Tech History: The Perimeter Frame – Body-And-Frame, Not Body-On-Frame

For too many years I’ve cringed when I see or hear the expression BOF (Body On Frame) as applied to American cars with perimeter frames, meaning pretty much all of them that had frames from 1965 on. And that cringe turned into something a bit more disdainful when I read endless articles and comments about how vastly more rugged a BOF car is than a unibody, and that’s why police cars were always BOF cars (forgetting that Mopar unibodies were very popular in that use), and BOF cars were much better at towing, etc., etc.

What is commonly forgotten is that the modern perimeter frame is very different from the ladder frame as used in trucks and American cars once upon a time. Ladder frames (and their derivatives) provide essentially all of the torsional stiffness of the vehicle; that’s why BOF trucks don’t need a body on them. But not so with the perimeter frame; it’s a rather delicate and purposely flexible affair that depends on its carefully arranged marriage with the body to provide the requisite stiffness. One could almost say that its side rails are there mostly to just tie together the front and rear subframes; these two play an important role in providing a strong mounting point for the suspension and drive train as well as provide filtering from impacts and vibrations.

I’ve long intended to write an article on the subject, but when Ate Up With Motor left a link to this Car Life article written by Roger Huntington, one of the better tech writers of the time, I decided to post that here along with  my commentary, as I could not say it better than he did.

This article was published in the fall of 1963 when GM’s new 1964 A-Body intermediates were arriving, sitting on perimeter frames unlike the unibodies of their Y-Body predecessors. Huntington starts with a bit of important history: Around the end of the fifties, it seemed like unibodies were going to be the next big thing. Ford had ditched the frames for its new 1958 Thunderbird and Lincoln, and Chrysler went wholesale unibody in 1960 (except the Imperial). And then of course the new 1960 compacts from the Big Three were all unibodies.

But that trend with the larger cars stopped after 1960, as Ford came to the painful realization that the ’58 Lincoln and T-Bird gained only a lot of weight with no real benefit. Chrysler was of course already committed, but it’s also important to point out their large cars were really hybrids, with a large subframe attached to the cowl to which the front suspension, drive train and body elements were mounted to. A 2/3 unibody.

GM’s decision to use a perimeter frame under the new ’64 A-Bodies was a clear indication that the unibody trend was certainly not going to get any additional traction at GM for anything bigger than a true compact, and the adoption of the perimeter frames for the A-Bodies as well as for their vast line of full sized cars in 1965 was the signal that there was a new direction with this relatively new frame design.

Huntington then does a brief history of the unibody in the US, pointing out that early versions like the Lincoln Zephyr had what was essentially a frame welded too its undersides. The post war “Step Down” Hudson used a similar technique.

The first real American unibody (in the modern sense) was the 1950 Rambler, which had a much more developed cowl and floor structure that provided the key strength rather than welded on frame members under the floor. AMC was the pioneer in this field in the US, and heavily influenced the Willys Aero, Ford Falcon, Valiant and Chevy II, with their coil springs mounted high and compressing against the rigid inner fender structure that was in turn an extension of the cowl. These cars were all relatively short and light compacts, and that’s an important point, as Ford painfully found out that this is much more difficult to do with a long wheelbase car, given the state of the art (pre-CAD).

Ford’s ambitious foray into unit construction with the ’58 Lincoln and Thunderbird resulted in having to add lots of  additional bracing to the cowl and floor as well as large amounts of sound deadening insulation, the result being cars that weighed 300-400 lbs more than their BOF predecessors. Now that wasn’t a better idea after all. Ford decided that frames were apparently the better way to go for anything with more than 115″ wb and weighing over 3200 lbs.

The big challenge with unibody construction was “in the area of ride and road rumble”. These are the qualities that were very high on the list of American big car buyers, and the big unibody Chryslers quickly developed a rep for being inferior in these regards. In a unibody, the suspension acting directly on the body effectively turns the body into a massive sounding board. This can be mitigated by large applications of sound deadening materials, such as the over 300lbs applied to the unibody Lincolns. But this just offsets the potential weigh advantage of unitized construction.

But then the existing frames used in cars had very substantial negatives too, the biggest one being the loss of interior floor space due to the frame rails. Historically, when cars were tall and they just sat on top of the whole frame structure like a truck, it wasn’t much of an issue. But as post-war cars fell under the spell of the longer, lower, wider mantra, this became a problem, with legroom, especially in the rear, being significantly impaired. Something had to be done; if not, the result would be what Studebaker was stuck with to the end: an intrusively high floor.

Huntington points out that about ten years earlier (ca 1953) the first efforts at dealing with this was to make the frame side rails less deep, from 7-8″ to 3-5″, and boxed, as needed. But that still impeded any significant lowering without seriously impacting rear leg room. Chrysler did that anyway, with their new 1957 models, which suffered from the consequences of sitting on their frames with no foot wells, and undoubtedly led to the decision to unitized construction in 1960.

GM took a different route, with their cruciform X-Frame, a subject that I’ve covered pretty extensively here, but will need to update some as a result of this article.  The goal was to have room for rear foot wells as well as a stiffer frame, which inherently has the potential to improve handling and reduce body creaking and such. It was first employed in the 1957 Cadillac, and eventually adopted by Pontiac (1958-1960), Buick and Chevrolet (through 1964, except the Riviera, which kept it through 1970). Although the X-Frame is commonly put down for being “willowy” or such, in reality, it was a very stiff frame, with torsional stiffness up some 10% and beam stiffness up 41%, compared to the full frame with a center X member as used in the ’56 Cadillac. Huntington said that it was still the stiffest frame in the industry at the time he wrote this article.

The X-Frame did require that the sills on the bodies it carried were substantially reinforced. As such, it was already something of a hybrid system, depending on increased body strength (sills) to work as a combined unit.

There were still two shortcomings of the X-Frame: it did not offer front seat foot wells, and it required a number of long transverse mounting brackets to attach the body to it. And it was claimed (by competitors) to not be as safe in a side impact. That was somewhat debatable, but it certainly was the case if the body sills were rotted out. And the X-Frame was neither light nor cheap.

The other major frame development arrived in 1956-1958, where the frame side rails were pushed out under the sills, to make room for a rear foot well. Since the frame still curved in under the front seat, front wells were not possible. Ford’s version was called the “cowbelly” or “Coke bottle” and it was first used on the 1956 Continental Mark II. It only had a light cross member under the transmission, and as such, was not exactly a paragon of stiffness, but it was cheaper than Chevy’s X frame and got the job done. This frame design was used by all big Fords and Mercurys through 1964. Convertible frames did get a center X member.

Buick’s and Olds’ version used a substantial X center member (and variations of that) along with these new semi-perimeter side rails. It undoubtedly was stiffer than the Ford frame, which may have something to do with the somewhat willowy reputation of these Fords and Mercurys. As noted above, all of these did allow some degree of a lowered rear floor or foot well.

The revolutionary breakthrough came in 1961, thanks to of the combined engineering efforts at Oldsmobile and Pontiac starting in 1957 or so. The result was the first modern perimeter frame, used on the big Olds and Pontiacs starting in 1961, then on the ’64 A Bodies and the ’65 B/C bodies, and by Ford starting in 1965. From then on, it was used right to the end of Ford’s Panther cars in 2011. It finally allowed lower floors front and rear, reduced costs, and offered improved ride, handling and sound/vibration reduction: the Holy Grail. And it did that by turning 40 years of frame development upside down.

Instead of being stiffer, the perimeter frame is purposely more flexible, to absorb suspension impacts, noise and vibrations from the road and drive train. Of course this also required it to be mated to a stiffer body, with numerous carefully (computer) designed rubber mounts so that the two elements worked in harmony. It’s the exact opposite of a traditional ladder or truck frame, where the frame is stiff and the body flexible.

And as can bee seen here, it eliminates all of the cross members in the passenger compartment, allowing for a low floor throughout.  Essentially, the side rails join together the front and rear frame elements, almost as if they were subframes, as those are required to be strong to support the suspension and drive train.

The key elements that make the perimeter frame work are the large and strong torque boxes that join the side rails with the front and rear sections, which have to be narrower for the wheels. These boxes are designed to twist slightly to absorb shocks and rumble from the corresponding suspension.

The 1961 Olds and Pontiac perimeter frame had 11% less torsional stiffness than the X-Frame it replaced. It was lighter, cheaper, and did little more than just tie together the body shell to the running gear and suspension. And it offered both a better ride and handling potential. The perimeter frame was a huge win, all-round, for the carmakers. No wonder everyone that was still using frames adopted it very shortly and was used for the next forty years, with very little change. The passenger car frame had reached the apex of its development.


Huntington points out that the torque-box can also be used in unibodies, and cites the example of the 1962 Fairlane. Doing a bit more research on that, it turns out that Ford used these to strengthen the original 1960 Falcon unibody “platform” as it was adapted to both larger cars like the Fairlane as well as versions with V8s and convertibles and wagons. Torque boxes were added to the front, the rear, and both, according to the use and need. The underbody of a V8 Mustang is significantly different than the six cylinder version; that applies to other variants using the Falcon platform.

So the next time you hear someone refer to a Panther as “BOF”, you can now correct them and tell them that it’s actually “BAF”.

Related reading:
Automotive History: An X-Ray Look At GM’s X-Frame (1957 – 1970)