GM: Gas Tank Fires: The Historical Case Against the Placement of Gas Tanks Outside the Frame Rails on the 1973-87 General Motors C & K Model Pickup Trucks
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GM: Gas Tank Fires: The Historical Case Against the Placement of Gas Tanks Outside the Frame Rails on the 1973-87 General Motors C & K Model Pickup Trucks

General Motors' negligence in the placement of its tanks in an exposed and unprotected position can best be fully understood when measured against the automobile industry's knowledge of fuel tank safety.

One of the greatest threats to human survival in automobile collisions is fire. Ruptured fuel tanks, filler tubes, and fuel lines are predominant factors in crash fire situations. Once substantial fuel spillage has occurred, the probability that fuel will come into contact with an ignition source is high. "Due to the extreme temperatures encountered, perhaps not more than twenty seconds are available for escape from burning automobiles, even with a protective fire suit."

This statistic illuminates the extreme danger associated with post-crash fire. The prevention of post-crash fire has been a consideration in automotive design for decades. Continuous research has led to successive design improvements which if followed will greatly reduce the threat of post-crash fire. A review of the literature on automotive design and engineering reveals a series of gradual improvements which have led to the current state of knowledge about fuel system design.

Before summarizing the most current design recommendations, it is helpful to look at when improvements in fuel tank location and construction came over the course of the past 30 years.

It is clear that researchers have known for years that placing the tank between the frame rails and forward of the rear axle is the safest place. That it is never put in a crush zone was well understood by the time the 1973 GM pickup truck was in its design stage.

As early as August, 1956, a U.S. patent for a saddle type fuel tank, number 3,006,358 was issued. The patent shows the tank located above and in front of the rear axle. The patent was issued in 1961 and assigned to Ford. The tanks were used in the Ford Skyliner hardtop convertible manufactured from 1957 to 1960. Patent 3,014,730, filed October, 1959 for Motor Vehicle Fuel Tank Installation was issued in December, 1961 and assigned to General Motors. The patent recognizes the need for

"good protection of the tank" and provides a "unique arrangement of the vehicle body and framing structure having advantages . . . in the event of a collision."

The location patented was used in the Corvair. The 1961 rear engine Corvair 500, 700 and 900 Series carried a 14 gallon fuel tank aft of the front axle, inside the frame rails.

On September 14, 1961 at the Fifth Stapp Automotive Crash and Field Demonstration Conference conducted at the University of Minnesota, General Motors employees Howard K. Gandelot, Engineer-in Charge of GM's Vehicle Safety Section presented a twenty minute motion picture comprised of clips from engineering record films of crash testing at the GM Proving Ground and spoke on the topic of "Considerations in Crash Energy Absorption."

The later published abstract of that presentation concludes that

"even in car-to-car collision impacts of 50 mph cars can be designed so that the crash energy is absorbed and dissipated with little or not damage to, and reduced deceleration in, the occupant compartments of colliding cars. "

The 1962 Chevrolet Corvette Service Manual shows the gas tank in an impact protected position located forward of the rear axle, above the frame rails. In a 1966 article by Locati and Franchini of Fiat, "Car Crash Fire Investigation," first delivered at the Fisita Congress in Munich, the authors report that a gas tank

"arrangement particularly safe in the different types of collisions is . . . where the tank is house inside sturdy bulkheads."

January 1966 saw the introduction of the Rover 2000 with a gas tank protectively positioned within the car's main structure, separated from the passenger compartment by a bulkhead and from the trunk by partition, between the rear wheels, above the axle.

On November 30, 1966 the U. S. Government issued its Notice of Proposed Rule Making for Initial Federal Motor Vehicle Safety Standards. FMVSS 301 provided that with a gas tank 90% full, in a 30 mph frontal barrier crash, gas loss shall not exceed one ounce per minute.

Thus, we see that as early as 1966, specific requirements for tank integrity were a concern of the federal government.

In September, 1967, Fairchild Hiller submitted its final report to the U.S. Department of Transportation, National Highway Safety Bureau, a public document entitled Investigation of Motor Vehicle Performance Standards for Fuel Tank Protection, which concluded that the safest position for a gas tank in a passenger automobile was above the rear axle, between the rear wheels,

"removed from the area of high probability of damage or repair." "One of the better methods for reducing the possibility of a failure of the tank during a crash would be to locate the tank in a well protected area:"

Removing the tank from the area of high probability of damage and rupture represents the most cost-effective modification and "represents the minimum cost of tank protection."

This study also concluded that the probability of fire was highest in side collisions [when comparing front engine cars, rear engine cars, conventional trucks and cab-over-engine trucks] for a conventional truck when struck in the side by a conventional car.

In the April, 1968 Journal of the Society of Automotive Engineers an article entitled "The New York State Safety Sedan Ready for Takers" illustrated a crash-resistant fuel system to minimize fire hazards by placing the tank above the rear axle.

In July, 1968 the Institution of Mechanical Engineers Proceedings on The State of the Art of Safety in Design - Continental Practice [Malschaert] contains the report of one foreign automobile manufacturer that

"among the very great number of reports of accidents with cars from our fifteen years of production, we have found no case where the petrol tank has failed in such a way that it increases the severity of the accident. Our tank is situated essentially between the two rear wheels, which may account for the excellent results."

The report also states:"The best place for the fuel tank is inside the structure so that it is protected by the body of the vehicle."

In July, 1968 Ridenour, et al., at a General Motors Automotive Safety Seminar produced a paper entitled "A Study of Automobile Fuel Tanks" which reports the testing of GM gas tanks with a 20 mph flat moving barrier using production fuel tanks being suspended edgewise in front of fixed barrier. Using gradually increasing speeds, "at 8 mph the production tank experienced a substantial failure," absorbing energy "many times that absorbed by the fuel tank in the 20 mph moving barrier into the rear of vehicle test."

In August, 1968 Fairchild-Hiller's "Phase I Final Report on Experimental Safety Cars Study" reports that

"the most desirable fuel tank modification, from a cost-effectiveness standpoint, is that of changing its position from below the aft end to a location above the chassis an between the rear wheels. In this position it is removed from the area of high probability of rupture and damage."

At the Twelfth Stapp Car Cash Conference on October 22-23, 1968, attended by GM engineers in Detroit Michigan, Severy, Brink and Baird reported their tests of passenger protection in full size 1967 Ford 500 Series four door sedan, commonly known as the Ford Galaxie. On slow motion film, in Test Number 106 was the "first UCLA collision experiment evaluating post-crash fire as a complication to collision survival" was presented. In the paper summarizing this research the authors noted:

"Somewhat offsetting the low probability aspects of post-crash fires, however, are the awesome and devastating aspects of such an adverse turn of events. Additionally, preliminary studies indicate that much progress can be made in reducing the probability of crash fires by incorporation of relatively inexpensive design considerations having to do with the fuel tank and related fuel system."

In October, 1968, Severy, Brink and Baird [UCLA] reported in "Vehicle Design for Passenger Protection From High Speed Rear-End Collisions," SAE 689774, reports that

"preliminary studies suggest that an improved location for the fuel tank would be the area cradled by the rear wheels, above the rear axle and below the rear window. This location is least often compromised from collision of all types."

As for station wagon gas tanks located in rear quarter panels, the authors state: "The problem requires attention and its solution is not so difficult as to warrant further delay."

In June, 1969 Fairchild-Hiller 's "Final Report on Fuel Tank Protection" notes that station wagon fender tanks, exposed in collisions in identical fashion as GM's side tanks "have little protection and are highly susceptible to damage in rear side impacts. * * * A tank surrounded by sturdy structure is usually safer."

In October, 1969, "Crash Survival:Help from the Tracks" in Nelson, Automotive Industries, Vol. 141, No. 7, noted that for the 1965 Indianapolis 500 race

" . . . all competing cars were equipped with fuel cells. These bladders are made of tough, rubberized fabric stuffed with foam. * * * Impact resistance fuel cells remained intact and eliminated the threat of fire. * * * NASCAR has required the fuel cells since 1966, USAC made them mandatory in 1965."

In January, 1970 an article entitled "Fire in Road Accidents" by Vaughan, Department of Motor Transport, New South Wales, Australia reports:

"The first means of preventing fuel spillage is by location of the fuel tank away from areas likely to suffer structural collapse in an accident."

In the April, 1970 Consumer Reports:

"It is known now that it's safer to place the fuel tank well forward of the rear bumper and that to permit the fuel tank's upper surface to serve as the floor of the trunk is to invite puncture by sharp and heavy objects. Auto manufacturers should be required to locate their fuel tanks in relatively safe positions, to separate them from the trunk floor and to seal off the trunk[against fumes.]"

Stegel and Nahum in their June 1970 SAE paper "Vehicle Post-collision Considerations" report: "There are numerous examples of . . . post collision fire as a result of side mounted fuel tanks."

In June, 1970 Rapin in SAE paper 700413, "Vehicle Structural Crashworthiness," in discussing designing a vehicle for crashworthiness, notes that the types of accidents to be taken in to account by the designer should be front impact, rear impact, side impact, rollover, free fall, and truck underride. In designing for impact:

"Rear part - the most important point is the protection of the fuel tank. It is absolutely necessary to avoid its penetration by bending or buckling members of the structure submitted to shock load."

In the October, 1971 edition of Machine Guide, an article entitled "Safety Tank Design for the Family Car" reports:

"The system includes a cannister-mounted flexible bladder tank with foam baffling and self-sealing fuel-access fittings."

The December, 1971 Final Report by Neva Johnson of Dynamic Science for the U.S. Department of Transportation, National Highway Traffic Safety Administration [hereafter referred to as NHTSA], entitled "An Assessment of Automotive Fuel System Fire Hazards," a public document available to the industry through the National Technical Information Service, analyzed 27 new vehicles and 35 crashed vehicles' fuel systems and concluded:

"The only fuel tank that was not crushed during the rear end barrier test was one which was located above the rear axle behind the rear seat."

This is a "much safer position" and "would in all probability, allow the standard metal tank to survive a 30 mph rear barrier impact without failing." The report critically explains that "it is not the crash acceleration forces that cause system damage which may ultimately lead to fire, but rather structural deformation and direct impact, either by outside objects or other vehicle components."

The basic design criteria formulated to minimize tank explosions:
  • Keep the system design as simple as possible to reduce the number of components which may be damaged.
  • Locate the system components in those areas least vulnerable to impact damage.
  • Increase the resistance of the components to direct impact damage
  • Separate the fuel system from the fire ignition sources (electrical and exhaust as much as possible to reduce ignition possibilities in case of system damage.

As far as locating the gas tank in vehicles the report advised that

"a much safer position is the forward section of the trunk compartment above the rear axle as structural deformation or direct impact does not reach this area except during severe collisions. The fuel tank should not extend to the side of the car but should be cradled between the rear wheels to protect it from side impacts."

In May, 1972 Volvo reported on its Experimental Safety Car: "Volvo's engineers first calculated and analyzed what happened to occupants and the car in a frontal, side and rear collision." The result was to safely position the fuel tank "above the rear axle, at the front of the luggage compartment, with a filler neck at the right front corner of the rear deck."

In May, 1972 U. S. Patent 3,661,419, by Mitsibushi, Rear Body Construction for an Automobile provides for the location of the fuel tank above the rear axle, with upswept floor pan.

In June, 1972 General Motors reported on its Experimental Safety Vehicle Program which began in July, 1970 and which included 11 separate automobile designs. The report states:

"The fuel tank is made of two stamped sheet metal halves of a general trapezoidal shape. The tank is mounted over the rear axle to provide protection in the 50 mph rear end crash. * * * The capacity of the tank is 23 gallons, plus 2 gallons of expansion space."

In 1973 the U. S. Army's System Safety Newsletter, in an article entitled Summary of U. S. Army . . . Experience, reports that the incidence of post-crash fires in helicopters equipped with crash-worthy fuel systems dropped form more than one fire in 11 mishaps to one in 50.

Severy in his article on Automotive Collision Fires, 1974 Society of Automotive Engineers Transactions, Section 4, Vol. 83, 3588 et seq. reports:

"Trucks have a higher crash-fire frequency than passenger vehicles, notwithstanding their superior size and weight. Even a cursory examination of the crash-vulnerable fuel system of most trucks provides the explanation for this undesirable record. The archaic "outside plumbing"designs of truck fuel systems completely obviate any safety advantage the use of diesel fuel may provide over gasoline. A notable exception to these archaic designs is found on the 1973-1974 Ford F-100 - F-350 trucks and the 1973-1974 GMC Motorhome with the fuel tank located between the rugged frame channels. This represents the safest and the most practical location for truck fuel tanks, diesel or gasoline."

Professor Severy notes that the location of the fuel tank and accessories is governed by consideration of the following: 1. protection from front, side, rear, and rollover impacts

In 1974, Ford in its Pickup Buyers' Guide states: "Ford moved the gas tank from the cab to a protected position between the frame rails." In that same year, 1974, the 18th Stapp Car Conference was held. As a result of this conference it was concluded that there are three fundamental fire prevention concerns which confront the automotive safety researcher and automotive designer. These are:

1. Prevent release of fuel

2. Eliminate sources of inadvertent ignition of fuel

3. Isolate motorist from flames, heat and toxic gas to provide an opportunity for escape.

By 1978 automotive engineers had a substantial body of research experience in designing fuel systems. The design criteria which follow are drawn directly from this research base.

Preventing Release Of Fuel

Collision induced fuel spillages result from separation of tank connections, often facilitated by the method of connection to the filler tube, vent and fuel feed lines. In designing a crash-resistant fuel system it should be kept in mind that the greatest number of accidents with fatal burns involve rollover accidents which are likely to cause severe damage to the vehicle. Thus, the ideal crash resistant fuel system must contain its contents both during and after an accident of such severity as to be beyond the boundary of any conceivable survivable accident for the vehicle under consideration. Such a system will focus on three major criteria: 1. Location; 2. Materials; and 3. Connections.

Tank Location

The most basic step in the prevention of fuel spillage is to protect the fuel tank from foreign object penetration through the use of structural members of the vehicle. "Tank locations very close to the rear or front bumpers or tanks at hinge points of structural collapse for locations more remote from the bumper represent intrinsically dangerous installations."

The fuel tank should not extend to the side of the vehicle but should be cradled between the rear wheels to protect it from side impacts. The recommended location of the fuel tank for front-engine vehicles is the space between the rear wheels behind a sealed, metal firewall separating the rear seat backrest from the fuel tank space. The section immediately surrounding the fuel tank should be strengthened to provide protection from penetrating objects."

Special consideration must be given to trucks which have a higher crash-fire frequency than passenger vehicles, due in part, to the archaic "outside plumbing" designs of truck fuel systems. The 1973-1974 Ford F-100 - F-350 trucks have the fuel tank located between the rugged frame channels. "This represents the safest and most practical location for truck fuel tanks."

Tank Structure and Materials

Crash induced fuel spillage occurs either through failure of the fuel tank, fuel lines or both. The tank itself is in danger of tearing due to fluid surge, being trapped in the structure and torn apart, or being cut by jagged metal. The following recommendations to these problems were offered by Harry Robertson in, "A New Look at Fuel System Design Criteria."

To avoid the cutting of tank by jagged metal in a crash, construct the tank of materials which exhibit inherent resistance to cutting. When a penetration does occur, the tank must be resistant to further tear. The 18th Stapp Car Conference concluded that "increasing the thickness of the tank provides a direct approach to improved fuel integrity. Research has shown that plastic tanks of high-density poly-ethylene have about the same dynamic rupture resistance as conventionally constructed metal fuel tanks." In addition designers can place the tank where it can deform or displace rather than remain stationary and be cut.

To avoid tearing of the tank in a collision when the fluid surge pressure exceeds tank material strength and the tank bursts, manufacturers can install the tank in such a manner that when it is being compressed in one area it can displace into another area, construct the tank of a tear retardant material that can withstand internal fluid pressure surges caused by crash loads, and to reduce the likelihood of corner failures due to pressure concentrations, all flexible tanks should have an outside corner radii of at least 1 inch and all inside corners should approximate a 6 inch radius. Often in a crash the tank becomes trapped in the structure and torn apart. To avoid this failure: 1.As the tank passes through structural members following impact, it can become snagged or trapped and torn apart. This snagging threat can be reduced by designing a tank with smooth contours. Rectangular or cylindrical shapes offer the most protection. Tanks with protuberances or composed of several interconnecting cells (such as "saddlebag" tanks) offer the least protection.


Attaching the tank to the surrounding structure through the use of sheet metal screws passing through the tank flange can force the tank to accept the distortions undergone by adjacent structures to the point of rupturing the tank. This type of attachment should be avoided in favor of one that allows the tank to displace rather than deform with the surrounding structure.

Fuel tanks have a tank outlet, quantity indicator and possibly an interconnecting hose between cells which are secured by bolting or other rigid methods to the surrounding structure. These lines are susceptible to tearing, being cut and being pulled from the tank. The following are potential problems and recommendations for preventing this crash failure. During deformation these rigidly attached components move with the surrounding structure, while the tank remains stationary. This relative movement can tear the fittings from the tank. Accordingly all plumbing should exit the tank at one central location in a relatively protected area. Steel wire covered flexible hose (aircraft type) is extremely resistant to being cut or torn. Extra length should be left to allow the hose to shift and displace with the structure rather than be pulled free from its couplings.

It is common for coupling failure to occur at the fuel tank and fuel spillage from coupling failure can be prevented through the use of self-sealing breakaway fittings. The connection can be designed in such a manner that the impact causes the fuel hose to disconnect from the fuel tank and seals both openings simultaneously. The use of a fuel-line pressure control valve will prevent excessive pressure from either bursting the fuel line causing coupling failure. While complete elimination of a filler neck would be ideal, there are very few tank locations where a filler neck is not required.

The following recommendations apply to design and attachment of filler necks. Filler caps should be countersunk into the fuel tank so that they cannot become snagged or torn off by the surrounding structure. Also, a two-stage filler cap should be used in which the initial rotation voids tank pressure through the vapor canister. The longer the filler neck the greater the opportunity for full or partial separation. The filler neck must be prevented from breaking or pulling from the tank during a collision. The chances of such failure are increased if a completely rigid filler neck is used. The solution to this problem is to make a flexible connection at the tank-filler outlet or to incorporate a flexible section between a rigid tank outlet and the filler cap. "If a rigid neck and flexible tank outlet connection are used, the filler neck must extend into the tank at lea accommodate a change in geometry."

Clearly by 1974 all automakers had available the technology and knowledge necessary to design and construct safe fuel systems.

It is difficult to believe that any responsible engineer or manufacturer would choose to ignore these safety suggestions and build a system susceptible to post-crash fire. The 1973 G.M. saddlebag tank, which extends into the mudsill beneath the doors, had great difficulty passing G.M.'s own internal standard, requiring that it survive in a 30 mile per hour moving barrier (flat surface) side impact. The internal standard is known as DD-8A-1 and was approved by the G.T.C. and the S. R. B. committees of G.M. in approximately December, 1971. These committees are known as the "alphabet soup committees."

This internal G.M.C. standard was pushed by the Oldsmobile Division, the engineering leader for G.M., and was more stringent than the then existing Federal Motor Vehicle Safety Standards[FMVSS 301] and required that all G.M. vehicles, whether or not subject to FMVSS, pass a 30 mph front, 30 mph side and 30 mph rear impact with a moving barrier, leaking no more than one once per minute. The thirty mile per hour front impact into a wall requirement, leaking no more than one once per minute evolved from a 1966-7 General Services Administration standard and was the first FMVSS adopted in 1968 for the 1969 production year.

In 1973 the 30 mile per hour into a wall rear impact standard, leaking no more than one once per minute came into effect. Thereafter the standard was changed to provide for a flat moving barrier.

In order to get the saddlebag tanks to pass its own standard, G.M. had to use a non-production vehicle, which was modified by smoothing, rounding and padding internal structures near the tank which routinely severed and gashed the tank in G.M.'s crash tests.

This test occurred in 1972 or so and was saved as proof of the vehicle's safety in side impacts. [National Highway Transportation and Safety Administration standards require self-certification with no reporting to the government, although on request from NHTSA a manufacturer must produce its crash test films, stills and reports to confirm compliance.] Thereafter, at some unknown time, Pete Estes, a G.M. executive, quietly ordered the end to the internal G.M. standard and allegedly this all occurred without formal approval by G.M.'s alphabet soup committees which controlled engineering decisions.

Twenty-eight months later, in early 1974, G.M. again tested a production pick-up following the then proposed federal standard of 20 miles per hour, moving barrier, side impact. The result: 156 ounce per minute failure at 20 mph. At all time during the design and manufacture of the 1973-87 Chevrolet C and K series truck, G.M. had the knowledge necessary to produce a safe fuel system. Whatever the reason for G.M. choosing the saddlebag design, it clearly was not a matter of not knowing better and safer alternatives.

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