do you know of any series that uses 4130 or other low-alloy steels in rollover structures where the structures are welded without any post-weld stress relief? I keep bringing this up because I often hear that 4130 is "fine" in safety structures, despite the obvious worries about the HAZ and resultant tiny elongation.
What do the European Touring Car guys use? How about American Le Mans? The World Rally Championship?
The only real-world examples that I can find of as-welded-
4130 used in a critical spaceframe come from homebuilt aircraft. I'm not sure aircraft are really relevant here because they only exceed yield in a crash, and I'll bet aero designers regard crashing as Not An Option -- unlike racing cars, where crashing is widely regarded as an inevitability.
I would expect yield can also be exceeded when flying in extreme turbulence as well as during aerobatic maneuvers that exceed the plane's capabilities.
I remember a news video of a large airplane that lost both wings in mid air while making an approach to drop a load of water in a California fire. I bet yield was exceeded in that case, although it was probably in aluminum instead of steel.
The NASCAR rulebook actually specifies, "...magnetic steel"....and .095" wall by 1.75" diameter.
There would be no weight savings available with CHMO. since one must drop guage size to gain any weight advantages, and NASCAR rules specify guage size.
Since sizes are specified - and there is no advantage to be gained from use of CHMO - the engineers and designers choose to avoid the preparation, fabrication, post-fabrication, and energy absorption issues associated with CHMO steels.
A lot of Midgets and Supermodifieds use CHMO because they have few minimum tubing diameter/wall thickness rules, so they can take advantage of being able to use one or two guages smaller CHMO in place of mild steel.
My personal observation of most of these cars after a collision is broken tubes within one-half-inch of the weld, while on many mild-steel oval track cars I often find post-collision bends of several degrees which actually include the weld that have absorbed much more energy than the broken tubes have.
I believe CHMO still has some popularity in drag racing also.
Some of the more exotic cars use aluminum honeycomb and/or carbon fiber superstructures, which intergrate chassis and rollover protection into a single tub unit.
Here's an excerpt from the NHRA rulebook. I never found any further info on what an "approved tig process" is, but you can email their tech dept and ask at
Info on roll bars and roll cages in full bodied cars. From:
"4.10 ROLL BARS
All roll bars must be within 6-inches (15.2 cm) of the rear, or side, of the driver's head, extend in height at least 3-inches (7.6 cm) above the driver's helmet with driver in normal driving position, and be at least as wide as the driver's shoulders or within 1-inch (2.5 cm) of the driver's door. Roll bar must be adequately supported or cross-braced to prevent forward or lateral collapse of roll bar. Rear braces must be of the same diameter and wall thickness as the roll bar and intersect with the roll bar at a point not more than 5-inches (12.7 cm) from the top of the roll bar. Sidebar must be included on driver's side. The side bar must pass the driver at a point midway between the shoulder and elbow. All vehicles with OEM frame must have roll bar attached to frame; installation of frame connectors on unibody cars does not constitute a frame and therefore it is not necessary to have the roll bar attached to the frame. Unibody cars with stock floor and firewall (wheel tubs permitted) may attach roll bar with 6-inch (15.2 cm) x 6-inch (15.2 cm) x .125-inch (3.2 mm) steel plates on top and bottom of floor bolted together with at least four 3/8-inch (9.53 mm) bolts and nuts, or weld main hoop to rocker sill area with .125-inch (3.2 mm) reinforcing plates. All 4130 chrome moly welding must be done by approved TIG Heliarc process; mild steel (or ST51) welding must be approved MIG wire feed or approved TIG heliarc process. Welding must be free of slag and porosity. Any grinding of welds prohibited. See illustration (Drawing
Roll bar must be padded anywhere driver's helmet may contact it while in driving position. Adequate padding must have minimum 1/4-inch (6.35 mm) compression or meet SFI Spec 45.1.
4.11 ROLL CAGE
All cage structures must be designed in an attempt to protect the driver from any angle, 360-degrees. All 4130 chrome-moly tube welding must be done by approved TIG heliarc process; mild steel tube welding must be approved MIG wire feed or TIG heliarc process. Welding must be free of slag and porosity. any grinding of welds prohibited. Additionally, roll cage must be padded any where the driver's helmet may contact it while in the driving position. TAD, TAFC, Pro Stock, Funny Car & Top Fuel padding must meet SFI Spec 45.1.
Full Bodied Cars
On full bodied car, with driver in driving position, helmet must be in front of main hoop. If helmet is behind or under main hoop, additional tubing same size and thickness as roll cage must be added to protect driver. Main hoop may be laid back or forward but driver must be encapsulated within the required roll cage components.
All cage structures must have in their construction cross bar for seat bracing and as the shoulder harness attachment point; cross bar must be installed no more than 4-inches (102 mm) below, and not above, the driver's shoulders or to side bar. All required rear braces must be installed at a minimum angle of 30-degrees from vertical, and must be welded in. Side bar must pass the driver at a point midway between the shoulder and elbow.
Unless an O.E.M. frame rail is located below and outside of driver's legs (i.e. '55 Chevy, '65 Corvette, etc.) a rocker or sill bar, minimum
1 5/8-inch (41.2 mm) x .083 (2.1 mm) CM or .118 (3.0 mm) MS or 2-inch x
2-inch x .058-inch (50.8 x 50.8 x 1.5 mm) CM or MS rectangular, is mandatory in any car with a modified floor or rocker box within the roll cage uprights (excluding six square feet of transmission maintenance opening). Rocker bar must be installed below and outside of driver's legs, and must tie into the main hoop, the forward hoop, frame, frame extension or side diagonal. Rocker bar may not tie into swing out side bar support. If rocker bar ties into side diagonal more than 5-inches (127 mm) (edge to edge) from forward roll cage support or main hoop, a 1
5/8-inch (41.2 mm) x .083 (2.1 mm) CM or .118 (3.0 mm) MS brace/gusset is mandatory between the diagonal and forward roll cage support or main hoop.
Swing out side bar permitted on O.E.M. full bodied car 7.50 (*4.50) E.T. and slower. The following requirements (a through d) will be enforced on cars certified after 1/1/98, on all cars 1/1/99.
a. 1 5/8-inch (41.2 mm) O.D. x .083-inch (2.1 mm) (CM) or .118-inch (3.0 mm) (MS) minimum. Bolts / pins must be 3/8-inch (9.5 mm) diameter steel, minimum and in double shear at both ends.
b. Male or female clevis(es) permitted. Male clevis must use two minimum
1/8-inch (3.2 mm) thick brackets (CM or MS) welded to each roll cage upright; female must use minimum 1/4-inch (5.4 mm) thick bracket (CM or MS) welded to each roll cage upright. Pins must be within 8-inches (204 mm) of the vertical portion of both the forward and main hoops. A half cup backing device must be welded to the vertical portion of the main hoop (inward side) or the upper end of the swing out bar (outward side), minimum .118-inch (3.0 mm) wall (CM or MS) extending at least 1 5/8-inch (41.2 mm) past the center of the pins. A clevis assembly utilizing a minimum .350-inch (8.90 mm) thick male component and two minimum .175-inch (4.45 mm) thick female components may use a 1/2-inch (12.7 mm) diameter grade 5 bolt, and does not require a half cup backing device.
c. Sliding sleeves of 1 3/8-inch (35 mm) x .083-inch (2.1 mm) CM or .118 (3.0 mm) MS, with minimum 2-inch (51 mm) engagement, are permitted in lieu of the upper pin/cup.
d. All bolt/pin holes in the swing out bar must have at least one hole diameter of material around the outside of the hole. "
-- Regards, Carl Ijames carl.ijames at verizon.net