(originally posted 1/19/2012. Updated 12/21/2022 with information from “The Cadillac Frame – A New design Concept For Lower Cars” a paper submitted to the SAE by GM engineers that was provided to me by George Ferencz)
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 conceived as a way to facilitate lowering the total vehicle height yet still allowing for reasonable leg room via deep floor wells for the passengers’ feet to drop essentially to the bottom of the car, unimpeded by frame rails. This was not possible with the ladder frames then in almost universal use, except unibodies, of course.
Placing ever-lower bodies on ladder frames created serious negative impacts in interior space efficiency, as Chrysler found out with its Forward-Look 1957 models. Both leg and head room were limited, and the seating position was close to the (high) floor. The solution at Chrysler was to convert their cars to unibodies in 1960, resulting in improved interior metrics, as shown in this comparison graphic. GM considered unibodies, and would use them on their compacts starting in 1960, but dismissed using them on their large cars for a number of reasons, of them being that it would be much more difficult to offer a wide variety of body lengths and styles. For instance, engineering unibodies for the extended length Cadillac 75 limo and commercial chassis would have been very expensive and difficult. Also, it was much easier to “tune” a frame to the corresponding body to minimize noise and maximize comfort. This undoubtedly explains (at least in part) why Chrysler continued with the full frame under the Imperial through 1966.
The 1957 Cadillac Eldorado Brougham was only 55.5″ high, and the other ’57 Cadillacs were also going to be lower than their predecessors, so the Cadillac engineers needed a new solution in regards to the body and frame. The existing ladder-type frame was simply not going to work. The height of ladder frames had already been reduced a few years earlier to allow lower bodies, by utilizing box sections and other stiffeners, but there was no further reduction possible.
Cadillac did build an experimental car in 1950 with something akin to the perimeter frame that would eventually supersede and replace the X-frame, but torsional rigidity was severely compromised. Adding X-members to the frame created the desired rigidity, but then there was the problem with intrusion into the leg room area.
The X-Frame was the result of a number of years of experiments between Cadillac engineers and A. O. Smith Corp., who actually built all of GM’s frames. It combines aspects of two very distinctive frame designs: the backbone frame and the X-braced ladder frame.
The backbone frame originates the 1908 Rover 8 h.p., which used a rigid steel tube as its primary member. Since it had a rigid rear axle, the suspension at the rear was by mounting the body on semi-elliptic springs.
Hans Ledwinka’s revolutionary Tatra T-11 of 1921 (full story here) took the central tube chassis and made it more effective by adding independent rear suspension via swing axles. The 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 did not create problematic interior intrusion 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. Note: this is quite different than a perimeter frame as used in most American passenger cars. The ladder frame is still very much under the body, and provides almost all of the overall rigidity, unlike a perimeter frame.
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 widely used to stiffen ladder frames as needed, specifically for convertible or commercial car 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 to varying degrees to the overall vehicle rigidity, which is why convertibles require additional reinforcement to their frames.
The X-Frame that GM came up with involved a very strong center backbone section, which could be built in different lengths to accommodate varying wheelbase lengths. Converible and extended wheelbase frames used thicker steel in the center section as well as some other reinforcements. The 1957 convertible frame weighed somewhat less than the 1956 convertible frame, but had roughly equal torsional rigidity and 100% improved maximum beaming deflection. The non-convertible frames had improved torsional rigidity.
This ’61 Chevy X-frame shows the details of its design and construction. It did result in a somewhat larger central tunnel in the interior, a distinctive feature of all X frame cars and a trade-off for the deep foot wells on either side. Obviously those confined to sit in the center of the seat did not see any advantage.
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 issue 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 this was not the case 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.
The result was…a high rear floor, right up to the bottom of the cushion. Of course the center tunnel was quite modest.
As a point of comparison, this picture of a ’58 Cadillac rear seat clearly shows the lower floor but larger center tunnel.
Rather curiously, for 1959 Buick even dropped the X center-reinforcement, going with a modified semi-perimeter frame, with a K-type front section. It looks very similar to Ford’s “cow belly” frame, in that it did offer room for rear seat foot wells, but not for the front seat area, as the frame narrowed down there.
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 Frame through 1964.
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 live with the false assumption 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.
The X-Frame was plenty stiff, and the handling of the cars that used it was not due to anything about the frame, but was a matter of suspension geometry and tuning.
Olds was the only member of the GM family to not use the X-Frame at all. The ’57-’58 Olds frame looks quite similar to the same vintage Buick frame.
But for 1959, Olds came up with what is essentially a variation the X-frame, incorporating wide side rails to do the work that the reinforced body sills were asked to do on the true X-frame cars. These side frame rails (or similar) have been widely adapted to high-performance X-frame cars, creating an overall stronger, more rigid frame. And that came without any sacrifice in the X-frame’s deep floor wells. Call it the improved X-Frame.
The other unique aspect about the ’57 – ’60 Olds chassis is that it had leaf springs in the rear suspension, the only GM full-sized rwd cars to do so during the whole modern era, except for the ’71 – ’76 “clamshell” wagons, which did so for space reasons. The ’57 Cadillac used leaf springs too.
For 1961 through 1964, Oldsmobile used a perimeter ladder frame. Note how much emphasis Olds places on the safety of “Steel Protection All Around”.
The frame Olds used those years (’63 in drawing above)…
very much predicts the frame all the GM full-sized cars went to in 1965 (Riviera excepted). This one is from a ’69 Cadillac. Olds’ reputation for engineering advancements among the GM divisions seems to have gone all the way down to frame design.
Interestingly, Pontiac did not show off its frames in its brochures, unlike the rest of the divisions. John DeLorean, who was then running Pontiac, was an engineer by training, but was also an excellent marketer. He thought GM was generally lacking in modern marketing techniques at the time, and perhaps that explains why Pontiac frames didn’t make it into brochures. Who cared? Folks were much more interested in how they looked, and even how they drove. Pontiac accentuated its very visible Wide Track stance, and left folks guessing about the frames.
But as the above frame spec chart shows, Pontiac followed Chevrolet and Cadillac with X frames for ’58 – ’60, but then went to a perimeter frame for ’61 – ’64 that looks very similar to the ’61 – ’64 Olds frame. (Canadian Pontiacs, essentially Chevrolets under the skin, did keep the X frame for ’61 – ’64)
Chevrolet’s “little brother” relationship to Cadillac was more than skin deep, inasmuch as the two of them were the most consistent users of the X-frame; from ’58 through the ’64 MY, in Chevrolet’s case. Here’s the ’59 version in full display. The ’58 was unique to that MY; the ’59 – ’60 frames were identical, as were the ’61 – ’64 frames.
The ’59 Chevy became famous in 2009 when it was crashed into a new Malibu, and (not surprisingly) fared rather poorly. The X frame played little or no part in that, given all the other aspects that were so different between these cars.
The X-frame is somehwat notorious for rusting, and there is a pretty brisk business in replacement frames, often reinforced. One wouldn’t want to start throwing a Chevy low-rider hydraulically four feet into the air with a rusty frame.
In Ralph Nader’s “Unsafe At Any Speed”, the X-Frame merits some less-than flattering attention. Here’s some excerpts from that:
A case in point is the “X” or “cruciform” type chassis frame. This frame was introduced in 1957, primarily to reduce the problem of restricted headroom and difficult entry into the “low-profile” automobiles that were becoming popular after the mid-fifties. The X frame construction does not have side rails along the passenger compartment, as did most previous conventional frame designs. From the time the cruciform type frame was introduced, it was widely used by General Motors on Chevrolet, Buick, and Cadillac. The Ford Motor Company continued to use frames with side rails, and it was evident that the two companies held strongly different opinions about the two designs.
In the fall of 1959, a photograph of a Chevrolet Impala that was broken in half after striking a tree broadside was widely circulated in newspapers throughout the country. The frame had severed at the intersection of the X. The report of the General Motors investigators who rushed to the scene attributed the severance of the frame to the semi-airborne position of the car as it struck the tree. This had apparently allowed the engine mass to act as the head of a sledge hammer. At the General Motors engineering center in Michigan the conclusion was that “automobiles are not designed to withstand such tremendous lateral forces – this would be extremely uneconomical.”
General Motors spokesmen continued to defend the cruciform type frame as offering substantial resistance to side impacts because of the rocker panel and floor pan underbracing members — even though by 1965 all General Motors models except the Buick Riviera had abandoned the design in favor of the perimeter type. In 1960 the General Motors technical center offered proof that a unitized structure with side rails can also split into two pieces. A picture of a Ford Thunderbird, torn in half after slamming against a telephone pole and tree, was offered as evidence to critics of the X type frame.
This comparison enraged Ford engineers. Fletcher N. Platt, a highly talented research engineer at Ford, retorted that the Thunderbird case involved a telephone guy-wire that had “acted as a knife on the entire body structure.” In contrast, he said, “the Chevrolet that broke in half failed at the center of the X frame after hitting a tree.” Platt said, “The X frame has no advantages from the standpoint of passenger protection. It requires less material to support the four comers of the car, but it is obviously less rigid and provides little lateral [side] protection to the passenger compartment.” He suggests consulting any “‘unbiased’ structural engineer regarding these two designs.” Mr. Platt might not consider Mr. Harry Barr, vice president for engineering of General Motors, qualified for the designation ‘unbiased,’ but Mr. Barr did admit grudgingly, under questioning, that the Oldsmobile perimeter type frame had some advantages over the Chevrolet X type frame in side-impact crashes at speeds of about fifteen miles per hour. Further proof that some General Motors engineers agreed with Ford’s Platt came in the form of an internal memorandum prepared by the Oldsmobile division in 1963 in which the Oldsmobile “guard-beam” frame was described as offering an “extra margin of protection” over the X type frames of Chevrolet, Buick, and Cadillac.
I was unable to find the picture of the broken-in-half ’59 Chevy cited here. Undoubtedly, a side impact, especially against something like a tree or pole, would be the X-frame’s most vulnerable aspect. There are other anecdotal negatives commonly cited on forums, such as cracking of the rear body sills, as well as other minor structural infirmities, especially with advanced age. The two-piece drive-shaft used with the X frame also comes for a lot of hate, especially the weakness of the center bearing, and the joys of replacing it.
But Ford clearly saw a marketing benefit to the X-Frame controversy, and touted their wide perimeter frame as a safety advantage over the X-Frame in their marketing materials, as in this on from 1960.
On the other hand, when a car T-bones another, the bumper and front body mass is almost invariably above the height of the frame side rails, so the resistance to impact is mostly in the body structure itself. It’s hard to make a very compelling argument against the safety of the X-Frame, considering that unibodies obviously don’t have frame side rails either. Passive safety is the result of numerous improvements that have taken place since then, and obviously modern car bodies are vastly stronger and safer.
Our exhaustive look at the X Frame is complete. If you made it this far, you’re now something of an expert. Hopefully, others will add further facts or insights.
Related CC reading:
Tech History: The Perimeter Frame – Body-And-Frame, Not Body-On-Frame