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Unread 01-09-2015, 12:02 PM
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Default Re: Vorshlag Budget TT Build: Project DANGER ZONE

continued from above

Its hardness combined with its flexibility (from some other alloying elements) and high tensile strength make steel far more useful than either type of iron: it is more durable and holds a sharp edge better than the softer wrought iron, but it resists shock and tension better than the more brittle cast iron. After about 1856 (the invention of the Bessemer converter) and into the 1870's (Andrew Carnegie's grasp of the vital importance of chemistry in steel making) steel alloys became cheap to manufacture and exploded in use, replacing wrought iron rails in railroad tracks and other uses.

Some of the sources I used, other than past knowledge: This and this and that, and more.

Modern Roll Cage Steel Choices - 4130 vs 1020/1080 Alloys

There are two basic steel alloys used in roll cage structures and we will start with the "stronger" and more expensive alloy allowed: 4130. AISI 4130 alloy steel is about 97% Iron and has 6 other alloying agents that make up the last 3%. Chromium (0.80 1.10%) Molybdenum (0.15 0.25%) are the two key elements added that give this metal its higher tensile and yield strengths, and are the two most expensive elements in the alloy as well - hence the nickname "Chromoly Steel". These alloys are harder to weld properly (generally they are only TIG welded) when compared to Low Carbon/Mild steels. The yield strength of 4130 is 66,700 psi (67 KSI) and when this metal is used, a little LESS of this alloy is needed to achieve the same total assembly strength as Mild steel. It has a good strength to weight ratio, but the same density as all steels (all steel alloys and iron have nearly the exact same density, .284 lbs/cubic inch, due to the fact that all steel alloys are still almost entirely made of iron). In the past, roll cage rules allowed for thinner 4130 tubing to be used relative to Mild Steel, but that is no longer the case for most road racing bodies.

I used to use a lot of "A36" mild steel 15-20 years ago when I designed oilfield equipment, which had a minimum yield strength of 36KSI, which is relatively soft and very cheap. The modern 10XX series steels have gotten better and a lot stronger - closer in strength to 41XX Chromoly steels, but without the negatives. AISI 1018 and 1020 "Low Carbon" or "Mild" Steel alloys (also known simply as Carbon Steel) are lower cost and slightly weaker than 4130, but these 10XX series alloys have excellent weldability and offer a good balance of toughness, strength and ductility. Once cold worked (via the DOM or CDS process) these Mild steels become even stronger and stiffer.

1018 steel (0.14 - 0.20% Carbon) has a yield of 54KSI and 1020 steel (0.18-0.23% Carbon) has a 51 KSI yield - which isn't that far off of 4130 (67 KSI). Ultimately 4130 is about 20% stronger than Mild steel. But 1018 tubing that is DOM cold worked gets stronger, and is rated at 70KSI, and 1020 DOM tubing is rated is 65 KSI. Cold working 4130 tubing via the DOM process turns it up to 90 KSI yield... roughly 22% stronger.

The key benefit to racers building roll cages out of Mild steel over Chromoly is that 10XX alloy steel is much more forgiving with respect to weld embrittlement and tends to "crash better" than the harder "alloy" steels. When you weld the 41XX series alloys the molecular structure of the alloy changes near the heat affected zone, especially if you put too much heat into the weld (and some welders like to "weld hot", which can really make the weld area change), so 4130 cages are almost exclusively welded with the trickier TIG welding process (a Tungsten tipped torch with a shielding gas and a separate metal rod, with a variable control on the welding arc).

Mild steel isn't nearly as susceptible to this issue and can be TIG or MIG welded and generally does not lose much strength at the welded joints. 10-20 years ago 4130 was all the rage for roll cages but lately 1018 or 1020 Mild steel is the norm, as long as they are DOM. To me nothing beats a properly designed, TIG welded, Mild Steel DOM tubing roll cage. This has the best combination of variables and the least number of compromises.

DOM vs ERW Tubing?

ERW or "Electro Resistance Welded" tubing is how steel tubing and pipe is made (at least initially) - where a continuous, flat roll hot rolled steel is bent around round (or square or rectangular) dies and welded at a seam (see image above). That's how lower cost pipe and tubing is left - with this visible welded seam on the outside and often a physical "ridge" on the inside of the tube (see below). This problem is - seam ultimately becomes the weakest point in this type of tubing. And the hot roll plate material isn't ever as strong as cold worked steel.

A visible seam and often a raised ridge is the result of the welding process from ERW tubing. All tubing starts as ERW...

The DOM process (Drawn Over Mandrel) takes ERW tubing and "cold works" it by drawing over a round mandrel and through round dies, inside and out. This makes the now DOM tubing "seamless" (its really hard to find the seam with your eyeball) and work hardens the steel structure - adding strength and removing the stress riser at the seam. ERW tubing was previously allowed in roll cages (up until just a few years ago) and the various CCR/GCR rules sometimes still reference ERW for "grandfathered" cages built before it was outlawed, but nowadays all roll cages are spec'd as seamless tubing - either 1018/1020 Mild Steel (DOM or CDS) or 4130 Alloy steel (DOM or CDS). There's another cold working seamless tubing process nowadays called CDS (Cold Drawn Seamless), but I can't seem to find any CDS tubing in common roll cage sizes - yet. The CDS specification seems to be more common in Europe. It could be the exact same process as DOM, but some U.S. tubing companies specify them separately, so I don't know.

The stiffness difference between ERW and DOM is shown in this video, with the same diameter and wall thickness tubes of both types in a side-by-side bending test. The lower strength of ERW + the stress riser of the seam are why it isn't specified in roll cages any longer, but it was a pretty recent deletion from roll cage specifications.

The FIA has updated their specified tubing to 350 N/mm2 (50.76 KSI) tensile strength (see page 46, rule 8.3 of Appendix K here), and material is simply listed as "Cold drawn seamless carbon steel". They used to only spec 4130 alloy tubing (or the European equivalent) but even the French have seen the benefits of using Mild Steel DOM/CDS tubing. As we have seen with changes to rules specs, ERW is no longer allowed and the advantages in welding Mild Steel outweigh the weight savings or 20% strength benefits of 4130 Chromoly.

Picking the Tubing Material, Tubing Sizes and Cage Design Layout

OK, that got a bit long, but it was hopefully worthwhile tech. Now that we know why we use steel, know more about the alloys, understand the benefits of the cold working and seamless processes that are required in the steel tubing specified, and why more cages are using mild steel DOM - let's pick the cage tubing size for this build and show some Corvette cage pictures already!

Many of the cars we work on at Vorshlag lately, that are built around NASA specs, weigh over 3000 pounds so we're often using 1.75" diameter x .120" wall thickness DOM Mild steel tubing. And since the 1992-96 C4 Corvette is listed as a base class of TTC and a Minimum Competition Weight of 3203 pounds, I assumed that we had to use this tubing size. This is nearly the heaviest cage tubing in all of the NASA CCR, but lower weight cars can use thinner tubing diameters and wall thicknesses, as shown below (copied from the 2015 NASA CCR).

NASA 15.6.18 - Roll Cage Tubing Sizes

For the purposes of determining roll bar tubing sizes, vehicle weight is as raced, but without fuel and driver. Minimum tubing size for the roll
cage is:

Up to 1500 lbs:
  • 1.375 x 0.095 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM
  • 1.500 x 0.080 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

1501 - 2500 lbs:
  • 1.500 x 0.095 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

2501 - 3000 lbs:
  • 1.500 x 0.120 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM
  • 1.750 x 0.095 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

3001 - 4000 lbs:
  • 1.750 x .120 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

Over 4000 lbs:
  • 2.000 x 0.120 Seamless Alloy (4130), Seamless mild steel (CDS Mechanical) or DOM

Since we do not need to include the weight of the driver (200 pounds) or fuel (20 gal x 6 pounds/gallon = 120 pounds), that means our goal weight of 3203 really translates to a caged race car weight of about 2900 pounds. So we can use the lighter 1.75" x .095" wall DOM Mild Steel tubing. I like this for two reasons. First, we have a bunch of this tubing already in stock at the shop. And two, we typically bend 1.75" tubing, so our tubing bender has this set of dies already installed. Right now we're building two cages at once, both with 1.75" diameter tubing (one is .095 and the other is .120" wall), so we won't have to keep switching the dies. This thinner wall tubing is also easier to bend.

We use a JD2 Model 32 manual tubing bender and dies

The weight is not insignificant: 1.75" x 120" wall DOM weighs 2.089 pounds/foot of tube length. The 1.75" x .095" wall DOM weighs 1.679 pounds/foot (19.6% lighter and about the same amount cheaper). The other choice for this weight is 1.5" x .120" wall DOM, which weighs more at 1.769 pounds/foot. Sure, we could have stepped up to a larger tubing size, but those CCR minimums are there for a good reason... mega-sized tube in a smaller/lighter car makes for less room to the driver and less energy absorption in a crash, so we're going with the recommended tubing range for a 2900 pound car, then picking the larger tubing diameter of the two options given there, which is slightly lighter.

Cage Layout and Design

There are three cage design options we can choose form the NASA CCR, shown in 15.6.8, -.9 and -.10. We're going with the "Forward Hoops" version from 15.6.8, shown above. This is the most common of the 3 methods (another is the "Halo style") and makes for the most room for the driver's head in a car like this. So about halfway through the day on Friday the 2nd, our fabricator Olof stared work on the cage install. He will build a majority of this cage while Ryan finishes a cage on another car at the same time.

Before you can start bending any tubes you have to clear out the interior. This car was already gutted, which saved us 15-20 hours of labor. That work is never included in the "cage" price, which some people don't always understand. If you want to save some money, bring in a NAKED car with zero interior bits, like this. I came in early that Friday and removed the driver's seat and targa top, then the guys pulled the rear hatch glass off.

The driver's seat normally weighs more than this, but it was alreay partially gutted by Matteucci and only tipped the scales at 34 pounds - I've weighed a lot of modern power front seats in the 60-75 pound range. The targa top weighed less than I had thought at 22 pounds. This is the plexiglass "See through" version, but I'm looking for a fiberglass version (both were offered from the factory) which we could paint white to match the car, but they sell for $$$ used. The weight is mostly in the metal frame structure, so the Plexiglass vs Fiberglass is probably a wash - except the fiberglass OEM version is likely stiffer. We might replace the Plexiglass with a custom Carbon Fiber skin (stiffer than Fiberglass). Is it legal? Well since we can run with the targa top removed (wouldn't that make its construction insignificant?) and as I read TT rule 8.3.B, we can lighten the "roof, hood, body panels and doors" as long as they "maintain their BTM (Base Trim Model) size and shape". The "no points" listing for I.h.20 says the same thing, with more details with respect to carbon/fiberglass doors being legal as long as the BTM body lines, hinges and handles are still operational. And an in-house built Carbon Fiber roof would be, you know, cool...

The rear glass was much heavier at 46 pounds. That bit will likely never go back onto this car, as we have a formed, 3/16", trimmed Plexiglass rear hatch replacement inbound that should save 30+ pounds. I will show that in my next post, if it gets here before the NASA race Jan 17th. This is legal per the "No Points Modifications" rule I.b.8, as long as it has the factory BTM shape and no uncovered holes.

Once the interior was cleared out enough to start Olof began cutting bits of fiberglass out of the way. See why I had the pictures of the C4 frame structures up above? That was to help us find where the frame is - which isn't obvious in some areas as there are big gaps between the shape of the interior fiberglass structure and the metal underneath.

If you ever get a roll cage quote on a Corvette, now you know why it costs more than a traditional steel unibody car - because you have to cut access holes to get to the frame. And they need to be fairly big holes, to give the fabricator access to weld a reinforcement plate to the frame. Then you have to close up the holes in the fiberglass later... all of that is extra work.

Once you have access to the frame structure it has to be cleaned of all paint (we use a pneumatic wire brush tool called a "Crud Buster" along with a flap disc on an electric angle grinder). Then the plates are drawn in cardboard and transferred to steel, in this case 1/8" thick hot rolled plate (minimum thickness is .080" for these plates, but we tend to use .125", since it is stronger and easier to weld).

Olof cut the plates and tack welded them to the frame at the main hoop, which is in an unusual spot. Normally the main hoop mounts to the floor behind the driver's seat - often 6-8+ inches behind the back of the seat. But in a Corvette, for tall-ish folks, the back of the seat ends up right at the rear bulkhead, so the main hoop has to go up on the rear deck area. The frame extends up here and we've checked with NASA inspectors on Corvette cage hoop placements and have also built cages in Corvettes like this. Miata cage main hoops are done the same way - just the nature of these 2 seat cars and their compact interiors.

On Monday the 5th, Olof designed and bent the main hoop, with help from our head fabricator Ryan. They got the hoop TIGHT up against the high strength steel roof structure, and placed it back the correct distance from my head. We had already done a number of seat mock-ups at this point and we knew where I needed to sit - with the seat almost touching the rear bulkhead. This put the main hoop where it is above.

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