Undercar Information

Form 2623T, 1/90

The evidence is now in to suggest that vehicle defects, particularly brake system defects, are an avoidable cause of many automobile accidents.

According to studies by the National Highway Traffic Safety Administration, vehicle defects were the sole cause of one out of every 43.4 fatal accidents studied. Vehicle defects played a partial role in a much larger percentage of accidents.

A multi–disciplinary study conducted by the State of Indiana concluded that vehicle defects were possible causes in 25.2 percent of all accidents studied. In addition, the Indiana study conclusively demonstrated that nearly half of all defects found were in the vehicles' brake systems. Approximately 42 percent of the brake defects were wheel–to–wheel or axle–to–axle imbalances or other performance degradations that can be diagnosed through testing.¹

Even if brake tests resulted in only a one percent reduction in vehicle accidents, annual savings would include 460 fewer deaths, 34,000 fewer injuries, and $740 million less damage.

While the technology needed for testing brake performance is widely available and economically justifiable for most auto service centers, it has not yet achieved universal use.

Compared to other automotive system testing techniques, brake testing in the United States remains relatively untapped. Just as their early predecessors did, today's technicians rely overwhelmingly on automobile test drives to check for pull, vibration, noise, and other signs of defects. To inspect the braking system, they demount one or more wheels and make a visual inspection, even though far more reliable, cost–efficient testing methods are available.

This report will analyze the current status of brake testing, including the mandated requirements for brake testing in Europe and Japan. It will discuss the safety issues associated with brake systems and the role that brake testing plays in the overall car service mix. It will also provide a technical review of brake systems, including the latest proportioning techniques and anti–lock brake systems, and will delineate the variety of brake testing methods available today.

The European Connection

In the late 1970s, high speed highway driving and congested, narrow-street urban driving prompted the regulatory bodies in a number of European countries to mandate strict brake inspection as part of a periodic vehicle inspection program. This effort grew rapidly during the 1980s.

Safety was the primary impetus behind advanced brake testing technology in Europe and Japan. Systems developed there provide a quick “go/no-go” check of a vehicle's braking ability, offer a diagnosis of the braking system, help a shop merchandise its brake services and provide a safer vehicle for the car owner.

Periodic brake inspections are mandatory in a number of European countries, where high speed highway driving and congested urban driving prompted the development of sophisticated brake testing technology.

The Need for Brake Testing in the United States

While the United States has lower speed limits and better infrastructures than many European countries, its automobile accident rate is not correspondingly lower. Last year, 45 million vehicles in the United States were involved in accidents that claimed 46,000 lives and injured 3.4 million people. On average, 125 auto–related deaths occur every day, the equivalent of a major airline crash each day of the year. The government estimates the resulting economic cost to be $74 billion.

According to studies, brake system defects are the single largest cause of automobile defect–related accidents in the United States.

To help reduce the frequency and severity of automobile accidents, 20 states and the District of Columbia provide Periodic Motor Vehicle Inspection Programs to detect automobile safety defects.

An agency of the Department of Transportation (DOT), the National Highway Traffic Safety Administration (NHTSA) was formed in 1970 with a mandate from Congress to reduce deaths, injuries and financial losses due to automobile accidents.

Under the mandate, the NHTSA established state Periodic Motor Vehicle Inspection (PMVI) programs. Because of its ability to control congressional funding to the states, the NHTSA can influence state vehicle safety programs. Currently, 20 states and the District of Columbia provide PMVI programs.

One strong national supporter of PMVI programs is the Coalition for Safer, Cleaner Vehicles (CSCV). Composed of automotive industry associations, consumer groups, state officials, individual companies and others, the CSCV believes “available data conclusively establishes the contribution of vehicle safety defects to crashes and the ability of effective PMVI to detect and correct such defects.”

Brake Service Market

Since brake system defects are one of the largest causes of automobile defect–related accidents, it is understandable that brake service is one of the most common of all the jobs in the typical shop's service mix. It is estimated that approximately 80 percent of the repair shops in the U.S. offer brake service. In several market segments – mass merchandisers, new car dealers, and tire dealers – the number is essentially 100 percent.

General repair shops average fewer than 10 brake jobs per week; mass merchandisers average 35. The overall average is 11 jobs per week.

In 1989, a typical four-wheel brake job cost approximately $165.67, with $85.67 in parts and $80 in labor. Profit runs in the 30-40 percent range, the highest percentage of any common repair service performed.

Approximately 80 percent of auto repair shops in the United States offer brake service, averaging 11 jobs per week.

Type of Service

Average $ per job type

In 1989, a typical four-wheel brake job cost $165.67, with $85.67 in parts and $80 in labor — the most profitable of all ordinary repair service.

Typical Brake System Layout

Modern brake systems have numerous parts, many of which can not be effectively evaluated with visual inspection.

Brake System Review

Although today's automotive brake systems are far more complex than those of 20 or even 10 years ago, the basic principles of operation remain unchanged. To be stopped, the energy of a moving vehicle must be converted to heat through the process of friction.

The diagram illustrates what happens when the brake pedal is compressed. First, brake fluid is forced from the master cylinder to each of the vehicle's wheels. Then, the fluid pushes pads on the front brakes and shoes on the rear brakes against the rotors and drums which are attached to the wheels. The friction between the stationary pads/shoes and the revolving rotors/drums causes the rotors/drums to slow and stop. This slowing action, in turn, slows and stops the wheels and the car.

Because of static weight distribution and rear-to-front weight transfer during braking, front brakes have more stopping power than rear brakes. The faster the stop, the greater the weight transfer. As a result, most systems use controls to regulate front and rear braking action. Two valves are common:

Metering Valve– Located in the front brake circuit, the metering valve remains closed until the front brakes reach a specific pressure. The valve's action prevents the front discs from applying until the rear shoes contact the drums, ensuring that front and rear brakes are applied at the same time.

Proportioning Valve– Located in the rear brake circuit, the proportioning valve limits pressure to the rear drum brakes so they will not lock during hard braking. Both the metering and the proportioning valves are often combined with a brake warning light switch in one assembly called a combination valve.

A separate mechanical parking brake system functions independently from the service brake system. Operated either by a foot pedal or a hand lever, the system typically uses a cable to actuate a lever assembly in the rear brakes, which in turn pushes the brake shoes against the rear drums.

The antilock brake system (ABS), a device which prevents brakes from locking, continues to gain popularity among the world's automakers. Like other braking systems, the ABS permits normal braking operation during routine stops. But during hard braking – when a brake could lock and cause a wheel to skid – ABS acts to prevent the condition.

Using an electronic monitoring device, the ABS senses the rotation of each individual wheel as it relates to the rotation of all the other wheels. If one wheel starts to slow before the others during braking, the monitoring device, working with a computer, signals a hydraulic modulator to release and restore brake line pressure to prevent the brake from locking. This process continues until that wheel catches up with the others and normal braking resumes.

Brake Testing Methods

A variety of brake testing methods are used today, offering varying levels of accuracy and reliability. The advantages and disadvantages of the most common methods – road tests, visual inspections, roller tester systems, and plate tester systems – will be explored here.

Road Test

Road testing is one of the most popular methods for checking vehicle braking capability. In a road test, the technician simply drives the vehicle, applies the brakes, and observes the results. This test can be performed on public streets, on the service facility's parking lot, or as a vehicle is driven into the service bay.

The road test, while one of the most popular methods of brake testing, is also one of the least objective, relying purely on the experience, skill, and opinion of the driver.

To enhance test consistency, the most sophisticated road tests are performed on a measured section of pavement on an unused public street, parking lot, or test lane. The Motor Vehicle Manufacturers Association (MVMA) and the American Association of Motor Vehicle Administrators (AAMVA) recommend that the brakes be applied firmly as the vehicle is driven 20 mph. The driver observes the action of the vehicle: vibration, nose dive, pull, and other signs of defects – and measures the vehicle's stopping distance. The vehicle must come to a straight stop within 25 feet (1971 and earlier vehicles) or 20 feet (1972 and later vehicles). Seldom performed alone, road tests are often combined with a visual lining inspection.

Advantages– A repair shop incurs no equipment cost for this method of testing. Since it is a dynamic test, the driver observes real–world operation. If a measured stopping distance is used, it provides some objective results.

Disadvantages– While offering some objective results, a road test performed on a test lane is dependent on a number of human variables. It relies heavily on driver judgment – speed, pedal pressure and moment of brake application – each of which can vary markedly from test to test.

A test performed on an unmarked section of pavement is even less objective and reliable. Results are based purely on the experience, skill, and opinion of the driver. The test reflects only the driver's opinion of the brake “feel” and “sound.” Further, road testing is time–consuming and dependent on road conditions and weather.

Visual Inspection

The visual inspection is currently the most common method of brake testing used in the U.S. today and is the method most often used by state PMVI programs. This “quick stop and pull-a-wheel” method of testing typically specifies that one or two wheels are examined. Profit-motivated repair shops pull all four wheels to be sure all potential problems are spotted.

Advantages– With this type of test, the technician sees and feels the actual brake system, making it the only positive means of checking brake pad and shoe lining thicknesses. Even on vehicles with lining wear indicators, a visual inspection is the only way to verify system operation. While not necessary for every brake test, periodic lining thickness inspections are recommended at certain mileage intervals, especially on older and high mileage vehicles.

Often performed in conjunction with a road test, a visual inspection allows a technician to examine the actual brake system for lining wear and other problems.

In a visual inspection, a technician can also detect scoring in the drum and rotor, check the brake hardware, observe leaks, and check the condition of brake hoses. By pulling all four wheels, a technician can also compare left-to-right and front-to-rear wear as an aid to identifying problems.

Disadvantages– An overwhelming disadvantage of the visual inspection is that it alone cannot verify brake system operation. Seldom performed by itself, the “pull-a-wheel” inspection is generally accompanied with a “quick stop” road test.

Other than measuring lining thickness, the results of a visual inspection are mostly subjective. Unless the state PMVI program sets standards, for example, it's difficult to determine what lining thickness is acceptable. A damp wheel cylinder boot may be cause for concern for one technician; another could see it as absolutely no problem. Opinions also vary widely on brake hoses and other system elements.

Roller Tester Method

Sometimes called a dynamic brake analyzer or a brake test dynamometer, the roller-type brake tester is a method of in-shop testing rarely used in the United States. The most commonly used European systems use two motor-driven roller assemblies, which are installed side-by-side in the shop floor. The roller assemblies test a vehicle's braking force one axle at a time. (Systems with four roller assemblies capable of testing front and rear axles simultaneously are also available.) Typically, the front axle roller assembly is stationary while the rear axle roller assembly can be moved to match the vehicle's wheelbase.

During the testing operation, a vehicle is driven onto the rollers, which are spun at a speed of two-to-four mph. The brakes are then applied, as the roller tester measures the braking force at each wheel. Results are displayed on a panel or meter that comes with the brake testing system.

Some systems provide graphs which plot the braking force of each wheel relative to time, and the brake pedal force versus time. These graphs can be an aid in analyzing brake system performance. A printer which provides a hard copy of data is often part of the system package. The roller tester is equipped to check parking brake operation as well.

Advantages– The major advantage of the roller–type tester is its ability to sustain measurements. A technician can maintain a steady brake pedal pressure for as long as necessary to check and double check operation of the brake system. Brake pressure can be varied to simulate a variety of stopping conditions and to isolate problem areas. This method, for instance, makes it easy to spot out–of–roundness problems. Since the brakes can be warmed up, it is also possible to identify problems that relate to heat, such as brake fade.

During a roller-type brake test, a vehicle is driven onto spinning roller testing equipment. As the brakes are applied, the braking force at each wheel is measured.

Because pedal force is easily controlled, repeatability and overall accuracy are very good. Since the vehicle is stationary, both the single axle and the double axle systems fit in a standard bay.

Disadvantages– The main disadvantage of the roller tester is its inability to take into account weight shift during braking. Consequently, it cannot measure real-life front/rear brake balance. A vehicle with an 80/20 front-to-rear brake force distribution during actual braking, for example, could conceivably check out with a 52/48 distribution on a roller tester – obviously a false indication.

Incapable of producing a nose-dive stop, the system cannot accurately check the performance of all the components that come into play during dynamic braking-theoretically, every single element of the braking system. These components include the proportioning valve, pressure control valve, and metering valve.

Because they use low wheel speeds, roller testers cannot test some anti-lock brake systems. Anti-lock systems that operate by sensing rotation on all four wheels, for instance, cannot be tested on a single-axle type roller tester. Likewise, constantly engaged all-wheel drive vehicles cannot be tested on a single-axle type roller tester.

Because they require in-the-floor installation, roller testing systems are expensive to install and difficult to relocate. Compared to a road test or visual inspection, the three-to-five minute test time is relatively quick. Its speed of operation, however, is still slow when compared to other more efficient testing systems.

Plate Tester Method

With a plate brake testing system, either two or four steel plates are installed on the shop floor, with a display console alongside. (Four plates allow all four wheels to be tested simultaneously; two plates test one axle at a time.)

During testing, a technician drives a vehicle onto the plates at four-to-eight mph and then applies the brakes. Working with a computer, force transducers in the plate assemblies measure the braking forces, calculate the results, and display them on a panel or a CRT. Some systems provide a front-to-rear and side-to-side brake force reading. Some graphically record individual wheel brake forces, which technicians can then use as an aid in understanding system performance. A printer to provide hard copies of the data is usually included as part of the system package. A brake pedal force gauge is often available.

Using a computer and steel plate assemblies to simulate “on the road” conditions in the shop bay, plate brake testers check brake balance, deceleration, sideslip and parking brakes-in thirty seconds or less.

Advantages– The major advantage of a plate tester is its ability to instantaneously and accurately measure front-to-rear and side-to-side brake forces. Because it takes into account the rear-to-front weight shift during braking – just as it would occur on the road – it can detect problems other testing methods miss. The braking force graphs show the operator how each of the four wheels is performing versus the others, and thereby help technicians identify the component problems that come into play during dynamic braking.

With plate brake testers, technicians can test brake balance, parking brake performance, deceleration and sideslip in thirty seconds or less, making it the most time-efficient testing method available. Because the system's steel plates bolt to the floor, plate testers are inexpensive to install and easily moved to modify shop layout.

Disadvantages– Simulating “on the road” condition in the shop bay, plate brake testers provide the fastest and most accurate method of brake balance analysis.

However, there is still no substitute for a wheel off inspection to detect worn linings or confirm a leaking wheel cylinder. Also, low brake fluid must be detected visually.

Finally, other occasional brake problems such as out of round rotors and drums, and brake failure due to heat must be analyzed by means other than the plate brake tester.

Quick Comparison

Summary

While road tests and visual inspections will always play an important role in the service mix of auto service centers, these testing methods can no longer be depended on to provide a comprehensive analysis of today's complicated braking systems. Because of the stringent brake testing requirements in other parts of the world, the technology needed to test these sophisticated systems is now widely available and economically justifiable. With insurance companies and organizations like the CSCV lobbying for improved brake safety inspection, now is the time for America's auto service centers to consider adding automated brake testing systems to complement other hi-tech vehicle diagnostic equipment.

Brake Testing System Comparison

¹ Jocelyn, Treat, and Miller, A Study to Determine the Relationship Between Vehicle Defects and Crashes (May 1, 1973), contract DOT-HS-034-2-263.