Drag/Track Car Cage

[JustinOlson]

So I've been doing a lot of reading trying to figure out the requirements of SCCA and NHRA cages. I'm looking at building a cage that satisfies both requirements so that I can take it to the drag strip and run fast, but also have no issues passing tech for track events.

 

SCCA Tubing Requirements for Roll Cages:

 

Up to 1500 lbs. 1.375 x .095 DOM / Seamless / Alloy

1501-2200 lbs. 1.500 x .095 DOM / Seamless / Alloy

2201-3000 lbs. 1.500 x .120 DOM / Seamless / Alloy

1.625 x .120 DOM / Seamless / Alloy

1.750 x .095 DOM / Seamless / Alloy

 

 

NHRA Tubing Requirements for Roll Cages:

1.625 x .118 DOM

1.625 x .083 4130 Chromoly.

 

With these requirements in mind I created a table to show the weights of different materials when used in the cage:

 

 

The two lightest options are 1.625" x .125" mild steel at 132lbs and 1.75" x .095" 4130 chromoly at 108lbs. So for $100 difference I will have a cage that 24lbs lighter and will pass tech in both NHRA and SCCA. I am capable of tig welding and feel 4130 is the best option in this case.

 

My only major issue is how to sport the main roll hoop as NHRA and SCCA do it differently:

 

SCCA:

 

NHRA:

 

I figured I could take the SCCA main roll hoop with diagonal bar and add the feet like the NHRA have going in. Would this satisfy both requirements? Input welcome.

 

Regards,

Justin

[boodlefoof]

Justin,

 

If I remember correctly, the NHRA .118'' requirement requires that all parts of all tubes meet this wall thickness. When you begin bending the tubing (particularly the tight bends at the roof), the outer edge of the bend will stretch and become thinner. Most people use .134'' tubing to maintain a .118'' minimum wall thickness all around.

[johnc]

I am capable of tig welding and feel 4130 is the best option in this case.

 

If this is your first cage and/or the first cage you'll be TIG welding, I would use DOM instead of CroMo. DOM is more tolerant of too much heat.

[JustinOlson]

Here's a good link about welding 4130:

 

http://www.lincolnelectric.com/knowledge/articles/content/chrome-moly.asp

 

http://www.tigdepot.com/faq.html

[JustinOlson]

1020 Mild Steel, cold rolled

Component Wt. %

C 0.17 - 0.23

Fe 99.08 - 99.53

Mn 0.3 - 0.6

P Max 0.04

S Max 0.05

Density - 0.284 lb/in^3

Brinnell Hardness - 121

Yield Tensile Strength - 50.8 ksi

Ultimate Tensile Strength - 60.9 ksi

Shear Modulus - 11600 ksi

Modulus of Elasticity - 29.7 ksi

1212 Mild, cold drawn

Component Wt. %

C Max 0.13

Fe 98.52 - 99.07

Mn 0.7 - 1

P 0.07 - 0.12

S 0.16 - 0.23

Density - 0.284 lb/in^3

Brinnell Hardness - 167

Yield Tensile Strength - 60.2 ksi

Ultimate Tensile Strength - 78.3 ksi

Shear Modulus - 11600 ksi

Modulus of Elasticity - 29.0 ksi

4130 Chromium-Molybdenum, Normalized at 1600 F

Component Wt. %

C 0.28 - 0.33

Cr 0.8 - 1.1

Fe 97.3 - 98.22

Mn 0.4 - 0.6

Mo 0.15 - 0.25

P Max 0.035

S Max 0.04

Si 0.15 - 0.35

Density - 0.284 lb/in^3

Brinnell Hardness - 197

Yield Tensile Strength - 63.1 ksi

Ultimate Tensile Strength - 97.2 ksi

Shear Modulus - 11600 ksi

Modulus of Elasticity - 29.7 ksi

4130 Steel, water quenched 1570°F, 1000°F temper

Component Wt. %

C 0.28 - 0.33

Cr 0.8 - 1.1

Fe 97.3 - 98.22

Mn 0.4 - 0.6

Mo 0.15 - 0.25

P Max 0.035

S Max 0.04

Si 0.15 - 0.35

Density - 0.284 lb/in^3

Brinnell Hardness - 302

Yield Tensile Strength - 142.0 ksi

Ultimate Tensile Strength - 151.0 ksi

Shear Modulus - 11600 ksi

Modulus of Elasticity - 29.7 ksi

4140 Steel, normalized at 1600°F, air cooled

Component Wt. %

C 0.38 - 0.43

Cr 0.8 - 1.1

Fe 96.785 - 97.84

Mn 0.7 - 1

Mo 0.15 - 0.25

P Max 0.035

S Max 0.04

Si 0.15 - 0.3

Density - 0.284 lb/in^3

Brinnell Hardness - 302

Yield Tensile Strength - 97.9 ksi

Ultimate Tensile Strength - 148.0 ksi

Shear Modulus - 11600 ksi

Modulus of Elasticity - 29.7 ksi

 

http://www.matweb.com/index.asp?ckck=1

[thehelix112]

Nice link, but not particularly applicable. You're not going to be quenching or tempering anything other than air cooling, and according to the interweb (large grain of salt applies), such heat treating is not required for the wall thicknesses you are likely to be using (<3mm) (pun).

 

I am in the process of designing my rollcage, will have to make some sketches and post pics shortly.

 

Best of luck,

 

Dave

[JustinOlson]

" The fatigue strength of the Chromoly verses mild steel issue involves more than just the materials involved. When mild steel is welded, the resulting micro-structure in the heat affected zone (HAZ) is usually a combination of large grain Pearlite and Proeutectoid, which are generally relatively ductile. On the other hand, welding say 4130 or other alloy steel, an undesirable and very brittle Martensite micro-structure is formed. To reduce this brittle martinsite to a more ductile coarse pearlite (like in the mild steel case) you would need to anneal the steel. Unfortunately, once the cage is welded into the body structure, annealing is very difficult if not impossible. This is why a DOM cage is practical and desirable in all but a small percentage of the applications.However, if a Chromoly cage must be built, it is imperative that all joints be gusseted to reduce the bending and other distortional stresses that act within the joints. "

 

http://www.honda-tech.com/zerothread?id=936638

[thehelix112]

We need a metallurgist. From what I can find, ``with more modern welding process like TIG and MIG, the cooling rates can be much faster and care must be taken to avoid forming high hardness and brittle Martensite on cooling transformation. On heavier sections preheat and post weld heat treatment should be used.'' (http://www.netwelding.com/Welding%204130.htm)

 

The key point here is heavier sections. The lincoln electric link you posted does mention the lack of heat treating and post heat treatment is only suitable for <3mm wall thicknesses.

 

As a side note, as that netwelding.com article says you should have convex welds to keep the weld area not in tension.

 

Dave

[Pop N Wood]

I don't have the links handy at this computer, but I have read many sources that says using a torch to weld cromoly helps with the annealing process. They claim it is actually prefferable to TIG.

 

I will have to find the links.

[johnc]

with more modern welding process like TIG and MIG

 

That's funny! Both processes have been around for at least 60 years. Modern? Try Laser, Friction Stir, and Explosive welding.

 

Also, throwing quotes out of various welding articles on the Internet just confuses the issue. We are talking about a motorsports application and NHRA has the most experience with 4130 chassis and cages. The Lincoln Electric article linked above is quoted almost verbatim in the NHRA approved TIG welding process for 4130. Some fabricators stress relive as an extra step but its not really necessary.

[thehelix112]

John,

 

I posted the quote and link because I learned a lot from the article. Sorry if you didn't do the same, didn't mean to waste anyone's time.

 

So the conclusion is that we can weld 4130 with 80S-D2 filler using as small as possible convex welds in a still warm room and everything will be fine?

 

Dave

[johnc]

Sorry if you didn't do the same, didn't mean to waste anyone's time.

 

It wasn't a waste of time. The article had some good information in it.

[JustinOlson]

Welding 4130 Steel for Race Cars

 

During the WW II era 4130 high strength steel was used for some aircraft components. At that time oxy-acetylene was the welding process of choice for many of these items. The preheat and slow cooling inherent with that process made welding the nominal 0.30 carbon steel relatively straight forward (assuming one could oxyacetylene weld!). However with more modern welding process like TIG and MIG, the cooling rates can be much faster and care must be taken to avoid forming high hardness and brittle Martensite on cooling transformation. On heavier sections preheat and post weld heat treatment should be used. With the proper post weld heat treatment strengths of 200,000 psi can be achieved with reasonable toughness by tempering the Martensite that forms in the heat treating process. However when welding race car tubing, preheat is not often used nor are the parts post weld heat treated.

Most of the tubing used for race car construction is referred to as normalized. This refers to the heat treatment and cooling rate the tubing was subjected to in manufacture. Most normalized tubing will range in tensile strength from 100,000 to 115,000 psi. This can be welded with the proper filler metals to achieve similar strengths. Although there are more weldable grades of steel (those with lower carbon content from 0.06 to 0.15) in the 100,000 to 115,000 psi tensile strength range readily available for plate and sheet, 4130 remains a commonly used grade for tubing. Just be sure to take the precautions noted when welding.

 

PROPER FILLER METAL CHOICE FOR WELDING 4130

 

In the mid 1970’s, while managing an R&D group for a leading welding filler metals manufacturer, a phone call was received from a dragster chassis builder. They wanted to weld 4130 tubing and needed a filler metal suggestion. Being a “car buff,†a number of alternatives were considered to provide the optimum solution. After careful review of their requirements and desired welding practices, the solution was defined. They were welding 4130 normalized tubing, it would not be heat treated after welding, preheat was not desirable and most of the weld joints were intersecting tubes that required fillet welds. The best filler material to use was a low carbon alloy now called ESAB Spoolarc 65 (meeting an American Welding Society (AWS) ER70S-2 specification). The main objective is to produce porosity and crack free weld deposits. This welding alloy has a very low carbon content, nominally 0.06, which can handle dilution into the relatively high (in terms of weld metal), 0.30 carbon in the 4130. The resulting diluted weld deposit has a tensile strength of approximately 590 to 620 MPa ( 85,000 to 90,000 psi.) The actual strength will depend on the amount of dilution with the 4130, weld bead size and material thickness. This is usually an under match for the 4130 tubing which could have a 760 to 800 MPa (100,000 to 115,000 psi) tensile strength depending on how the material was processed. [Added Note: some normalized 4130 tubing may be only have a 90,000 psi tensile strength, it depends on the manufacturer] However, if extra joint strength is required, a slightly larger fillet size or gussets can be employed. In addition, this welding wire contains small amounts of aluminum, titanium and zirconium. Although these elements were initially added to handle welding over mill scale, they also contribute to a less fluid weld puddle. The benefit to the welder is, it is easier to make out of position welds. Note, it is suggested all welding on 4130 be performed on ground surfaces free of oil or grease (to keep the hydrogen levels as low as possible).

Several years after making this suggestion, when looking at a catalog from the dragster chassis manufacturer, it was interesting to note they were advertising their use of the ER70S-2 filler metal for their 4130 welding. In fact, they were offering it for sale for those customers purchasing frame parts and doing their own welding!

The Internet was searched to see what current recommendations were being made for joining 4130 tubing. Several hundred sites were found that recommend the ER70S-2 welding rod/wire alloy. It was the predominant recommendation. Typical of the Internet however, there were many improper descriptions of why this alloy should be used and several incorrect recommendations.

Need a higher strength deposit? If a higher strength weld is required for perhaps a butt weld that cannot be reinforced, strengthened with a gusset, or put in a less critically stressed area, there are possible solutions. The use of Spoolarc 83, which contains 0.50 Moly, will provide a weld deposit with higher strength. When diluted into the 4130 base material a weld tensile level of 760 to 800 MPa (110,000 to 115,000 psi) can be achieved. If this higher strength welding wire is employed, a minimum preheat of 65 degrees C (150 degrees F) is suggested. Weld strength can increase to a level slightly higher than with Spoolarc 83 (AWS ER 80D-2). Do not use an austenitic stainless steel such as an ER308L, (which is recommended on some Internet sites). Diluting this or similar austenitic stainless alloys with 4130 can lead to cracks. Also, consider that providing a higher strength weld deposit cannot compensate for the reduction in strength that will occur in the parent metal immediately next to the weld deposit.

 

If the part will be heat-treated after welding to achieve very high strength, a matching chemistry filler metal to the 4130 should be employed. Because of the relatively high carbon, a minimum of 200 degrees C, (400 degrees F) preheat and very slow cooling after welding should be used to avoid cracking. After welding, the part can be heated to 870 degrees C (1600 degrees F), quenched in oil or water then tempered back to say 370 degrees C (700 degrees F). A complex cycle, but this will result in a tensile strength of approximately 1380 MPa (200,000 psi). Since the weld is the same chemistry as the base material, it and the heat-affected zone will have the similar properties as the base material when heat-treated. All critical welds of this type should be inspected for internal soundness to assure they are free from cracks.

End Of Abstracted Article

OTHER PROBLEMS ENCOUNTERED

In addition to the filler metal selection issues mentioned, some additional cautions should be followed. Many fabricators use TIG

welding and make very small, concave fillet welds. There seems to be a feeling that the smaller the better. This raises several concerns. First there is little filler metal used to make these very small welds. Therefore the weld consists mostly of the high (by welding standards) carbon from the 4130 base material. This can cause cracking since there is no preheat or postweld heat treatment being used. Also cooling rates for these small welds, especially when using TIG, can be quite high. Therefore one suggestion I had made in the article (removed from this abstract) was that some stainless steels filler materials could be used. This is also mentioned on a number of Internet sites. However with these small fillet welds there is only a small amount of stainless filler in the deposit and possibly a significant amount of the high carbon base material. This combination can lead to a crack sensitive deposit. It is suggested stainless filler metals not be used for welding 4130.

Making an analysis of the resulting weld chemistry for varying amounts of filler metal dilution creates a scary scenario at low amounts of any stainless filler alloy. When I discussed the use of stainless filler metal making these small fillets in 4130 tubing with a friend who is an acknowledged "worldwide stainless welding expert," he cringed! As he said, the suggestion that 312 stainless filler be used is based on at least 40 to 50% filler metal diluted in the high carbon material. If you make almost an autogenous TIG weld (no filler metal) and add just 20% of even 312 stainless you get a Martensitic deposit. You do not obtain the desired microstructure on which folks base their recommendation for a particular stainless alloy rod being acceptable. I have had race car fabricators say they like to use stainless filler because it makes the weld stand out and look good on unpainted frames they sell! Not a good reason since it could also contain cracks!

With only small additions of these filler alloys to the weld deposit there is a high percentage admixture of 4130. In these very small deposits this can create a crack sensitive metallurgical structure. In fact for these small welds the use of ER70S-2 becomes even more of a preferred suggestion. ER70S-2 with its low carbon and leaner Manganese and Silicon alloy than some other of the rods/wires often recommended as usable such as ER70S-6, creates less of a dilution problem. Small cracks and the presence of a brittle Martensitic structure in these welds can lead to failure or can cause a brittle fracture when subjected to a crash. See the welds in the photo of the dragster chassis. I don't know what filler wire was used to weld these joints, what little there was, but the fillets are very small. It does not appear very much if any bending took place in the structure before they failed!

Another problem created with small concave fillet welds is when they cool the surface is put in tension. This makes it susceptible to cracks especially near the toe of the weld where it is very thin.

Bottom line is use larger flat fillets to assure less dilution with the 4130 and a less crack sensitive shape.

CHECK WELD QUALITY

It is very important to check weld quality and understand the types of defects that could be encountered. Check your weld procedures and keep them consistent. You should make some sample welds and bend them to destruction to assure failure occurs only after considerable bending has taken place. Look for porosity or cracks that may have been present in the weld. It would be a wise investment to hire the services of an American Welding Society (AWS) Certified Welding Inspector (CWI). There are some 20,000 registered. In fact many of them are members of the 50,000 member AWS. They can advise on procedures and what to check for such as small undercuts at the weld toe of fillet welds that can lead to premature failure.

Consistently following the proper weld procedures and knowing how to check for possible weld problems is of major importance.

Closing Suggestion

When welding 4130 chrome moly in the normalized condition, AWS ER70S-2 filler metal, with its low carbon content is the proper choice. Make sufficiently large fillets and make them flat, not concave. If the part is to be heat-treated after welding, then a filler metal matching the 4130 chemistry should be employed. This requires preheat and special precautions to avoid cracking.

 

 

http://www.netwelding.com/welding%204130.htm

[Pop N Wood]

Sort of makes my little link seem insignificant

 

http://www.tinmantech.com/html/faq__tig_vs__gas.php

[JustinOlson]

After reading a bit more about the safety of mild steel relative to chromoly, I'm beginning to believe its not worth saving 25lbs to go to 4130 for the main cage. So at this point 1.625" X .125" 1020 DOM seems like the road to take. Easier to weld, cheaper, safer, more interior room. Good enough for me.

 

"Remember that cages mean to prevent intrusion. When your brain hits your skull in a 45g collision it won't matter that your cage was super rigid, you'll just be a well preserved corpse with jelly for internal organs. Energy absorbtion is the name of the game.While AISI 4130 steel typically has a higher tensile strength that is only a part of the picture. All steels have a very similar stiffness (modulus of elasticity) and Mild steel (AISI 1020 for this example) will deform 600% more before fracture than Chromoly.

(My reference is the ASM International Metals Handbook H.E. Boyer, T.L Gall)

What you should discuss is energy absorbtion. A stiff but brittle metal will not absorb as much energy before fracturing as a similarly stiff but more ductile one. The differences in specifications laid out by NHRA, SCCA, NASA, etc reflect the difference in materials properties in an attempt to place them on a relatively level playing field with respect to energy absorbtion. THAT is a fact which reflects a larger picture."

-Niles

[dr_hunt]

After reading a bit more about the safety of mild steel relative to chromoly' date=' I'm beginning to believe its not worth saving 25lbs to go to 4130 for the main cage. So at this point 1.625" X .125" 1020 DOM seems like the road to take. Easier to weld, cheaper, safer, more interior room. Good enough for me.

 

"Remember that cages mean to prevent intrusion. When your brain hits your skull in a 45g collision it won't matter that your cage was super rigid, you'll just be a well preserved corpse with jelly for internal organs. Energy absorbtion is the name of the game.While AISI 4130 steel typically has a higher tensile strength that is only a part of the picture. All steels have a very similar stiffness (modulus of elasticity) and Mild steel (AISI 1020 for this example) will deform 600% more before fracture than Chromoly.

(My reference is the ASM International Metals Handbook H.E. Boyer, T.L Gall)

What you should discuss is energy absorbtion. A stiff but brittle metal will not absorb as much energy before fracturing as a similarly stiff but more ductile one. The differences in specifications laid out by NHRA, SCCA, NASA, etc reflect the difference in materials properties in an attempt to place them on a relatively level playing field with respect to energy absorbtion. THAT is a fact which reflects a larger picture."

-Niles[/quote']

 

Actually, the quibbling over strength of this and that is really not material to most speeds we encounter in racing. A well designed cage that follows the guidelines of SCCA and NHRA are more than ample to save yourself in the case of an accident. Saving 20 or 30 pounds really isn't a big deal and won't make diddly squat difference in your times IMO. If your down to shaving pounds to get more speed then you should be done tweaking the motor for all you can get and there is no more to achieve from your powerplant. 99% of the cars on the track haven't reached that point yet!

 

I think you made a good choice and your point is valid!

[johnc]

Remember that cages mean to prevent intrusion. When your brain hits your skull in a 45g collision it won't matter that your cage was super rigid, you'll just be a well preserved corpse with jelly for internal organs.

 

A roll cage is designed to maintain a safety cage around the occupant to prevent crushing injuries. A roll cage IS NOT an energy absorbing structure and should not be designed as such. It is an energy transferring structure and should transfer impact energy throughout its structure to other parts of the vehicle. BTW... If your brain impacts your skull at 45g that means the energy absobing safety systems (helmet, seat, HANS device, harnesses, crush zones) in the vehicle failed.

 

All steels have a very similar stiffness (modulus of elasticity) and Mild steel (AISI 1020 for this example) will deform 600% more before fracture than Chromoly.

 

If your mild steel roll cage deforms 600% it was improperly deisgned and you are dead.

 

The differences in specifications laid out by NHRA, SCCA, NASA, etc reflect the difference in materials properties in an attempt to place them on a relatively level playing field with respect to energy absorbtion. THAT is a fact which reflects a larger picture.

 

BS! Read my lips - a roll cage IS NOT en energy absording structure, it is an energy transferring structure. The crush zones on a vehicle are designed to absorb energy until these zones are crushed to the safety cage. New vehicles fail the NHTSA and DOT crash tests if there is damage to the safety cage around the occupants. We as cage builders are creating the exact same thing.

[JustinOlson]

Here is the reply back from niles. I posted what you posted back to him:

 

"

A roll cage is designed to maintain a safety cage around the occupant to prevent crushing injuries.

cages mean to prevent intrusion.

So we agree on that one.

 

A roll cage IS NOT an energy absorbing structure and should not be designed as such. It is an energy transferring structure and should transfer impact energy throughout its structure to other parts of the vehicle. BTW... If your brain impacts your skull at 45g that means the energy absobing safety systems (helmet, seat, HANS device, harnesses, crush zones) in the vehicle failed.

We also agree here!

I didn't say the cage should be your primary energy absorbing structure, I hope nobody misinterpreted that from my post. I merely indicated that in a serious accident a cage will deform a little bit with the body. The specifications for tubing sizes laid out by the sanctioning bodies are to make sure that the structure is up to the task of protecting the occupants. The heat treatments that take chromoly's strength up so high make it brittle. Nobody wants a cage that breaks. If the ultimate cage was the one with the highest stiffness, we'd all have high carbon tool steel cages.

 

 

If your mild steel roll cage deforms 600% it was improperly deisgned and you are dead.

Percentages are relative terms. To say something deforms 600% is meaningless. The mild steel I used as an example will deform more before it breaks than the chromoly steel in the example. That doesn't mean you want either to deform. It means that when they develop the requirements for a cage to pass tech, they make sure that the thickness of the steel they specify is such that the cage can serve its purpose: protecting occupants and redirecting loads without breaking.

 

BS! Read my lips - a roll cage IS NOT en energy absording structure, it is an energy transferring structure. The crush zones on a vehicle are designed to absorb energy until these zones are crushed to the safety cage. New vehicles fail the NHTSA and DOT crash tests if there is damage to the safety cage around the occupants. We as cage builders are creating the exact same thing.

You seem upset. Can I reccomend a good beer?

 

Really the only purpose of my earlier post was to say that there's more to the picture than just "which has a higher tensile strength." The specs for the rules are determined so the cage is strong enough, regardless of what you build it from."

[thehelix112]

The heat treatments that take chromoly's strength up so high make it brittle.

 

I thought thats the point of yield and ultimate strength metrics? And who says chromo is brittle? If you are talking about the joins after welding, maybe you should re-read some of the links a few of us posted earlier.

 

Aside from thats, I interpretted your original post as johnc did.

 

Dave

[johnc]

You seem upset. Can I reccomend a good beer?

 

Maybe I got a little excited.

 

I finished the cage in the Corolla yesterday and I set my own personal record by building a cage in 18 hours of work over two days. FYI... The cage I built is out of DOM tubing.

 

And, come to find out, this car is a car magazine entry in the "24 Hours of LeMons" event I posted about in the Vendor's forum. I can't mention the car magazine because the entry is unofficial and there may never be a sticker on the car or an article written about the event.

 

Back to the topic at hand:

 

Given the material limits specified by the sanctioning bodies, material choice is the least of a cage builder's concerns when it comes to building a safety cage. Safety cage failure is much, much more often a function of design and manufacture then of material selection.

 

CroMo tubing ultimately has less ductility then DOM but the material itself is not brittle. Ductility of 4130 is only 15% less then 1020 tubing (typical DOM) as bare material yet you are getting a 75% increase in yield strenght. Where problems crop up is when 4130 tubing is improperly welded.

 

One anecdote:

 

When I was at the Lincoln Electric Motorsports welding school they showed us a Joe Gibbs Racing Winston Cup car where a door bar failed in a side impact and a tube was sheared off, bent in and pointed at the driver's chest. Luckily there wasn't a second impact. Lincoln determiend that the door bar failed because of insufficient penetration on the weld. Gibbs racing inspected their other 45 cars and replaced all the door bars because the same problem was found in every car. Every person who did any welding at all at Joe Gibbs racing, was sent through two weeks of motorsports welding training at Lincoln.