Heat treated chassis? No thanks...
Discussion
Funny how when John Force has a crash like this everyone makes a big deal about it, yet when lesser known drivers such as Hilary Will had a similar crash during testing early 2007 no one mentions about the chassis construction and pre-treatment.
It's a well known fact that chassis' break, it's as simple as that. Schumacher had a similar crash a few years back, Sheavill's crash was exactly the same. You can't expect anything to last forever which go under that much loading and stress.
It's a well known fact that chassis' break, it's as simple as that. Schumacher had a similar crash a few years back, Sheavill's crash was exactly the same. You can't expect anything to last forever which go under that much loading and stress.
A very interesting article Jim.
From my limited understanding of metallurgy I have often wondered why SFI chassis are required to use chromoly tubing when the only benefit it has over CDS is that you can use thinner wall tubing to obtain the same strength. CDS is actually less prone to cracking so will withstand repeated load cycles better.
The use of heat treated tube appears to be a further continuation of the above, finding ways to make a structure of the same nominal strength but lighter. The article suggests that the full implications of this have not been examined and that heat treated is dangerous. I hope that a full study is undertaken by the NHRA/SFI to determine properly just how a chassis should be built for strength and structural integrity regardless of weight.
From my limited understanding of metallurgy I have often wondered why SFI chassis are required to use chromoly tubing when the only benefit it has over CDS is that you can use thinner wall tubing to obtain the same strength. CDS is actually less prone to cracking so will withstand repeated load cycles better.
The use of heat treated tube appears to be a further continuation of the above, finding ways to make a structure of the same nominal strength but lighter. The article suggests that the full implications of this have not been examined and that heat treated is dangerous. I hope that a full study is undertaken by the NHRA/SFI to determine properly just how a chassis should be built for strength and structural integrity regardless of weight.
I'm posting freestyle here and don't have the right source books in front of me, so I can't spiel out the numbers, but this is all down to appropriate material selection and correct weld process.
4130 Chrome moly, whether in normalised or heat treated form is stronger than CDS when direct compared by like diameter and wall thickness.
Proving there is no such thing as a free lunch, 4130 Chrome moly achieves this greater strength by being harder than CDS. Note that Normalised 4130 is "softer" than heat treated 4130.
With either of the 4130 chrome molys this greater hardness means it is more brittle than CDS material which has far better ductility than 4130 chrome moly. This means that it will take more cyclical loadings for CDS to fail.
Taking the above into account we then make it far worse by joining 4130 in a manner that was not envisaged when it was originally specified.
Bear with me here; the reason that you hear people talking about Mil-Spec or MS 4130 Chrome Moly tubing is because the specification of the material was originally laid down by the US Military and they envisioned using the material in aircraft frames and that the joining method would be gas welding that both simultaneously formed the the weld pool to homogenously join the material and heat the surrounding area up to give a post weld heat treatment to anneal the heat affected zone around the weld and lessen the chance of welding stresses leading to the early failure of the joint.
Checking the rule book for drag racing we mandate TIG welding process, which can easily lead to much higher welding temperatures and uncontrolled welding stresses with no provision for post weld heat treat !
Ask any coded welder about his work and he'll tell you that he works to a WPS, or Welding Procedure that advises best practice for every different weld he performs. It includes weld preparation, electrode selection, fuiller rod selection, interpass temperature, size and type of weld bead to be laid and any applicable post weld heat treatment.
None of the above is in the rule book. I guess NHRA just assumes that anyone who picks up a TIG welding torch will get it right first time....
4130 Chrome moly, whether in normalised or heat treated form is stronger than CDS when direct compared by like diameter and wall thickness.
Proving there is no such thing as a free lunch, 4130 Chrome moly achieves this greater strength by being harder than CDS. Note that Normalised 4130 is "softer" than heat treated 4130.
With either of the 4130 chrome molys this greater hardness means it is more brittle than CDS material which has far better ductility than 4130 chrome moly. This means that it will take more cyclical loadings for CDS to fail.
Taking the above into account we then make it far worse by joining 4130 in a manner that was not envisaged when it was originally specified.
Bear with me here; the reason that you hear people talking about Mil-Spec or MS 4130 Chrome Moly tubing is because the specification of the material was originally laid down by the US Military and they envisioned using the material in aircraft frames and that the joining method would be gas welding that both simultaneously formed the the weld pool to homogenously join the material and heat the surrounding area up to give a post weld heat treatment to anneal the heat affected zone around the weld and lessen the chance of welding stresses leading to the early failure of the joint.
Checking the rule book for drag racing we mandate TIG welding process, which can easily lead to much higher welding temperatures and uncontrolled welding stresses with no provision for post weld heat treat !
Ask any coded welder about his work and he'll tell you that he works to a WPS, or Welding Procedure that advises best practice for every different weld he performs. It includes weld preparation, electrode selection, fuiller rod selection, interpass temperature, size and type of weld bead to be laid and any applicable post weld heat treatment.
None of the above is in the rule book. I guess NHRA just assumes that anyone who picks up a TIG welding torch will get it right first time....
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