M.E. ON HEAT TREATING AND TUBING
My name is Dr. Rory R. Davis, P.E.. I am a licensed professional mechanical engineer in the states of California and Nevada. I have already written a technical summary memo, dated August 28, 2007, that clearly stated the case for returning to Cond N tubing, and the reader is assumed to have read that memo. To repeat from that memo,
“The logical choice for future car frame building is obviously to return to Cond N tubing, eliminating failure risk to the maximum extent possible at minimum cost. Hardened frame tubes provide little benefit for too much risk, and greater quality control cost.”
The case was clearly made that hardened tube introduces risk that elongation is too low, and to avoid such a situation, much more involved quality control testing is required to qualify tube before it can be used in car building. It is known that back half tubing changes have insignificant effect on Top Fuel car flexibility, and the potential weight savings of using thinner wall hardened tubes instead of Cond N tubes is insignificant. This is because, once buckling is precluded by frame design with small support members, tube strength in weld heat affected zones governs, which does not allow significant thickness decrease of tubes to be used, no matter whether they are heat treated or not. THE BOTTOM LINE IS, THE DANGER OF USING HARDENED TUBING IS NOT WORTH THE INSIGNIFICANT REWARD. This same logic holds for both Top Fuel and Funny Cars, obviously.
A further reason to prohibit hardened tubing from use is also now clear. Despite all the discussion of the need for tight quality control to ensure good material elongation for tubing used in cars, it appears that 2007 cars were built by McKinney using induction hardened tubing that showed in testing to have unacceptable elongation. Sample data was apparently ignored or at least misunderstood, despite the fact that the data was available, and other data was available at the same time for both Cond N and other hardened materials that DID show acceptable elongation requirements. Clearly, the people involved in this industry do not currently have the knowledge and skills necessary to use hardened tube responsibly. The use of hardened tube should be prohibited based on this fact alone. As recently discussed with Dan Olson, hardened tubing can only be used with extensive quality control testing to PROVE good minimum elongation of approximately 10% or more. Everyone should recognize and never forget or ignore this fact.
There continue to be car chassis failures, some involving loss of life and serious injury, in which poor elongation tubing is considered to be a major contributing factor, if hardened tube was present. The following immediate actions must be taken (partly explained to Dan Olson recently by phone):
1) A “patch” of all NHRA Funny Car frames that contain hardened tubing should be made immediately to mitigate the potential failure of poor elongation hardened tubing that may be in the cars. McKinney frames are particularly important to patch (due to known poor elongation of the induction hardened tubing used in their fabrication), but others using any hardened tubing whatsoever should also be patched the same way, because no other sources of hardened tube have been fully qualified to date. The suitability of the patch should be verified by Ford Motor Co. Finite Element Analysis (FEA), which is currently the best available for Funny Cars.
2) All NHRA Top Fuel Car frames should be reviewed for adequate small lateral support tubes in the lower back half, to preclude a buckling critical frame. Any frames without adequate support tubes should be immediately patched. With this patch, the risk of poor elongation tubing in Top Fuel is greatly mitigated, because the critical failure mode becomes strength of weld heat affected zones.
3) Top Fuel chassis specification should be updated before 2008 cars are fabricated. It is strongly recommended to return to Cond N tubing, and prohibit hardened tubing. THERE IS NO GOOD REASON TO GO THROUGH THE EXTENSIVE QUALITY CONTROL REQUIRED FOR THE USE OF HARDENED TUBING. Most importantly, the use of small support tubes to preclude a buckling critical design must be codified in the spec. All the data necessary to make this spec update is available now, and the update could be quickly implemented.
4) Funny car chassis specifications as written should be enforced, including prohibition of hardened tube in 2008 cars. If not perfectly clear in the writing of the spec, that prohibition should be made clear now for 2008.
5) Understanding of the detailed characteristics of Funny Cars chassis must be brought to the same level as for Top Fuel cars, via testing and analysis, with possible design and spec changes for the 2009 season, or 2008 season if humanly possible.