Many drivers can recall a near miss saved by the quickapplication of brakes, while a smaller number remembers poundingthe brake pedal too late and crashing. What does this subset havein common? Significant numbers of them will blame their accidentson brake failure. Most of them will be wrong.

The reality is drivers judge distances poorly, especially whentraveling at freeway speeds. Typically, they have no idea how theirbrakes behave in critical situations and consequently believecrashes result from mechanical malfunction, when in truth theirbrakes performed as designed, but their following distances weretoo short for their speeds. The standardization of electronicallycontrolled antilock braking systems (ABS), which prevent wheellockup and skidding during heavy braking, has compounded theproblem because ABS can make strange grinding noises when itactivates, leading untutored drivers to suspect braking issues.When coupled with a human tendency to avoid blame, brake failurebecomes the commonplace excuse for many crashes.

As a result, a significant percentage of the workload forforensic automotive investigators is brake-failure examinations.Despite the frequency of the claims, actual malfunctions in thesecomplex systems are rare, although they can happen. Investigationrequires the input of a trained automotive specialist who knowswhere to look for problems in braking components to find thetelltale evidence of failure. Often issues associated withneglectful maintenance by owners impede brake function, especiallywhen optimal performance is required.

Brake Physiology

In a typical automotive disc-brake system, when the driver depresses the brake pedal,that pressure is transferred to a system of fluid-filled brakelines. The fluid then transmits the pressure to the calipers,clamping the brake pads against the brake rotors, which areattached to the wheels. The friction generated between the pad andthe rotor provides the force that stops the vehicle. Loud squeakingfrom the brakes when applied usually signals brake pads that arereaching the end of expected use.

Wear and Tear

Pad or shoe wear from normal usage is the most common brakemaintenance issue, and examining the parts of the braking system istypically the first step for a forensic investigator. He or shewill look for unusual wear in the pads or shoes. That effortentails removing the vehicle's wheels to access the brake pads andmay require removing the pads for more precise measurement.

When an investigator removes the brake pads,he can also inspect the pad surface. A smooth, reflective qualitymay indicate “glazing” caused by overheating, which decreasesstopping power. Overheating may also result in brake rotordiscoloration, and is usually caused by excessive brake “riding” bythe driver.

Less commonly, brake failures may be caused by excessive rotoror drum wear. With the wheels removed for the pad inspection, thebrake rotors or drums can then be checked for thickness, diameter,and warping to ensure that they are within manufacturerspecifications. Rotors and drums need replacement less frequentlythan pads and shoes, but excessive wear can contribute to brakefailure, especially if the pads or shoes are similarly worn.

Examining Fluid Issues

In a typical hydraulic brake system, a brake fluid leak canresult in complete brake failure if the fluid level drops too low.Such leaks can develop from excessive wear because of vibration orage, or from collision or roadway hazard damage. For example, aninvestigation of a suspected brake failure in a medium-duty truck that sustained significant crash damage in afreeway accident revealed that the truck had a brake-line designthat allowed the primary fluid lines to rub against one another.The constant friction of the rubbing lines culminated in a suddenfluid leak and a complete loss of braking power.

Inspection for fluid leaks is simple and should be done bymechanics as part of routine maintenance. A forensic investigatorexamining a crashed vehicle will have little trouble spotting aleak, but determining its cause may be more complex, sometimesrequiring laboratory examination with advanced equipment.

Another fluid-related cause of brake failure is the introductionof small air bubbles into the brake lines. Air bubbles occur whenoverheating brake fluid reaches the boiling point (brake fluid agesand has a limited usage life) or a slow leak allows air into thesystem. (A boiling point or other chemical test can determine ifthe vehicle's brake fluid has exceeded its useful life.) Theresulting bubbles interfere with the brake system's ability totransmit braking force to the wheels, causing the brake pedal tofeel softer than normal and making it very difficult to applymaximum braking effort.

Such a condition can contribute to an accidentif a driver does not have sufficient stopping distance when facinga hazard. Once again, the way to avoid failures because of taintedor deficient fluid brake failures is proper preventativemaintenance practices by the vehicle owner. This may includechanging the brake fluid in accordance with manufacturerrecommendations and regular brake bleeding to eliminate any airintroduced into the system.

Potential Mechanical Problems

The most unexpected cause of brake failure that investigatorsmay encounter is a mechanical failure in the brake pedal assembly.In rare cases it is possible for the master cylinder (the componentthat distributes the brake pedal force to the wheels) to becomedetached from the pedal linkage. This condition is immediatelyobvious to an inspecting investigator, so the real item of interestwill be pinpointing the cause of the linkage failure, whether it isrust, tampering, or metal fatigue.

Tapping Accident Data

Increasingly, crash data retrieved from a damaged vehicle'sevent data recorder (EDR) is providing useful insight to accidentinvestigators. If the module is accessible in a brake failureexamination and has collected the crash event data, then theforensic investigator can gain insight about driver actions in theseconds that preceded and followed impact. A brake failureallegation could wither away if that data reveals that the drivernever engaged the brakes prior to a collision. More likely, the data will deliver the context thatwill guide the further investigation of a brake failureallegation.

Perhaps the record will show that the driver was not speedingand applied the brakes suitably, but did not stop quickly enough toavert disaster. That could open the possibility of a brake problemthat should be identified in the claims resolution process.

A brake failure from a maintenance issue could implicate arepair shop, thus identifying a subrogation opportunity, whereas an original equipment failurecould result in a product liability action.  

Managing Brake Failure Claims

While brake failure is probably the most common excuse for acrash, cumulative experience shows that driver error probably trumps actual brake malfunctions as theprimary cause of accidents because of braking issues. Although itis easy to be swayed by a dramatic recitation of the crash story,if there is any doubt about the logic or sense of the eventsequence, consult an automotive expert.