Automotive Mileposts  

1974-1976 Cadillac, Buick, and Oldsmobile
Air Cushion Restraint System (ACRS)

Image: 1974 Cadillac Air Cushion Restraint System instrument panel

Above: 1974 Cadillac instrument panel modified for factory Air Cushion Restraint System installation. Note the wide steering wheel center padded hub, the four spoke steering wheel with horn buttons mounted on each hub, and the relocated glove compartment box, moved to the lower center section of the instrument panel. A small storage compartment is located on the left side of the steering column, and the ash tray below the radio was also redesigned and relocated.

Image: 1974 Cadillac ACRS instrument panel

1974-1976 GENERAL MOTORS AIR CUSHION RESTRAINT SYSTEM AT A GLANCE:

Years Available:
1974, 1975, and 1976 only
Model Availability: 1973 Chevrolet Impala and Caprice (experimental fleet units)
1974-1976 Cadillac (all, except Eldorado Convertible and Fleetwood Limousine)
1974-1976 Buick Electra 225 and Riviera
1974-1976 Oldsmobile Delta 88, Delta 88 Royale, Ninety Eight, and Toronado
Number Built: 10,000 total over 3 years; see approximate numbers below:
1973 Chevrolet: 1,000 (experimental test units)
1974 Electra: 1,000
1974 Riviera: 329
1975 Electra: 123
RPO Code: AR3
Price: (Cadillac)
1974 - $225.00
1975 - $300.00
1976 - $340.00

GM Air Bag Chronology:

1972: The Chevrolet Division of General Motors Corporation builds 1,000 1973 Chevrolet Caprice and Impala models equipped with experimental air bags. They are provided to fleet customers under an agreement for testing purposes.

1973: First air bag fatality occurs. An unrestrained infant placed on the front passenger seat of one of the experimental Chevrolets is killed when the passenger-side air bag deploys in a wreck.

1973: In October, a 1974 Oldsmobile Toronado rolls of the assembly line in Lansing, Michigan. It is the first production car built with the Air Cushion Restraint System (ACRS) option that was intended for retail sale.

1973: On December 6, 1973, the first Buick equipped with ACRS was completed at Factory 4 in Flint, Michigan. It was a 1974 Electra 225 Limited that was painted Crystal Lake Blue Metallic.

1974: GM begins building Cadillacs, Buicks, and Oldsmobiles with ACRS, and states they expect to sell 100,000 a year.

1976: After three years of marketing, the option is dropped after just 10,000 vehicles with the air bag option were built during the 1974, 1975, and 1976 model years.

The Air Cushion Restraint System (ACRS) introduced by General Motors as an option for some of its full-size Cadillac, Buick, and Oldsmobile models in the fall of 1973 was a two-part system, with an air cushion designed to protect the driver, and another the front seat passenger. The driver's side restraint cushion and inflator were completely contained within the large center hub of the steering wheel, which differed greatly in appearance from the stock steering wheels. The four-spoke steering wheel incorporated a horn button on each spoke, and the center color coordinated portion was embossed in a manner that allowed it to tear away in the event the air cushion was deployed. For 1974, the ACRS option was initially offered only in conjunction with the standard (non-tilting) steering column. Additionally, a vinyl covered, foam-cushioned knee restraint extended below the instrument panel on the driver's side. In the event of a collision, this padded restraint helped maintain the position of the driver in the seat so the steering wheel cushion could properly restrain the driver without injury.

The air bags were designed to restrain the driver and front seat passenger in certain front end collisions equal to an 11-mile-per-hour barrier crash and up to the severity of a 30-mile-per-hour barrier impact. Consider this: at 30 miles per hour, the car is moving at 44 feet per second, and at this speed, hitting a solid barrier such as a concrete wall or bridge abatement would force the car to a complete stop within one fifth of a second! And that's from the instant of contact through the rebound from the barrier. In a crash of that severity the driver and front seat passenger would be thrown forward violently, so in order to restrain them before they could impact the car interior, the ACRS must sense that a severe crash is occurring and deploy the air cushion restraints within forty thousandths of a second!

In order to accomplish this, the ACRS system used two separate crash sensing devices. One of them was mounted at the front of the car, approximately at center, behind the front bumper. The second was mounted inside the car, under the instrument panel. In the majority of front end collisions, the front bumper is the first component on the car to make contact, and if the contact results in an instantaneous sustained deceleration at the bumper of 11 mph or more, switches in the Bumper Impulse Detector close immediately and deploy the air cushions inside the car.

The second sensor, the in-car crash sensor, is designed for collisions where there is no front bumper impact. It was able to deploy the air cushions in all frontal impacts. This sensor incorporated three pendulum-type "G" switches with two calibrated at close to 18 G's as measured at the sensor. This is equivalent of an 11 mph barrier impact at the front bumper. In extremely severe frontal crashes, the right front and center passengers need additional restraint, so if the crash deceleration reaches 30 G's at the in-car sensor, the third pendulum switch closes and begins to generate gas for the passenger restraint cushion. This second stage, high level of inflation would only occur if the car impact reaches a severity level equivalent to an 18 mph frontal barrier crash.

The passenger side of the air cushion system consists of an inflator assembly, gas distributor, and a large air cushion unit. The inflator includes compressed gas with two gas generators. In a low level deployment, all of the compressed gas is released and one gas generator is ignited. The ignition of the second gas generator occurs only in high level deployment situations, where the crash deceleration reaches 30 G's as measured at the in-car sensor. When deployed, the passenger side air cushion extends laterally from the steering wheel to the right front door, and it includes a built-in knee restraint. The knee cushion remains firm during the collision, to help maintain the passenger's position in the seat, but the torso cushion is porous and leaks down at a controlled rate to dissipate the passenger's impact force.

A chemical gas generator deploys the driver's restraint cushion, and the driver's cushion measures approximately 22 inches when fully deployed, and it maintains an air pressure of 3 psi to cushion the driver from impact with the steering wheel. The impact force of the driver hitting the steering column is dissipated by the energy-absorbing steering column, a standard feature on GM cars since 1967. The steering columns used in ACRS vehicles were specific to those cars, but the same basic principle of absorbing energy was similar to other GM columns.

Ignition switches are unique to GM cars with ACRS, and the correct one can be very difficult to find today. Also, a warning label was mounted in the glove compartment advising that the car was part of a National Highway Traffic Safety Administration (NHTSA) study, and a toll free number was provided to call, day or night, should the vehicle be involved in a collision.

ANATOMY OF AN AIR BAG DEPLOYMENT

Image: GM's Air Cushion Restraint SystemIn a 30 mph frontal barrier crash, the sequence of events, as represented in thousandths of a second, is as follows:

TIME - ZERO: This is the moment of initial front bumper contact, no velocity change at the bumper has been noted, so there is no crash information available.
TIME - SIX THOUSANDTHS OF A SECOND: A deceleration of 11 mph is sensed at the Bumper Impulse Detector. Instantly, the gas generator starts and the driver's cushion deploys. At the same time, the compressed gas in the passenger's side inflator is released, and one gas generator is ignited.
TIME - 15 MILLISECONDS: The complete car begins to decelerate rapidly, and the occupants inside the car begin to be thrown forward. By this time, the passenger side knee cushion is almost fully inflated, and the large torso cushion is filling rapidly.
TIME - 25 MILLISECONDS: The driver's side cushion has been fully inflated, and the crash deceleration at the in-car sensor has reached 18 G's.
TIME - 32 MILLISECONDS: The crash deceleration reaches 30 G's at the instrument panel, and the in-car sensor now activates the second gas generator in the passenger inflator. By this time, the passenger has moved forward to the point of full knee restraint, and their torso is being enveloped and held by the large restraint cushion.
TIME - 50 MILLISECONDS: At this point, the crash force has thrown the driver fully forward into the knee restraint and steering wheel cushion.
TIME - 80 MILLISECONDS: The force at the driver's chest and knees peaks, and the steering column begins to compress to dissipate the impact energy.
TIME - 100 MILLISECONDS: The crash and forward motion of the car has nearly ended, and the occupants are being held in maximum restraint. Occupants are no longer moving forward, in relation to the car interior, although forward movement of the car structure behind the front seat occupants may still be occurring.
TIME - 120 MILLISECONDS: The car is bouncing backward, away from the barrier, and most of the driver's impact force has been absorbed by the energy-absorbing steering column and knee restraint. Some air remains in the driver's cushion, and this combined with the car's rebound impulse forces the driver rearward into the seat back and head restraint.
TIME - 150 MILLISECONDS: Driver rebound has stopped, and motion of the driver is completed. Passenger rebound is still underway, as the passenger's torso moved further forward without the steering column to stop them.
TIME - 200 MILLISECONDS: Most of the passenger's impact energy has been dissipated by the controlled leakage of the large torso cushion, but gas generation continues, and in combination with the rebound motion of the car, the passenger is forced rearward into the seat back and head restraint.
TIME - 201 MILLISECONDS: Gas generation ends, and passenger rebound is completed. The air in the restraint cushions dissipates, and the cushions deflate rapidly so they don't interfere with passenger movement.

To fully appreciate the speed of the sequence of events just described above, the reader must consider the entire sequence of a 30 mile-per-hour barrier crash is completed in the same time as the blink of an eye. One-fifth of a second from instant of impact to rebound from the barrier! Pretty amazing, isn't it? And even more difficult to comprehend is that ACRS must not only sense the crash, but it must also deploy the restraints in the first fifth of that fifth of a second! That's 1/5th of 1/5th of a second. Within 40 milliseconds after contact with the barrier, the system must accomplish all of this.

To say that this system is impressive is an understatement. Technology and testing comparable only to the space program went into the design of the ACRS. Did it work? Absolutely! And, the system was so well built and designed that even decades later, GM cars equipped with the Air Cushion Restraint System in the mid-seventies that were involved in serious frontal collisions still had the air bags deploy and cushion the impact, just as designed! Automotive Mileposts has had accounts of a very severe accident involving a 1974 Oldsmobile Toronado in the late 1990's where a fatality occurred in the other car, but the elderly woman driver of the Toronado walked away from the crash! Another report came in from the Insurance Institute for Highway Safety, which tested two of the 1973 Chevrolet experimental fleet test cars in 1993. One had been stored for decades in a shed, had over 100,000 miles on it, and had to be towed to the test site. It received only a new battery. Nothing else was done to it. When the car was crashed into the barrier, both air bags deployed as designed.

The second test, conducted by the Vehicle Research Center of the Insurance Institute for Highway Safety, crashed a 1973 Impala built in 1972 head-on into a barrier at 25 mph. This car also had over 100,000 miles on it. The testers noted that neither the clock nor the radio worked on this car, but both air bags worked perfectly during the crash. This really goes beyond impressive.

People can say what they want about the quality of American made cars in the 1970s, but this is proof to us that superior technology did exist, and was used. General Motors is to be commended for taking the first steps in putting safer cars on the road at this time, and it's a pity more people didn't understand the value and benefits of owning a car equipped with air bags. As classics, we feel GM vehicles equipped with this option are worth seeking out. Not only for the rarity of the air bag option, but also for the increased safety factor.