Walters Forensic Logo  Publications Contact Us Home 
 

ANTI-LOCK BRAKING SYSTEMS - A FORENSIC NIGHTMARE?

 Although the concept has been around for many years, Anti-Lock Braking Systems have only been used successfully in domestic automobiles since 1985. The concept of Anti-Lock braking is such that during brake line pressure, each wheel continues to turn, thus achieving maximum braking force. This allows for improved control during heavy braking. The benefits of Anti-Lock Braking Systems (ABS) are:

1)

Reduced stopping distance under braking conditions.

2)

Increased directional stability under braking conditions.

3)

Added steering control during maximum braking.

4)

Compensation for different vehicle load conditions.

 

The benefits of ABS braking are most notable during panic braking, braking on slippery surfaces, braking during a turn and directional control needed during braking.

Cars equipped with ABS can and often do leave perceptible measurable tire marks in emergency situations. This is not commonly known, since lockup is generally avoided or limited to only a split second.

There are two ABS systems presently in production on passenger cars and light trucks.

1)

The two wheel system reduces stopping distance in many conditions by acting on the rear wheels. This improves directional stability compared to conventional vehicles. However, the front wheels may still lock.

2)

The four wheel system eliminates lockup on all wheels.

 

Typical locked wheel marks are essentially straight, prominent from end to end, and end rather abruptly. Road surface conditions influence the length and darkness of these marks. With ABS, the typical brake mark resembles the straight line skid of conventional vehicles except in one respect. ABS leaves marks that are lighter and therefore harder to detect. Since these marks are lighter, traffic conditions, weather and rain can wash away these marks easier than non ABS skids. Therefore, extra care is necessary when examining the scene where ABS applies.

Another important difference between ABS and conventional tire marks is in their pattern. Conventional marks are solid continuous streaks if not interrupted by pedal pumping or irregularities in the road surface. They often begin and end abruptly in addition to their prominence. ABS on the other hand may follow an intermittent light and dark pattern. The reason for this is that sensors relay wheel speed information back to the microprocessor in the vehicle's computer. There is a slight delay in the feedback system which allows the wheel to cycle from higher to lower brake force, resembling a consistent cyclical pattern. Flat spots are less likely on ABS since the tires are less likely to lock. Therefore, flat spots will not be evident unless the vehicle has spun sideways.

ABS provides steering control under extreme braking conditions by reducing the braking force sufficiently to allow the wheels to continue rolling. The tire's ability to sustain cornering forces is maintained. If the vehicle is in a broadside slide, skidmarks may be the same regardless of the braking system.

In summary, it is important to remember that anti-lock vehicles when braking heavily may leave tire marks and these tire marks can have distinct characteristics just as skids from conventional vehicles follow recognized trends.

ABS marks are often lighter than those of conventional vehicles, have intermittent dark spot patterns and have less tire effects.

ABS is sure to become more widely used. This paper was designed to help you recognize ABS marks for efficient and thorough accident investigation.

 
The information contained in this web site is intended for marketing purposes only. It is not all-inclusive, and does not fully describe the many and varied services that the company provides, nor does it completely describe the education, training, skills, or expertise of our staff.

 
 
 

Walters Forensic Engineering | 277 Wellington Street West, Suite 800 | Toronto, ON M5V 3H2
Information contact: engineering@waltersforensic.com