Roll Cage 4130 Chrome Moly is it legal for LSR vehicle

[Peter Jack]

The rules require a heavier wall thickness for mild steel tubing which will likely more than make up for the difference in material strength. DOM tubing is very high quality tubing, much tougher to bend than the equivalent ERW. It is probably much less likely to fracture beside the weld than the 4130N given the difference in wall thicknesses and the effect of the heat affected zone in 4130. The N is very important if you are going to use chromemoly tubing.

Pete

[panic]

I would want to be in a car that was built WAY over
enginered to be as safe as possible.

Easiest, cheapest, most foolproof method of making the cage stronger:
add 1/2" to the OD.

Comparing moly to mild, perhaps a 30% advantage in strength (viz., before separation) with the same wall thickness. If wall is thinner, strength is a match, but small disadvantage in stiffness.

Comparing minimum OD to +1/2" OD, using 1-5/8" as a base, same wall thickness to remove a variable:
1-5/8" at .083" = .360 "stiffness units" (dimensionless)
2-1/8" at .083" = .834 "stiffness units"

The 2-1/8" is about 32% stronger, and more than twice as stiff.
 

[ronnieroadster]

Yes Tman this is Ron San Giovanni.
 Peter Jack I agree when running on the salt weight is your Friend we learned that when helping the team get the D/BSTR record a few years back I was the one who made the final call to add weight depending on the salts condition prior to each run so this area of LSR racing is not new to me.
   But excess weight can also be your enemy during an accident there is always going to be a fine line about such a decision. My interest in weight reduction if I decide to use the chrome moly gives me the opportunity to place the weight where I feel its needed.
 Ronnie Roadster

[Tman]

Yes Tman this is Ron San Giovanni.
 Peter Jack I agree when running on the salt weight is your Friend we learned that when helping the team get the D/BSTR record a few years back I was the one who made the final call to add weight depending on the salts condition prior to each run so this area of LSR racing is not new to me.
   But excess weight can also be your enemy during an accident there is always going to be a fine line about such a decision. My interest in weight reduction if I decide to use the chrome moly gives me the opportunity to place the weight where I feel its needed.
 Ronnie Roadster

Thought it might be you from the HAMB, have admired your cars for years.

[jacksoni]

Last I spoke with Steve Davies (chief inspector) they strongly prefer mild steel to moly though latter still legal. Regardless of diameter and "stiffness" as Panic mentions however, minimum wall thickness with mild steel is nominal .125 (.120). Though have seen no suggestion, I would not be surprised to sometime see a switch to outlaw moly. As others have said, too much may be just enough cage strength wise ( or not enough depending on point of view  ). And I agree about the weight comments, I just don't like it in the back of the car.

[bvillercr]

Last I spoke with Steve Davies (chief inspector) they strongly prefer mild steel to moly though latter still legal. Regardless of diameter and "stiffness" as Panic mentions however, minimum wall thickness with mild steel is nominal .125 (.120). Though have seen no suggestion, I would not be surprised to sometime see a switch to outlaw moly. As others have said, too much may be just enough cage strength wise ( or not enough depending on point of view  ). And I agree about the weight comments, I just don't like it in the back of the car.

I surely hope they don't outlaw it, just because an inspector prefers one over another.   

[wobblywalrus]

My experience is with chrome-moly motorcycle frames.  They are not uncommon and they work satisfactorily, for the most part.  They are lighter.  Occasionally they would crack and I would weld them up.  My welds looked good and they worked OK about half of the time.  The welds that went bad did not break.  The metal near the weld cracked.  The cracks did not happen just after the welding.  They occurred later when the frame was stressed.  This is a big problem.  A person does not know they have a bad joint until it is stressed and it cracks.

Ask your welder if he knows how his work has held up during hard use after he welded it.  This is the critical thing.  

[Interested Observer]

Some comments on the above discussion:

“Certified weldor” --  Certified by whom, to do what, with what materials, with what technique, under what conditions?  Unless the answers to these questions fit your application, you’re whistling in the wind.

4130 --  As has been stated, 4130 offers some, but marginal advantage in strength over mild steel unless it has been properly frabricated and heat treated.  It also offers a proclivity for brittle fracture.  It is most useful for piece parts that can be machined from heat treated material, or roughed out, heat treated, and then finish machined.  For welding, what qualified weld procedure would you use?  What pre-heat? What qualified filler metal?  What welding technique and joint design?  What post-weld stress-relief for all the welds?  For space frame construction, do you know somebody with a pretty large heat-treat facilities?  Can you live with the resulting deformation?  Not to mention non-destructive testing of the joints for cracks, even assuming all the previous steps were properly taken.  Then, how do you repair the cracks without invoking the whole process again?  Welding alloy steels is not a trivial undertaking.

Carroll Smith is correct and some of the headaches outlined above are why the conventional wisdom, also stated in previous posts, is that mild steel is hard to beat in this application.  Using 4130 borders on foolishness.

Peter -- Mild DOM tubing has some strength advantage due to the cold-work involved during its forming, and can have a nice finish for the same reason, but can lose that effect in the heat affected zone of a weld.  It can also have more wall thickness variation than ERW tubing.

Panic’s “stiffness units”  --  What Panic is referring to is the bending moment of inertia of the tube cross-section.  (Would be nice to use more descriptive, or even correct, terminology).  This doesn’t apply to axial stiffness, and may or may not be an indication of the actual bending capacity of the as-fabricated tubing frame elements.  The joint design would have to be adequate to support the capacity of the tubing, and there is a real possibility of local bucking of the tube wall before the cross-sectional capacity is reached.

Wobbly  --  The cracks were probably there, it just took a while to become easily noticeable.  That’s why alloy welds need to be stress relieved and NDE’d.

[dw230]

"I would not be surprised to sometime see a switch to outlaw moly."

I don't foresee that happening in the near future -certainty not in 2012.

DW

[johnneilson]

"I would not be surprised to sometime see a switch to outlaw moly."

I don't foresee that happening in the near future -certainty not in 2012.

DW

I realise that this is the information age, but what the hell is "outlaw moly"??

I do know that there are more/divesified alloys available in yuur-up. I dealt with some in karting years ago, all seemed to respond well to being treated like 4130 (post annealing and pre heating).

John

[jdincau]

I realise that this is the information age, but what the hell is "outlaw moly"??

outlaw as prohibit

[Elmo Rodge]

I realise that this is the information age, but what the hell is "outlaw moly"??

outlaw as prohibit

Yeah. He's using it as a verb, not a noun.    Wayno

[johnneilson]

Duh..........

here in the state of Confusion (California), English is a second language.

The Brits have been using a form of alloy that is not 4130, supposedly made for race car chassis (easy to weld/repair etc.)

J

[jacksoni]

"I would not be surprised to sometime see a switch to outlaw moly."

I don't foresee that happening in the near future -certainty not in 2012.

DW

May have stirred up something here  . I would emphasize my use of "sometime".  In 2010 with a new car we were really given a hard time in inspection about the moly cage though were eventually passed on it. It did meet rulebook specs and then some. Anyway, I was just speculating here based on these discussions and not really meaning to be predictive. Certainly if such a change were made I would expect a lot of grandfathering and warning about it.

[Glen]

The top fuel and funny cars are Chrome moly and they front and back half them 2 or 3 times a year due to fatigue and bending.