Between 1957 and 1970, General Motors built millions of cars with an unusual X-frame chassis that remains controversial to this day.
When the Cadillac division of General Motors rolled out its redesigned 1957 models in Detroit in late 1956, the chassis design (above) may have looked unusual by Motor City standards, revolutionary even, but it was hardly new to the global auto industry. The backbone chassis, as it was known, was used effectively by Mercedes, Tatra, and even on trucks in Europe before World War II, while the humble Volkswagen Beetle employed a hybrid variation on the design throughout its life.
The engineering principle behind the backbone frame, long story short, is that a tube has greater torsional rigidity than a rectangle, and a conventional ladder frame is a rectangle. Meanwhile, the GM engineers had their own special reason for adapting the design. By tucking the frame structure inside the driveshaft tunnel, the floorpan could be moved closer to the ground and thus the roofline as well, enabling the low, sleek styling that the times demanded. By getting the frame rails out of the way, this modified backbone or “tubular center X-frame chassis,” as Cadillac called it, allowed the ’57 models to sit a full three inches lower overall than the year before.
For 1958, the Chevrolet and Pontiac divisions at GM adopted the X-frame concept as well. Chevrolet (above) coined the name “Safety-Girder Frame” and claimed 30 percent greater rigidity than the previous year’s ladder frame, while Pontiac called its setup “Aero Frame.” All the GM X-frame chassis were essentially similar, with the layout lending itself nicely to coil springs on all four corners or, alternatively. self-leveling air-spring suspension. Cadillac introduced the pneumatic system on the Eldorado Brougham in ’57 and Chevrolet offered it as optional equipment in ’58
The 1958 Chevrolet Engineering Features handbook covers the Safety Girder strategy in some detail. According to the handbook, the rocker panels, body crossmembers, and cowl were specially reinforced so that “the body and frame supplement each other to create an effective integrated combination.” In other words, the body and frame were semi-unitized, sharing the structural duties (above). The handbook goes on: “The big, ‘side rail’ rocker panels, supported by the underbody crossmembers, also supply the necessary structural strength to sustain collision impacts on the sides of the body.”
However, that is not how the American motoring community and the court of public opinion interpreted the design. Fairly or not, many people decided that with no outer frame rails as such, the vehicles offered little or no protection in a side impact. (Since instrumented crash testing as we know it did not then exist, we have no useful information to offer either way.)
There were well-publicized crashes involving X-frame cars that included injuries, fatalities, and lawsuits. In the most well-known case, Evans v. General Motors Corporation, involving a 1961 Chevy wagon in which the driver was killed, the court offered no judgement on the frame’s safety. Instead, it ruled that a manufacturer was under no legal obligation to make its cars “accident-proof or fool-proof.” Not long after that, the U.S. Congress passed the National Traffic and Vehicle Safety Act of 1966, which handed automakers a clear duty to exercise reasonable care for passenger safety in vehicle design.
Pontiac used the X-frame from ’58 through ’60, then switched to a more conventional chassis. Buick (lead photo) came in later, selecting an X-frame configuration for its full-sized cars for 1961 through 1964, and on the Riviera all the way through 1970. The Oldsmobile division opted out, never producing an X-frame. Cadillac and Chevrolet stuck with the X-frame through 1964, then switched to a perimeter frame with outer rails that hugged the rocker panels (1965 Cadillac shown below). We haven’t seen another X-frame chassis from the Motor City since.
This frame in side impact collisions was not as efficient as a parameter type frame. I recall seeing photos of GM cars hit in the side being severally damaged infact in one instance a 58 Impala actually was torn in half after hitting a tree. Cudios to Oldsmobile for not adapting this frame because it definitely didn’t offer the same protection.
Thanks for your thoughts. Personally, I couldn’t make that conclusion without more info. I don’t know that it is, but there is no reason the reinforced rockers couldn’t offer equal protection to a standard unit-construction body of the era. Before I committed such a claim to print, I would need to see impact test results, or at the very least a good simulation.
On a side note…early 60’s Canadian built Pontiacs came with the Chevy x-frame and drivetrain due to tariffs…just try ordering underfloor structure part from OPGI for a 63 Grand Prix and you’ll find it fits under your neighbors 61 Impala just fine but not an American built 63 Pontiac with the perimeter frame
I, too, have seen the much-circulated photo of the X-framed Chevrolet torn in half after colliding with a tree, but of course modern cars have been known to suffer the same consequences upon side impact with a tree. Nevertheless, it does seem likely there’s an inherent advantage to the perimeter frame in at least some side-impact situations.
In collisions when the vehicle is torn apart, it really does not matter what kind of frame it has – the shock of the impact would cause lethal damage to the organs anyway, and a few G’s more or less won’t make any difference.
Even todays modern unit body construction cars with crumple zones and crush areas can still be ripped apart by impacting with an immovable object. The remedy for this is to not hit trees.
Our 60 Pontiac had a 2 piece drive shaft. We discovered this when the center bearing support and U joint assembly (which joins the 2 pieces together) hidden inside the tunnel went bad. It looks like the set up under a truck where they need a long drive shaft and do it with 2 sections. The Pontiac drive shaft was not that long so they must have needed it to change direction slightly as it passes through the tunnel.
How could it be that Oldsmobile never used the X-frame and Buick did on the Riviera through the 1970 model year when both the Toronado and Riviera shared the E-body frame structure ?
The Toronado didn’t have a X frame. There was no center tunnel with front wheel drive. The Riviera was rear wheel drive.
They shared the same internal body structure but used different chassis.
The Riveria and Toronado had completely different suspension and chassis . The Riv has all coil suspension. The Toro had torsion bar front and single leave rear suspension.
Do you mean that a truss or triangle is stronger than a rectangle?
There’s a crash test floating around utube someplace featuring a 59 Chevy, it’s sobering @ best.
I have viewed it many times and we may even feature it here at some point. My take is that in an offset frontal impact as in the video, a conventional frame of the era would probably not fare any better.
Agreed! That video has come under a lot of scrutiny over the years.
It’s my thoughts that they picked the x frame car for that test because they knew the lack of side rails would allow the car to fold up like it did. It made a more dramatic result for the results they wanted to happen. Not to say a ladder frame would have fared better because I’m not an engineer, but common sense will tell you the lack of strength behind the front wheel allowed the impact to proceed farther into the cabin past the firewall area.
From what I understand of the test model ’59, it had no engine or transmission installed, thus lacking a lot of, if not most of the frontal strength.
The 1959 Bel Air vs the 2009 Malibu!
I own a low milage 1958 Cadillac, and I find that over uneven road surfaces, the chassis becomes unsettled and the car flexes quite a bit. I’ve owned dozens of other cars/trucks with ladder and perimeter type frames over the decades, and none of them felt this way. I’ve even had the trunk lid pop open when going over railway tracks!
Mild steel in any configuration will not provide structural integrity. Not to mention the lack of any other safety innovations. I can’t tell you how many windshields, steering columns, non latching doors, lack of any restraints , have caused fatal crashes.
That frame also nessated a two piece driveshaft. With no way to grease that center u-joint without removing the whole driveshaft and the support bearing.
Also, My father, a 124 mile a day commuter owned at leased 8 Chevys with the X-Frame. Two of them developed cracks in the area where the steering box mounts to the frame. The owner of the welding shop said that was common and offered to weld re-inforcements on the others in our “fleet”.
Having worked fire rescue for 30 years in Miami, I have seen many full frame, and unibody frames ripped in half on impact with trees, and utility poles. Lots of factors, speed, road conditions etc. Most car people re frame X frame cars because of twisting with higher horse power engines now available.
I remember that the headers on my 64 impala had to run off to the sides at awkward angle due to the x frame
I owned a 64 Belair and we drove from Indaina to Niagra Falla and then to New York. The car was 10 years old and in the rust belt.
I have a habit of taking ramps faster than I should. We drove a couple thousand miles and didn’t find out till we got home the frame had rusted so bad the drivers side came out in 2 pieces. You couldn’t even notice while driving. That section was replaced and re enforced and was driven for another 20 years.
New cars with crumple zones are still safer for passengers.
Well,… today it could be called “X-Frame” vs “No-Frame?!!”
My dad was a Ford dealer at the time. I remember that their marketing really pounded on how unsafe they claimed the GM cars were.