The Chassis Design is Good
On Sept. 27 DRO received and printed a story from Joe LaCour with some well researched facts and opinions regarding the NHRA’s decision based upon, evidently, the sole recommendations of chassis builder Murf McKinney and his staff of engineers. Reportedly at least two chassis builders and members of the SFI’s chassis committee including Brad Hadman disagree with Mr. McKinney and his engineers regarding the heat treating of 4130 (n) tubing.
We’ve recently received from a very reliable source, who asked not to be identified, that the Plymouth company that manufactures and sells chassis tubing to McKinney for chassis fabrication, agreed with the McKinney engineers’ decisions and manufactured a heat-treated version of the 4130(n) tubing be used as a fix for the chassis problems. McKinney Corp and the best minds that Murf McKinney could assemble came up with a material and design fix they believed would resolve the problem. No one is more upset than Murf McKinney about the course of events currently going on in drag racing because his company built the Force chassis. With that in mind I asked chassis builder Keith Burgan to submit this report to bring more facts about the McKinney Corp side of this critical issue. – Jeff Burk
Following the posting of Joe LaCour’s piece on the properties of 4130 tubing and its application to race cars, DRO editor Jeff Burk invited comments. Mr. LaCour’s treatise was penned prior to last weekend’s incident at the Texas Motorplex but is still relevant in that it addresses the issue of heat treated tubing as used in drag race vehicles.
Before we begin any discussion regarding the modification of the properties of the tubing I would like to address what I consider to be a major flaw in one of his arguments. After providing quite an amount of information about the properties of 4130N and other varieties of steel and the welding thereof Mr. LaCour sums up his conclusions and includes the following statement: “While on the subject, we should briefly touch on design. Many have commented on the past and recent catastrophic failures of Top Fuel Dragsters as if the ‘tremendous loads’ were to blame. Also mentioned was that the chassis is ‘designed to break apart in a crash’. This of course is all nonsense. The loads on a Top Fuel Dragster chassis, while higher than an average race car, are by no means tremendous. And in a crash, containment is always preferred to non-containment. The reasons for many of the failures are due to poor, or lack of, proper structural design.”
Although he has had considerable interaction with quite a number of well-known drag racers and crew chiefs, Mr. LaCour seems to think that it is a good idea for the car to stay totally intact in an accident situation. This belief does not take into account the energy dissipation involved in the at least partially controlled destruction of the extremities of the frame and the fact that the cockpit, while being extremely stiff, is also very light once the rest of the car has separated. This reduces the inertia of the cockpit and that means that any subsequent impacts with solid objects such as the retaining walls are not as violent. It also means that the cockpit will generally travel less distance before coming to a halt.
Last weekend’s incident perfectly illustrates this point. The catastrophic failure of the chassis in John Force’s car was not part of the chassis design as it exposed Force’s lower limbs, but note how quickly the rear portion of his car slowed down even before the parachutes snagged the wall. Both his and Kenny Bernstein’s chutes deployed at about the same time but Bernstein’s car, intact but for the body, rolled almost to the last turnout. Once again, Funny Cars are not supposed to break where Force’s did, while dragsters, by their design usually end up in three pieces with the driver in a relatively light, strong structure, away from the fuel tank and the bulk and mass of the motor.
The major point of Mr. LaCour’s dissertation, however, is that using “heat treated” 4130 is not good. A lot of the Internet “experts” that have been flooding the boards with comments, deductions and theories regarding the failure of the chassis on Force’s car seem to think that the tubing has been hardened to an excessive level. This is not the case. Yes, the tensile numbers are elevated, but not to an abnormal degree. The original reason for the post-manufacture heat treating was that the tensile numbers varied from batch to batch and that was making identically constructed cars react differently. In conjunction with the desire to narrow down the variations in the tubing, and at the recommendation of the tubing manufacturer the decision was made to modify the characteristics of the tubing to better suit the application. The manufacturer was willing to make tubing to the new specification but the minimum run was 5,000 feet of each size, so the decision was made to outsource the process. The tubing is not so hard that it is prone to failure at the welds or adjacent to them, it is simply a modified version that is held to a tighter tolerance.
Force’s car broke where it did due to major vibrations and oscillations induced by a chunking left rear tire. With the forward portion of the cockpit area held rigid by the motor plate and the fact that the rear of the frame is also very stiff due to the extreme triangulation provided by the rear end mounts and roll cage structure, the frame broke adjacent to those points, just ahead of the forward hoop and the seat uprights.
I am not a qualified engineer and I have not been privy to the FEA analysis that was performed during the frame’s design and subsequent to the failure, but I would assume that the bends in both the top and bottom rails just behind the motor plate allowed a modicum of flex at that point while the twisting of the frame due to the oscillations overstressed the tubing at the point of fracture.
Another point that has been raised in some circles relates to the fact that Robert Hight’s chassis had to be front halved after a bout of vicious tire shake at Maple Grove. In this case the tires did not chunk or delaminate but (crew chief) John Medlen told me that the shake was so violent that the sensor that Robert was wearing in his ear saw peak numbers in the range of 200 g’s. Once again this was way beyond the normal range and would, for me at least, explain the chassis failure in that instance.
As I noted earlier, I don’t profess to be an engineer but I do know that McKinney Corp has three qualified engineers on staff as well as having advanced design and analysis software to validate their theories. These guys only deal with race cars and the like; they don’t design bridges or buildings, and their boss, Murf, has some 30 years of hands-on race car experience as well. They often consult with outside sources to make sure that they build the best possible product and I would trust them with my life without hesitation.
To read the original article this story is in response to, click here.