(originally posted 1/19/2012) GM’s X-Frame, upon which millions of GM full-size cars sat from the years 1957 through 1964 (Buick Riviera through 1970), has generated plenty of controversy, speculation, and accusations. Since there seems to be no complete survey of the X-Frame – at least any available on the web – let’s lift off all those handsome and finned GM bodies by Fisher, and take a closer look at what’s really under there.
Starting with a real stunner: The X-Frame first appeared in 1957, underpinning the new C-Body Cadillacs and Eldorado Brougham (pictured). It was obviously conceived as a way to facilitate lowering the total vehicle height, through deep floor wells for the passengers’ feet to drop essentially to the bottom of the car, unimpeded by frame rails.
I’m not a frame expert/historian, but Cadillac’s approach in developing the X-Frame seems to be rather unique, although it combines aspects of two very distinctive frame designs: the backbone frame and the X-braced ladder frame.
The backbone frame originates with Hans Ledwinka’s revolutionary Tatra T-11 of 1921 (full story here). A strong solid steel tube was the carrying member for the whole car and its (lightweight) body.
The Tatra tube frame evolved into a combination central backbone-platform frame, as seen here in the mid-thirties Tatra 97. Needless to say, a very similar route was also taken by others, including Porsche for the now very-familiar (and similar) VW platform frame. In these, the body was rigidly bolted to the platform, to create essentially a unitized structure from the two halves.
The pure backbone chassis was taken up by others, none more famously so than by Colin Chapman, with his brilliant Lotus Elan. With a very deep central section, which worked well in a sports car, the Elan had unparalleled rigidity, the ultimate goal of any frame/body structure. Rigidity is the only way that a suspension system can be designed to optimize its function.
On the other end of the spectrum, sits the ladder frame, here immortalized in the frame rails from a Ford Model T. A certain amount of flex was an intrinsic part of the equation. Its origins are obviously in the heavy timber frames underpinning wagons, but the seminal 1901 Mercedes was perhaps the first to sport something akin to what became this timeless approach to automotive frame building
Jumping ahead about a century, here is a modern ladder frame, as now used in pickups and BOF SUVs.
The origins of using an X-member to reinforce a ladder-type frame has been credited to the fwd Cord L-29, and this excerpt from its brochure substantiates that claim.
But this 1939 Buick frame shows that its adoption had expanded by then, and for obvious reasons. It undoubtedly increased rigidity, at least in certain planes.
X frame centers were also used to stiffen ladder frames specifically for convertible use. This is a sedan frame for a ’57 Chevy.
And here is the center section for the convertible version. It’s important to note that the bodies of BOF (body on frame) cars contribute substantially to the overall vehicle rigidity, which is why convertibles require additional reinforcement to their frames.
There’s no readily available background on the engineering decisions that led to the first GM X-Frame, on the ’57 Cadillacs (’61 Chevy frame shown here). GM claimed improvements in rigidity, as well as the lower floor. It did result in a very large central tunnel, a distinctive feature of all X frame cars.
In order to make the X-Frame work, Fisher Body increased the strength of the rocker sills of the bodies, as well as side-to-side stiffeners in the floor. These can be seen fairly well in this shot of a ’58 Chevy. In essence, GM was transferring a substantial amount of the overall structure’s strength to the body; certainly any side impact resistance that this intrinsically vulnerable design might have had. The problem is not only whether the body sills had enough strength for that purpose in the first place, but these rocker sills were notorious for collecting moisture and rusting prematurely.
But before we discuss the X-Frame’s safety weaknesses, real or perceived, let’s do a survey of what all the GM divisions were doing frame-wise during this period. Contrary to some assumptions, not all the divisions used the X frame, either at all, or at least not during some of the time.
Vehicle design and construction at GM then was almost the exact opposite of today. Now, numerous vehicles (and brands) with distinctly different bodies share a “platform”, generally the key underbody structure, suspension and/or floor platform. Back then, Fisher body engineered a common body to be used by various divisions, but each division engineered its own vehicle otherwise, including the frame, suspension, drive train, etc. It really was ass-backwards; or it certainly came to be so, given how increasingly little folks actually appreciated what went on under the floor, at least very technically speaking. And it was inevitable that GM would eventually centralize these aspects.
But not during the X-Frame’s reign. The ’57 and ’58 Buicks, which shared Cadillac’s big C-bodies, did not go with the X-Frame, keeping an X-strengthened ladder frame. So where did the rear foot wells go? Did the Buick C-Body ride higher than the Cadillac? If I could find overall height specs on both, it might possible to ascertain that.
Rather curiously, for 1959 Buick even dropped the X center-reinforcement, going with a strict ladder-perimeter frame, with a K-type front section. To each their own!
But beginning in 1961, Buick fell in with the X crowd, even touting it as the “Safety-X-Frame”. Full-sized Buicks stayed with the X through 1964.
And the Riviera maintained its X-Frame all the way through the 1970 MY. Here’s a 1969 Riviera showing off its skeleton. And for those that assume that X Frame cars were intrinsically poor handlers, the Riviera in GS guise was generally highly regarded as one of the most capable handlers in its size class. Even the base Riviera was considered to be quite good in this regard. It’s more a matter of suspension tuning.
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