Reconsidering the 1972 NHTSA Report on the Corvair

If you’re at all familiar with the Corvair, Chevrolet’s air-cooled, rear-engined six-cylinder compact, you’re almost certainly aware that consumer advocate Ralph Nader famously lambasted it as a “‘one-car’ accident” in his 1965 book Unsafe at Any Speed: The designed-in dangers of the American automobile, whose first chapter is devoted to the Corvair. You may also have heard that an investigation conducted by the National Highway Traffic Safety Administration (NHTSA) in the early 1970s refuted Nader’s charges, declaring that the early Corvair’s handling was perfectly safe. However, the facts aren’t so clear cut — and neither is the ostensible exoneration of the Corvair.

Reading the NHTSA Report

The NHTSA report, first published July 1972, is entitled “Evaluation of the 1960–1963 Corvair Handling and Stability,” DOT HS-820 198. I’ll go out on a limb and say that far more people have heard of this report than have actually read it, but it is readily available if you care to give it a go. As a work of the U.S. federal government, it’s public information, and, with the possible exception of certain of the illustrations, is in the public domain in the United States. The U.S. Department of Commerce currently makes it available through the National Technical Information Service (NTIS), under catalog number PB211015. You can download a copy by going to the NTIS website at https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/PB211015.xhtml and clicking the PDF icon above and to the left of the title (under the horizontal blue bar, below the word “Actions:”).

It describes an official investigation convened in response to a series of increasingly strident letters from Ralph Nader, demanding that the NHTSA look into the handling deficiencies of the Corvair with an eye toward ultimately forcing GM to recall the early cars to upgrade their rear suspensions along the lines of the optional suspension equipment offered in 1962–1963 or the improved 1964 design.

This investigation was carried out over a period of about 18 months between 1970 and the summer of 1972, and included a series of tests designed to evaluate handling and stability in both steady-state and transient maneuvers. These tests involved not only an early Corvair (with standard suspension), but also a 1962 Ford Falcon, a 1960 Valiant, a 1962 Volkswagen Beetle, a 1963 Renault, and a 1967 (second-generation) Corvair.

The Report’s Conclusions

The part of the NHTSA report that most people have heard of and care about is its eventual conclusions, which were as follows:

Evaluation of the extensive data obtained from General Motors and from other sources, analysis of the NHTSA input-response vehicle test data, and recommendations from the Advisory Panel employed in this case indicate that:

  • The 1960-1963 Corvair understeers in the same manner as conventional passenger cars up to about 0.4g lateral acceleration, makes a transition from understeer, through neutral steer, to oversteer in a range from about 0.4g to 0.5g lateral acceleration. This transition does not result in abnormal potential for loss of control.
  • The limited accident data available indicates that the rollover rate of the 1960-1963 Corvair is comparable to other light domestic cars.
  • The 1960-1963 Corvair compared favorably with the other contemporary vehicles used in the NHTSA Input Response Tests.
  • The handling and stability performance of the 1960-1963 Corvair does not result in an abnormal potential for loss of control or rollover and it is at least as good as the performance of some contemporary vehicles both foreign and domestic.

To contextualize the lateral acceleration figures, the report later notes:

Data supplied by General Motors and Testimony by Dr. Thomas Manos, expert witness for plaintiffs in cases against GM, have established that a sustained lateral acceleration of 0.6g is the limit of control for the 1960-1963 Corvair; that is, 0.6g is the value beyond which the vehicles can no longer be maintained under control by the most skilled driver. This value, when viewed with other limit of control values, illustrates that the Corvair compares favorably with contemporary passenger cars. [19/25] … For the lateral acceleration range of 0.5g to 0.6g the 1960-1963 Corvair is in stable oversteer. The oversteer does not become self-energizing or unstable until the limit of control is exceeded. [23/29]

(The numbers in brackets are page numbers; the first number is the one that appears on the page itself, while the number after the slash is the page number of the PDF file.)

For the era, a lateral acceleration of 0.6g was not bad. Subsequent improvements in lateral acceleration (which is a measure of steady-state cornering power) reflect improvements in tire technology more than necessarily suspension design; the dynamic tests were conducted with tires as close to original equipment as could be obtained. (The testers were obliged to use 5.60-15 tires for the Renault because they weren’t able to find a source for the 5.00-15 or 5.10-15 sizes that were standard when the cars were new.)

The NHTSA Report Versus Corvair Critics

There is a key point that has long vexed me about the NHTSA report’s ultimate conclusions: Ralph Nader may have been the most vocal critic of the early Corvair’s handling, but he was definitely not the only one. So, even if one were to dismiss all of Nader’s charges as entirely unfounded — which I don’t think is reasonable — are we to do the same with the critics whose perspectives on the Corvair’s flaws partly informed Nader’s arguments, like OCee Ritch or Denise McCluggage (both of whom Nader quotes in Unsafe at Any Speed), or contemporary reviewers who made similar contentions about the Corvair’s handling, like Sports Car Graphic editor Jerry Titus?

McCluggage and Titus were respected race drivers as well as journalists, and even acknowledging (as the NHTSA report does) that vehicle handling is to some extent subjective, both writers seem amply qualified to comment on driving dynamics. When Denise McCluggage says that a car has a penchant for swapping ends if carelessly pressed, or when a driver of Jerry Titus’s skill says (as Titus does in the November 1961 Sports Car Graphic) that in stock form, the Corvair “not only oversteers abnormally when cornered fast, it does so with a series of ‘unloads’ that are extremely difficult to correct,” I’m not inclined to shrug it off as the uninformed nattering of a misguided zealot (which is how many auto enthusiasts view Nader).

So, in examining the NHTSA report, my question was, “Is there any way to reconcile that apparent discrepancy?” The testing was specifically designed to elicit the kind of misbehavior Titus, McCluggage, Ritch, and others had noted, so why didn’t it? Were those earlier critics all mistaken? Were the Corvair’s alleged handling issues illusory (the result, as GM implied in some of the lawsuits, of driver negligence), or were they somehow the dynamic equivalent of the weird noise your car keeps making that the service technician always insists they’re unable to replicate?

After some consideration, I think I may have the answer.

Titus Versus the Corvair

I want to focus specifically on the remarks of Jerry Titus, because he not only observed the Corvair’s handling peculiarities, but also took the time to try to analyze them — and, just as importantly, published his conclusions in articles to which I (and potentially you) have access. The most important are these two:

  • “Why Doesn’t the Corvair Handle?” (April 1960): This three-page article originally appeared in the final issue of a relatively small, long-defunct bimonthly car magazine called Foreign Cars Illustrated and Auto Sport (see http://99wspeedshop.com.s3-website-us-west-2.amazonaws.com/foreigncarsillustrated.html) and is reprinted on pp. 30–32 of the Brooklands Books Corvair Performance Portfolio 1959–1969, compiled by R.M. Clarke. (The availability of the latter volume isn’t what I’d call ideal, but it is around, and many Corvair enthusiasts may already have a copy.)
  • “Corvair with RPOs” (November 1961): This two-page item is a follow-up to the April 1960 article, this time published in the then-new Sports Car Graphic, of which Titus was the technical editor. It is also reprinted in Corvair Performance Portfolio 1959–1969, on pp. 40–41.

“Why Doesn’t the Corvair Handle?” isn’t a full road test, but rather a follow-up to Titus’s Corvair test in the magazine’s previous (January 1960) issue, where he had first noted the peculiar rear end “unloads” mentioned above. The article describes his efforts to analyze why that was happening and what might be done about it.

Because Titus was a racing driver, his analysis, unlike Nader’s, doesn’t presume that oversteer is inherently bad or more intrinsically dangerous than understeer. (The authors of the NHTSA report take a similar position.) Titus also doesn’t lay the blame for the unusual behavior on the Corvair’s obvious tail-heaviness: He notes that the previous “test vehicle’s overall balance was otherwise fine; there was no other indication that the weight distribution between front and rear might be a problem.”

After conducting some further track tests of a four-door Corvair sedan and examining photographs of the car’s behavior, Titus theorized that the issue was not weight distribution or inherent oversteer, but rather the jacking effect of the rear suspension geometry.

This discussion will make more sense if you can picture the layout of the rear suspension. The following illustration is from a 1956 patent filed by Chevrolet engineer Maurice Olley (US2911052, patented November 3, 1959), depicting the basic arrangement of the swing-axle suspension that would be used by the 1960–1963 Corvair and 1961–1962 Pontiac Tempest. Please bear in mind that this is a concept illustration that doesn’t necessarily reflect the actual dimensions or geometry of the production suspension, but it does indicate the basic layout. (I’ve added color to make it easier to decipher.)

Color-highlighted version of Fig. 6 of U.S. Patent 2,9111,052, showing top (plan), front, and side views of one side of a swing-axle rear suspension with the wheel located by a semi-trailing lower wishbone shaped like a capital "A" and by the axle halfshaft

The above illustration depicts the left rear suspension; the front of the car would be toward the left of the image. As you can see, the rear wheel is carried by an angled (semi-trailing) lower control arm, numbered “F” and colored orange. (The circle in the center of the control arm represents the lower seat for a coil spring, not shown.) The differential, numbered “15” and colored green, sends power to the wheel through a halfshaft, numbered “10” and colored purple. This has a universal joint on the differential side, allowing the halfshaft to swing up and down relative to the differential. The wheel hub/bearing carrier, colored blue, is rigidly affixed to the brake backing plate, numbered “76” and colored pink; thus, the outer end of the halfshaft is always perpendicular to the backing plate.

The alignment specs for the early Corvair call for about 1.5 degrees of positive camber (top of the wheel tilted outward) when unladen. In a turn, the outside rear wheel’s camber becomes more positive, causing an effect called tuck-under or roll-under. Titus notes that this isn’t necessarily a problem (and isn’t exclusive to swing axles), but with the geometry of the early Corvair suspension, it can cause the swing arm to lock against the universal joint and act as a lever, so that instead of weight transfer forcing the wheel downward, cornering forces lift the car’s weight up and off of the outside wheel, thus reducing its grip on the road surface. Titus explains it like this:

The resistance forces are transferred back into the chassis at hub level, through transverse arms in the same plane. As the roll-under increases, the distance from the axle center to the ground decreases and the anchor point of the arms moves above the hub center. The axle and the arms become, in effect, a lever which prevents chassis-roll onto the suspension. What happens instead is quite the reverse: with the weight mass concentrated on this inner pivot point, the chassis can move in only one direction—upward!

This upward motion exaggerates the wheel tuck-under, which Titus concluded was responsible for the peculiar rear wheel unloading and sudden breakaway he experienced.

There are two points Titus doesn’t mention in that article, but that I think tend to support his analysis:

  • First, while Titus doesn’t try to calculate the Corvair’s rear roll center height, he notes that it “appears to be only slightly above the inner axle centers.” A subsequent article on suspension fundamentals by Roger Huntington in the March 1963 Car Life presents a detailed breakdown of the early Corvair suspension geometry (which usefully supplements the actual suspension specifications, available in the Corvair Vehicle Information Kits from the GM Heritage Archive: https://www.gm.com/heritage/archive/vehicle-information-kits), which indicates that rear suspension’s static roll center is 13.6 inches (354.4 mm) above ground level — a few inches above the universal joints, just as Titus surmised.
  • Second, although Titus doesn’t mention the Pontiac Tempest, the 1961–1962 Tempest, which used an adaptation of the early Corvair’s rear suspension, had similar handling deficiencies. Unlike the Corvair, the Tempest wasn’t tail-heavy (the big four-cylinder engine put more than half the static weight on the nose), it had less rear roll stiffness, and all but early 1961 cars had a front anti-roll bar. The fact that the Tempest nonetheless exhibited the same tendency toward abrupt, violent breakaway as the early Corvair therefore strongly suggests that the principal culprit was the rear suspension geometry, probably for the same reasons Titus identified. (For further reference, the most comprehensive analysis I’ve seen of Tempest handling is by OCee Ritch, in an article in the 1963 Pontiac Performance Handbook (a Hot Rod special issue that Brooklands Books reprinted in 1989 as Performance Tuning for the Restorer: Pontiacs of the 60s), in which he calls the Tempest “evil handling.” That article’s photos of handling tests of a 1962 Tempest coupe at Riverside International Raceway vividly depict rear camber changes and jacking very much like what Titus describes for the Corvair.)

Weight and Geometry

A third point that’s particularly crucial here is why the Corvair (and the rope-drive Tempest) had so much positive rear camber when unladen. Titus notes that this was common in swing-axle cars to reduce rear tire wear, which is true, but that was only part of the object.

Both Chevrolet and Pontiac originally intended these cars to be cheap family sedans capable of carrying six people and their luggage. The early Corvair sedan had base a curb weight (unladen weight with fuel and fluids) of 2,375 to 2,400 lb (1,077 to 1,089 kg), but the design weight (which Chevrolet specifications also call “performance weight”) assumes an additional 600 lb (272 kg) of passengers and cargo, and the rated maximum load is about 900 lb (408 kg).

Adding more weight to a vehicle compresses its springs; with a swing-axle suspension, this also lowers the inner pivot points of the axle halfshafts, which causes the wheel hubs to rise and shifts wheel camber from positive (tilted outward at the top) to negative (tilted inward at the top). The 1960 Corvair specifications indicate that with the 600-lb design load, rear camber goes from 1.5 degrees positive to 1 degree negative, a change of 2.5 degrees. (In suspension alignment terms, this is quite a bit.)

This clearly presented Chevrolet (and Pontiac) with a conundrum: Starting with a static camber of 0 degrees (neither positive or negative), or a certain amount of static negative camber, tends to provide better cornering grip than starting with static positive camber. However, too much negative camber reduces traction and stability in straight-ahead driving, and promotes uneven tire wear. (Try walking with both your feet rolled slightly inward and you’ll understand why.) Since the swing-axle rear suspension was subject to so much camber change under load, starting with a static negative camber of 1.5 degrees when unladen would risk excessive negative camber (perhaps as much as 4 to 6 degrees) when heavily loaded.

Looking at the specifications, I think it would be fair to say that Chevrolet compromised the Corvair’s rear geometry somewhat in unladen or lightly laden condition in order to provide more favorable geometry when carrying a heavy load of people and luggage. The published specifications I’ve seen for the Tempest aren’t as detailed, but its rear suspension was also set up for static positive camber at curb weight, presumably for the same reasons.

The Titus Solution

In “Why Doesn’t the Corvair Handle?” Titus says that after various experiments, including increasing the front tire pressures (at the suggestion of Chevrolet Engineering) from the recommended 15 psi to 26 psi, he found the only thing that really improved the test cars’ nasty breakaway characteristics was to use sandbags to add weight to the rear floor, behind the driver. (The article is vague about the precise amount, saying only that it was “almost 200 pounds,” but the later Sports Car Graphic item indicates that he eventually settled on 170 lb (77 kg).)

Titus explains:

No, the weight in itself wasn’t helping the cornering. But placing it over the outside rear wheel changed the angle of the “swing axle” to one that was slightly uphill, running from the inner pivot out to the wheel hub. Camber became just a hair negative instead of excessively positive. With this setup, lateral forces no longer “locked” the suspension, and tread contact was good. The result: a smooth, fast, and controllable cornering effect.

Of course, loading the back of the car with sandbags was a crude solution, and not very practical. Titus suggested that a more permanent means of achieving the same results would be to modify the powertrain mounts so as to lower the inner pivot points, “de-cambering” the rear suspension to provide a bit of negative camber in unladen condition.

As Titus says in “Corvair with RPOs,” published a year and a half later, that was essentially what Chevrolet did with the optional suspension kit (RPO 696). This option didn’t alter the powertrain mounts, but it used shorter rear springs that changed the static rear camber from 1.5 degrees positive to about 1 degree negative. (The option also included limiter straps on the rear control arms to prevent excessive wheel tuck-under, plus a front anti-roll bar to counter the increase in rear roll stiffness caused by the straps and stiffer rear springs, but the de-cambering probably had the greatest impact.) Titus found that this greatly reduced the abrupt breakaway that had been such a headache with the standard suspension without adding significant extra weight, and also improved straight-line stability.

The NHTSA Report, Round Two

So, if the early Corvair’s stock suspension geometry made it prone to abrupt breakaway in fast turns, as Titus and others said, why did the NHTSA tests not demonstrate that tendency? To find the answer, I had been expecting I might have to pick apart the way the various dynamic tests were constructed, but the likely solution turned out to be much simpler than that.

The report section on the highway cornering tests describes how the cars were prepared for the tests, which includes these very revealing statements on p. 57 (p. 63 of the PDF):

After each vehicle had been mechanically prepared and instrumented, weight was added so that each wheel was loaded statically to match the manufacturer’s maximum recommended weight with passengers and luggage. … Vehicle wheel alignment was adjusted to the manufacturer’s recommendations at curb weight after the mechanical preparation. In each case, wheel alignment was again measured in the fully loaded condition.

Page 39 of the report (page 45 of the PDF) notes that the test cars were evaluated in fully loaded condition based on the suggestion of Nader associate Gary B. Sellers, in a letter to Rodolfo A. Diaz, then NHTSA acting associate director of motor vehicle programs. That letter, dated March 17, 1971, is reproduced in Appendix IV, at the very end of the PDF document. It outlines a series of suggestions for the test procedures, including (in point 5), “[t]hat the vehicle also be tested with the maximum recommended load (such as the weight of four to six passengers), as might often be experienced on a Sunday drive” [emphasis added]. The NHTSA evidently either missed or disregarded the stipulation “also,” and opted to evaluate the test vehicles only in fully loaded form.

Unfortunately, the report doesn’t specify exactly how weight was added — presumably with sandbags or other such dead weight — or list the actual curb weights and test weights of the vehicles tested. (As noted previously, the Corvair’s rated maximum load was about 300 lb (136 kg) greater than its “design weight.”) Likewise, the alignment figures aren’t specified.

Fortunately, complete factory alignment specs for the Corvair are readily available. The NHTSA test used a 1963 Corvair; the report doesn’t indicate the body style or trim level, although my guess is that it was a four-door sedan in the cheaper 500 or 700 series. As in 1960, the 1963 Corvair specifications call for a static rear camber of 1.5 degrees positive at curb weight with the standard suspension, but the rear springs of the 1963 car were stiffer than the 1960’s, which meant less camber change at design load: Rear camber at design weight (curb weight plus 600 lb (272 kg)) is listed as 1.0 degrees positive. If the test car was at maximum load (curb weight plus about 900 lb (408 kg)) rather than design load, I would expect rear camber to be closer to 0 degrees.

Either way, the point is that the NHTSA test Corvair, as prepared, carried a load heavy enough to significantly alter the rear wheel camber, and — likely — also the crucial relationship between the height of the wheel hub centers and the swing axle pivot points, in much the same way Titus had achieved by adding sandbags to his test car back in 1960.

To put it another way, by testing the Corvair in a fully laden or heavily laden condition, the NHTSA testers appear to have managed — probably inadvertently — to temporarily eliminate the suspension geometry problems that were the proximate cause of the handling issues Titus described.

An Alibi, but Not an Acquittal

This finally puts the NHTSA test results in some perspective: Their conclusions about the handling characteristics and stability of the early Corvair are probably reasonably accurate so far as they go — but they only reflect the behavior of a Corvair in heavily laden condition.

That would be reassuring for a Corvair owner who regularly took long trips with a car stuffed with kids and luggage (or who kept the rear footwells packed with sandbags just in case), but as Titus, McCluggage, Ritch, and others had observed, a lightly laden Corvair could be a very different story. With only the driver aboard, there would not be enough load on the rear springs to mitigate the suspension’s geometric shortcomings.

It’s evident that Chevrolet and Pontiac chassis engineers anticipated that Corvair and Tempest owners would use their cars to carry a full load of passengers, which is almost certainly why the factory alignment settings are what they are. However, many Corvairs and Tempests were sold as sporty cars; the best-selling variants of both models were the Monza and Tempest Le Mans coupes with bucket seats, not sedans or wagons. Without the additional weight of passengers and cargo to mitigate the jacking and tuck-under issues, it’s evident that the stock suspension geometry could still prove treacherous in ways the NHTSA tests don’t reflect.

One could of course still argue that the handling of the early stock Corvair wasn’t that much worse in lightly loaded condition than other American or imported cars of its time. However, that’s not what the NHTSA tests evaluated. Consequently saying the report completely exonerates the Corvair of any charges of flawed handling is drawing a broader conclusion than the data actually supports. It’s akin to insisting someone is definitely innocent of a crime committed between midnight and 6 a.m. just because they have a solid alibi for the first two hours!

Summing Up

Reading the NHTSA report, one senses a certain degree of administrative exasperation with Nader (which is perhaps understandable given the tone of his letters, several of which are reproduced in the appendix). Nonetheless, I don’t think there’s any question that the NHTSA approached the investigation in good faith; it was an elaborate effort occupying many months, and if the agency was frustrated with Nader, they nonetheless made an effort to accommodate his suggestions.

However, while I think the decision to test the cars in fully loaded condition was reasonable, I believe it also effectively limited the applicability of the results, in ways I’m not sure the testers even recognized. It also seems inadequate: Even family cars are often driven with only one or two people on board, and many vehicles spend most of their lives in solo commuting duty. That the early Corvair handled acceptably when heavily laden is commendable (and reflective of the original design priorities), but it was only one part of the story. Comparing the handling of the test vehicles with the driver only as well as with a full load would have been more representative — and, in the case of the test Corvair, might well have returned far less felicitous results.

FIN

AUTHOR’S NOTE

A version of this article first appeared on the Ate Up With Motor Patreon page on April 20, 2024. This version has been edited for formatting and to incorporate the note about why the NHTSA tested the cars in fully loaded condition into the body of the text (it was originally presented as an addendum), in the interests of clarity.

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  1. Excellent work! As a (objective) Corvair enthusiast since I first saw one in 1960 and having owned a ’63 as my first car, I have been pondering (and writing about) this issue for a very long time. The NHTSA report just never passed my sniff test; I assumed something about it must have been off for it to return such benign results for the Corvair. Did they drive it hard enough? Was the process tainted by some interference? Now we know. And frankly, it’s absurd, to have tested these cars at maximum weight, which was bound to influence the results, one way or another. It’s truly ironic that it made the Corvair’s issue significantly more benign. Most cars you would expect just the opposite.

    By its nature, the Corvair appealed to buyers looking for something sporty (and compact), and were decidedly unlikely to be carrying a full load of passengers. Testing it at maximum weight is simply absurd, unless it was also tested at light weight (1-2 passengers).

    My first car was a ten year-old ’63 Monza 4-door with the up-rated 102 hp engine and the 4-speed. I was 19 at the time, and was not properly aware of the RPO696 optional suspension. It definitely had a bit of negative camber at rest (and empty), but then the coils might have gone a bit soft. Or perhaps it did have the optional suspension, which given the higher output engine and 4-speed in 4-door, someone might have wanted a sportier Monza 4-door.

    I drove it pretty briskly, especially on a 600 mile drive down the Skyline Drive and Blue Ridge Parkway in mid-late October of 1972, shortly after I got it; the road was essentially empty back then. I steadily increased my speed in the endless curves, and I could very clearly feel the transition to neutral and then to oversteer, which I really enjoyed (I’ve been a fan of oversteer ever since, and removed the front anti-sway bar on my xB in part to enhance that quality). But never once did I feel any tendency for the rear to start jacking up. So maybe I wasn’t going fast enough, or it really did have the RPO696 suspension. And I’m thankful for that. I had read just enough at the time (1972) to know that snap oversteer was possible, and that rolled VWs especially were not that uncommon. But I also knew that driving technique was critical, and made sure to avoid braking in brisk curves, which was undoubtedly another way to bring the effect on.

    No more debates about the Corvair’s NHTSA exoneration; you’ve put that to bed, and very effectively.

  2. Great research!

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