Carl F. Thelin

 HEAD FIRST IS BEST

These are notes that I made for Bob Knoll at the Consumers Union Auto Test Division. CU is the publisher of CONSUMER REPORTS magazine. These are my thoughts about the benefits and handicaps of the Anti-Lock Brake System (ABS). Bob asked for these ideas soon after anti-lock brake systems began appearing on production cars in the US. In general, my 40 year background has always focused on the relationship between the driver, the environment and the vehicle. Even though I was the designer of experimental vehicles at the General Motors Engineering Staff, I was aware of the research done by the General Motors Research Staff on the environment and drivers. Further work at Calspan and now at Consumers Union reinforced this.                

             UNPREDICTABLE DRIVERS

Drivers have unpredictable responses to threats when they are under the influence of alcohol, drugs or lack of sleep.

            UNPREDICTABLE ENVIRONMENTS

Whenever the driver is confronted with a dynamic situation where he would be surprised by the response of the vehicle, there is potential for a collision or some other bad consequence. An example of unpredictable environment: the parking lot pavement is very slippery when wet because someone applied Jennite-type asphalt sealer. A long brake slide and maybe a spinout might be the response to ordinary brake application.

Another example: another vehicle exits from a private driveway located on the inside of a sharp curve. The on-coming driver with the right of way has a sightline too short to see and slow down in time. Emergency braking and perhaps a swerve into the other lane might be the action causing an unexpected behavior.

            UNPREDICTABLE AUTOMOBILES

The early models of the Chevrolet Corvair, which Ralph Nader described as "unsafe at any speed", were unpredictable for the ordinary driver. This was a car that had a self-steering characteristic of understeer (the harder you steered, the less response you got) at ordinary levels of cornering severity -- like almost all other cars. However, when the Corvair driver attempted to avoid a collision such as the one described just above, he found that the car handling behavior quickly transitioned into oversteer. That meant that the car responded more quickly than he expected, turning so sharply that it went beyond the limit of his control. With that too-sharp response the car might go off the opposite side of the road and overturn, or slide sideways into a collision. The Corvair also was unusual in that it could overturn while on the pavement, whereas most vehicles did this only if they tripped on something or went down an embankment.

           

            UNPREDICTABLE ORDINARY BRAKES

When drivers press on the brake pedal with a certain force, they expect that there will be a proportionate degree of slowing down. They expect there will be a predictable relationship between the force on the brake pedal and the deceleration of the car. Most drivers also expect that, in an emergency, they can press the pedal very hard and lock the brakes -- even though this is not good for the tires.  Another example of unpredictable behavior is the lock-up of the only the rear wheels when brakes are applied quickly on a wet road. There were many examples of this behavior. The lesson of the GM X-car brakes of the early 1980's is still relevant. Those cars braked well on dry roads. On wet roads they did not. The rear wheels stopped turning first and the car began to slide sideways. If the drivers were trained to respond correctly, they would have released the brakes as soon as they felt the rear wheels begin to slide and then reapplied the brakes more smoothly. Unfortunately, most drivers do not have frequent opportunities to practice emergency recovery, as do most auto-industry test-car drivers. Most people do not experience this sort of bad behavior until they are in the process of braking hard because they began braking too late for gentler braking.

            UNPREDICTABLE ANTI LOCK SYSTEM BRAKES

In the case of the Chevrolet Caprice Police cars with Antilock Braking Systems (ABS), the unpredictable response of the vehicle was a strong, noisy pulsation of the brake pedal. The danger of the unpredictable noise and vibration is that the driver may momentarily release pressure on the brake pedal. Even if he pumps the pedal, all he is doing is increasing the stopping distance.

But, on a car that needs ABS, when the brakes stop the back wheels from turning while the front wheels are still turning, the loss of lateral force control at the rear inevitably means that the vehicle will spin sideways. Then it may go off the road, roll over or crash sideways. The antilock brake system was developed to prevent this last unpredictable response no matter what the pavement was like.

            THE REAL BENEFIT OF ABS IS THAT YOU CRASH HEAD-FIRST

As previous work on roll-over propensity has shown, the problem is not simply that some vehicles are tall and skinny. Even a Lincoln Town Car will overturn if it slides sideways off the road onto soft earth, or down a steep embankment.  Rollovers that occur while the vehicle remains on firm pavement are limited to those that perform poorly on the NHTSA Tilt-Table test – they are what we might call top-heavy or too narrow. The real reason cars roll over is that they get sideways in situations where the majority of other vehicles would not. In addition to rear-wheel lockup, this is a problem with the handling. Quick steering and short wheelbase are indicators here.

But, even if the vehicle does not overturn, there is still a greater hazard to occupants when the vehicle slides sideways out of control. Whether it slides into a pole or another vehicle, or it is struck on the side by another vehicle, the risk of injury is greater. Thus the real benefit of the ABS is that many vehicles that would have been involved a rollover or a spinout will now crash head first, where the occupant systems have been optimized after many years of attention to FMVSS 208 and the others that address frontal crash protection.