Legal Cage question?

[2ndgenracer]

Curious what everyone is running for cages. I was going to run 1-5/8 .083 wall Cmoly for NHRA SFI spec, but I'm not sure any of these other road race events or sanctioning bodies will pass this diameter/thickness. For open road race or autocross what do they require? I've read somewhere that 1-3/4 .120 moly is recommended. The tubing costs more, plus the die cost, but this would pass NHRA with no problem.

It seems easy enough to just put in the 1-3/4 but I'm a tight ass and can save about $500 if I can just run the 1-5/8. Also if I do put in the 1-3/4 .120 wall I want to make sure that's the correct thickness. Please let me know what you guys have run into.

Thanks for your help,
Brett

[chicane]

SCTA-BNI 3.B.1 states:
The minimum requirements are 1-3/4" OD with a minimum 0.120" wall thickness for CRS... for roll bars.

For roll cages... the minimum requirements are 1-5/8" OD with a minimum of 0.120" wall thickness for CRS OR a minimum of 0.095" for E4130 chromoly... and depending on class... it may be legal to use 1-1/2" OD x 0.095" nominal wall thickness CRS.

But... any class that is capable of 175mph or higher must use the larger tube minimum requirements.

They too recommend the use of low carbon (mild) steel tubing. The pads should be of the same material, 1/4" thick top and bottom plate. For chassis weights under 2500lbs, the must have a parimeter of at least 18" inches (i.e., 4x5") and chassis over 2500lbs shall have at least 22" perimeter (i.e., 5x6").

You'll need someone with an updated SCCA rule book to look up Appendix "C" for the updated rules for the minimum requirements. I do know that ERW is no longer allowed in SCCA for "new" builds... and E4130 is now the main stay.

BTW... $500.00 is cheap. What is your life worth ?? Tight ass or not... saftey and its associated costs are a bargan to the alternative.

I am doing 1-3/4" x 0.120" DOM mains with 1-5/8" and 1-1/2" x 0.095" DOM supports... if the rules dont change. They wont... but I need to spec this to cover multiple sanctioning bodies.

[chicane]

On another note... this is just one of the reasons I went DOM, vice E4130 or ERW.

One misconception that a lot of people have is that different alloys of steel have different stiffness. This is not true. All steels have almost the same stiffness (modulus of elasticity) within 5%, (stainless is about 10% less). If you want a different modulus of elasticity then you need to use a different material ie: aluminum, titanium ect. The modulus of elasticity determines how far the material will flex under a given load (ie: rubber low modulus, glass high modulus). The modulus for steel is 30,000,000 PSI

Different alloys of steel in different conditions of heat tread and work hardening do however have different strengths. There are to values for strength that we need to look at, Ultimate strength and yield strength. The yield strength tells how much stress the material can handle before it permanently deforms. This is the spring back or memory that the material has. Ultimate strength is the stress level that the material can take before it finally fails, at this point however the part has already bent and is ruined. The other material property that we should look at is Elongation, it is described as a percent and is the amount of permanent deformation that the material will have before failure (ie: silly putty very high elongation, Glass almost zero elongation). More elongation will allow the part to get a pretzeled up and not fracture. On our tubing bender we have had some problems in the past with braking the martial during the bending process, the solution was finding a metal distributor that had the same alloy of material but with a higher elongation value.

The strength of steels is largely dependent on the heat treat. The difference between the alloys is how heat treatable they are. Lower alloys can’t attain as high a level of heat treat.

As far as cromo vs. mild steel here are some specs to look at.

1020 DOM Yield 70 Ksi Ultimate 80 Ksi Elongation 15%
4130 NORM Yield 70 Ksi Ultimate 90 Ksi Elongation 20%
4130 HT@800F Yield 173 Ksi Ultimate 186 Ksi Elongation 13%
ERW tube Yield 25 Ksi Ultimate 45 Ksi Elongation 35%

As you can see 4130 is only slightly better than DOM, also note that it has a higher elongation value. This means that everyone’s conception that 4130 will splinter on impact is false.

As far a weld zone effects there are a lot of conflicting theories regarding this, filler rod type, normalizing stress reliving, ect. One welding practice that is recommended for 4130 and rarely done is preheating. 1020 will be more forgiving to non professional or professionally funded welds.

I would put more energy in to the design rather than the material, avoid single sheer mounting and structures in bending. Don’t use ERW tube, use 4130 for bragging rights or smaller part that you can send to heat treat, other wise use 1020 DOM.

[deuce_454]

YES! finally, stick this post in the knowledge base! ... and listen to the man btw

[James OLC]

NASA CCR states 1.75 x 0.120 CM or DOM (no ERW) for 3001 - 4000 pounds

SSCC states 1.75 x 0.090 DOM or 0.120 ERW, CREW for over 2700 pounds
(roll bar required over 130 mph avg., roll cage required over 160 mph avg.)

SCCA GCR states 1.75 x 0.120 DOM or CM (no ERW) for 3001 - 4000 pounds unless grandfathered (pre 1994).

All of these rule books are available online with more detailed specifications.

We are starting the cage in the OLC Camaro in a couple of weeks, it will be 1.75 x 0.120 DOM throughout.

[2ndgenracer]

Done......1-3/4 .120 wall it is. Thank you for your response.

Brett