This article was published in the October edition of our monthly newsletter. The full newsletter is available at this link
A link to this article, and an invitation to join this discussion, have been sent to the following Mustang II IFS suppliers:
Fat Man Fabrications
Heidt's Hot Rod Shop
Jim Meyer Racing Products
Macgyvers Street Rod Products
Performance Suspension Technology
Rod Factory, The
Street Rod Engineering
Total Cost Involved Engineering
Are Mustang II Crossmembers Safe?
Multiple catastrophic failures of aftermarket crossmember kits have motivated a recent evaluation of the safety of these units.
Additionally, general concerns about the viability of Mustang II suspensions in home-built hot rods have come to the forefront.
Some rod builders are insisting on only purchasing certain types of units, from certain manufacturers, with specific modifications. Others are opting for installing a stock Mustang II crossmember for safety purposes, choosing to rely on Detroit engineering over aftermarket manufacturers.
Here are some of the questions that have been raised on the Hotrodders Bulletin Board in regards to Mustang II crossmembers:
--Are some types of Mustang II crossmember units safer than others? If so, why?
--What modifications can be made to aftermarket units to improve their safety?
--Are stock Mustang II units inherently safer than the aftermarket versions?
--Are crossmember manufacturers concealing crucial safety information from hotrodders?
Here are some comments from engineers, machinists, and professional hot rod builders on the Hotrodders Bulletin Board:
***On the general safety of installing crossmembers:
"ALL suspension changes can only turn out as good as the skill of the installer, regardless of the suspension used."
"I wouldn't consider aftermarket x-members over a stock unit for a daily driver for safety reasons."
"It doesn't matter who builds your chassis, a pro-builder or if you do it at home, when components are "adapted" it can put a different type of stress on parts that may have not been tested by the factory for that particular type of stress. Street rod chassis manufacturers and parts manufacturers don't have the facilities or money to test their units for structural integrity the way Detroit does so regular inspections are a must."
"You have to pay attention to detail when buying one of them, and even more when installing one."
***On specific cases of catastrophic failures of aftermarket crossmembers:
"Another example is the broken Heidt's x-members that have been publicized here recently. That design is flawed since it introduces a stress riser at a highly stressed point in the x-member then eliminates the strut rods which were intended to lessen the stress in that location. Give that design to any graduate mechanical engineer and he will confirm that fatigue cracks are guaranteed."
"In the cases of the very small minority who have had failures, these failures were most often the result of poor installation and/or using strut rod eliminator kits without proper strengthening of the crossmember."
"Being a mechanical engineer I have often wondered when something like this would happen. As many of you have observed there may be a problem with the fabricated front crossmember...not just this particular manufacturer but all in general, since they are all similar."
"I hate to say it but [Heidt's] design seems flawed to me, have a look under your factory vehicle and notice the design of the crossmember, I see none of those design features incorporated in this example."
(4 Jaw Chuck)
"As far as testing our products, we introduced our Superide Independent Rear Suspension kits recently. I installed the first one in my '57 Chevy pick-up. It was a great opportunity to test a new product, as we left the box off the truck for two weeks so we could do torque and force measurements easily. I pounded on it as much as I could for those two weeks, literally driving through every pot hole and railroad track I could find. This kind of testing was great, as it showed that the design was sound."
***On the statistical significance of crossmember failures:
"Five complete structural failures in 40,000 for a critical suspension component would mean a massive recall to a manufacturer like one of the big three. I think his product was never tested to failure if you ask me and he is terrified of that coming out in a court room."
(4 Jaw Chuck)
"An estimate of all the manufacturers in entire street rod industry would show that there were probably a total of 30,000 sets of control arms and 40,000 crossmembers sold in the last 10 years combined. Of all these products out there from all these companies, don't you think that if there was a genuine problem with this type of design that there would be hundreds and hundreds of failures, not just a hand full?"
"It sounds like Fatman has one heck of a sales crew and know just what to say, but 5 failures in 40,000 units is a VERY high number, that means that you have a 1:8,000 chance of a catastrophic suspension failure (if that was the standard new cars were made to 20,000 new cars would have a front suspension failure every year in the United Sates alone!!)"
To view the entire discussions from which these quotes were taken, visit the links below:
(includes a letter from Gary Heidt defending his products after a catastrophic failure of his crossmember)
(details a catastrophic failure of a Fatman Fabrications crossmember)
(discusses the general safety and viability of Mustang II suspensions for hot rods)
(discusses the failure of specific Mustang II units)
(details modifications to improve the safety of Mustang II units)