The type of friction material that is used in a vehicle's brake system has a huge impact on noise, stopping power, fade resistance and wear of both the linings and rotors. The "right" material will provide the best possible combination of these factors without sacrificing too much of one thing for another. It is difficult to achieve the best of everything in a single friction formula, but many of today's "application-specific" friction materials come pretty close.
According to a recent Babcox Research survey, more than 90 percent of brake technicians say they prefer to install "application-specific" brake linings.
Application specific is a buzzword that means that the replacement linings are engineered to closely match the friction requirements of different vehicle types and platforms. It may be an "off-the-shelf" compound, a modified compound or a brand new compound that provides the best combination of noise, braking performance and wear for a particular car or truck. In many instances, the linings are very similar to, if not identical to, the OEM brake linings.
One aftermarket brake supplier told us it now uses about 40 different friction compounds in its various product lines to achieve application-specific coverage. Another supplier said it currently has 25 different compounds in its line, and will be adding more as needed to keep pace with new models and changes at the OE level.
Other suppliers we interviewed would not quote a specific number, but said they do use a variety of different compounds to provide application-specific coverage.
BRAKE LINING REQUIREMENTSThe braking requirements of a Toyota Land Cruiser are obviously different from those of a Honda Civic. Vehicle size, weight, brake type (disc or drum), the design of the calipers and rotors, front-to-rear brake balance and how the vehicle is driven are all factors that influence the selection of a particular friction compound. Consequently, a friction formula that works well on Lexus might not be the best choice for a Lamborghini. So aftermarket brake suppliers have to use a variety of different friction compounds to cover the diverse mix of vehicles that are on the road today. There probably are several hundred different compounds being produced by friction suppliers worldwide.
The widespread use of anti-lock brake systems also has played a role in the development of application-specific brake materials. The threshold at which the wheels start to lock up depends on vehicle weight, speed, traction and the coefficient of friction of the brake linings themselves. Because the ABS control electronics are calibrated to the OEM brakes, aftermarket replacement linings should closely match the friction characteristics of the OEM linings.
The "J.D. Powers factor" is another driving force in the ongoing development of new application-specific friction materials. Every vehicle manufacturer wants its cars and trucks to be ranked at the top of the latest J.D. Powers survey. A high ranking means more sales. To achieve higher customer satisfaction ratings, the OEMs are leaning on their suppliers to do everything they can to minimize noise, vibration and harshness (NVH), which includes tweaking and fine-tuning the brakes.
The current fixation on quiet operation means no squeaks, squeals, rattles or noise complaints. The quieter, the better. But making brakes quiet is not easy. When a new friction compound is being developed, compromises often have to be made. A quiet compound may not be a long-lasting compound. Reducing noise often involves sacrificing some pad life. Softer pads are quieter pads but do not wear as well, so other strategies have to be employed to extend pad life.
CHOOSING THE RIGHT FRICTION MATERIAL
With so many application-specific products available in the aftermarket today, how do you choose the "right" replacement lining for a particular vehicle? The short answer is to pick one that delivers what you want. Any brand or type of lining that provides safe braking, quiet operation, good pedal feel and reasonable wear is a good choice.
Price should be the last thing you consider when making a choice. Why? Because the price of the linings is only a small percentage of the total brake job (assuming you are doing a complete brake job, which you should be). Your profit comes mostly from the labor plus the markup on the parts. If you have to limit the total cost of the job to make a sale or to compete with the shop down the street, do it on the labor or parts markup. Do not use cheaper quality linings to make up the difference.
PREMIUM BRAKE PADS
In recent years, most aftermarket brake suppliers have introduced new or upgraded "premium" or even "ultra-premium" brake linings. Some of these new lines have been targeted at the growing light truck/SUV market, while others are for a broad range of passenger car and truck applications.
Premium products are selling well because many consumers are concerned about safety and are willing to pay extra for top-of-the-line replacement linings.
Most of the premium grade linings also are marketed as application specific, but each manufacturer has taken a unique approach with respect to the type of friction compounds they use. Some tout a particular ingredient such as ceramic, titanium, etc. to market their brand, while others focus more on the features and benefits. Either way, both you and your customer come out ahead.
Though friction formulas and designs vary, one feature that most premium pads do share in common is that they provide superior all-round performance and typically equal or exceed the OEM linings in stopping power, feel, fade resistance, noise control and lining life.
Most premium pads also incorporate such features as chamfers, slots and built-in shims to control noise and vibrations. Some also have "low dust" formulas that reduce the accumulation or visibility of dust on alloy wheels. This is one of the characteristics claimed for some ceramic linings. The dust is a lighter color and shows up less on alloy wheels.
Some premium pads are also "preburnished" to eliminate many of the problems that can occur if the pads are not properly broken in. When brake linings are manufactured, the resins that bind the ingredients together are not fully cured. When the linings are later installed on a vehicle, the heat produced by normal braking bakes the linings and cooks out the residual chemicals from the resins to improve the friction characteristics of the lining. But if the brakes get too hot before the linings are fully cured, it can "glaze" the linings, causing noise and performance problems. So to eliminate the need for a break-in period, some brake suppliers are now adding an extra manufacturing step to fully heat-cure (burnish) the linings.
With most economy and standard-grade linings, you will still get good brake performance and safe braking. But you won't get the same degree of noise control, pad life or performance that you do with premium linings.
In most cases, the cost of a set of premium linings is only $5 to $10 more than the standard grade, which adds little to the overall cost of a brake job, but can go a long way toward reducing comebacks and improving customer satisfaction. Because of this, more and more technicians are opting to install premium pads.
ELIMINATION OF BRAKE NOISE
Noise (or the lack thereof) is also a major selling feature of most premium linings. With so much emphasis on quiet brakes today, some manufacturers have renamed their product lines to reflect this feature. By changing the ingredients in a friction compound, the linings can be made to dampen and suppress noise, something that is difficult to achieve with most hard semi-metallic linings. Thus, low metallic, ceramic and non-asbestos organic (NAO) formulas are commonly used where noise is an issue.
Noise is caused by vibration. Hard, high-metallic content pads tend to be noisy because steel is rubbing against cast iron. Reducing the metallic content of the compound and/or substituting other non-ferrous ingredients such as brass and copper for the chopped steel wool fibers can reduce noise. The use of various carbon compounds and ceramic fibers also are ways to make linings quieter without affecting stopping power or pad life.
Additional reductions in NVH can be achieved by using integrally molded insulators in disc brake pads, by using various surface coatings (titanium dioxide, copper, graphite, etc.), by slotting and chamfering pads, etc. Some replacement pads combine all of these elements while others do not use coatings or integrally molded insulators.
Chamfers are angled or beveled edges on the leading and trailing ends of the pad that reduce "tip-in" noise when the brakes are first applied. Chamfers also slightly reduce the surface area of the brakes, which increases the clamping force applied by the pads against the rotors. This further helps to dampen sound-producing vibrations.
Slots are grooves cut vertically, diagonally or horizontally in the pads to reduce noise by changing the frequency of vibration from an audible level to a higher, inaudible frequency beyond the range of the human ear. Slots also help reduce brake fade by providing a passage for gasses and dust to escape at high brake temperatures.
Some brands of brake pads also have a special surface coating that helps dampen sound and lubricate the pads during their break-in period. Some of these coatings form a boundry layer that stays on the surface of the rotor to help control noise.
BRAKE COMPOUND RESEARCH & DEVELOPMENT
When friction suppliers develop a new compound, they first model its characteristics as best they can on a computer. Then they mix up a batch and test it in the lab. If the compound passes the basic lab tests, the compound is made into linings for further lab testing. Then the compound is tested on instrumented vehicles. Finally, it is tested under real-world driving conditions, usually on some type of fleet vehicles such as taxi cabs or police cars.
Laboratory testing has become very sophisticated. In the old days, testing focused on basic hot and cold friction coefficients, and the shear strength of the material to make sure the pads would not tear apart. Now they test the linings in environmental chambers that simulate exposure to moisture and road salt, and on special "dual-mass" dynamometers that simulate the effects of a vehicle's weight and momentum on the brake linings. Testing today usually involves a full mockup of the brake system and suspension on which the linings will be used to see how the parts interact.
Evaluating noise is harder to do because it is subjective. Engineers can accurately measure noise levels in decibels and frequencies, but they can't always predict if the pads are going to make objectionable noises until they are on a real vehicle with a driver behind the wheel. Even then, they typically use several different drivers and ask each to grade the noise characteristics of the linings.
LAB TESTS OF BRAKE LININGS
Back in 1992, the Brake Manufacturers Council (BMC)began funding a Society of Automotive Engineers (SAE) task force to develop a new laboratory dyno test procedure for evaluating brake linings. In 1994, SAE published the J1652 test for evaluating friction materials on front disc brakes. Then in August 1999, SAE published the J2430 test procedure for testing linings on the front and rear brakes together.
The J2430 is a test that any manufacturer can use to evaluate their products. The test is very detailed and takes about 15 hours to complete. It does not have pass/fail standards, but is designed to reveal how a given set of linings compares to the FMVSS 135 requirements for new brakes.
Greening Laboratories in Detroit also has developed lab tests for "certifying" the performance of brake linings for several aftermarket brake suppliers. Its Dual Dynamometer Differential Effectiveness Analysis (D3EA) test procedure measures and compares the various performance windows (stopping power, fade resistance, etc.) of aftermarket linings against the OEM linings on a given vehicle platform. As long as the aftermarket linings fall within specified limits, they are certified as being OE-equivalent in terms of braking performance.
OEM brake linings on all new vehicles (including imports sold in the U.S.) must meet the FMVSS 135 standard issued by the National Highway Traffic Safety Administration. This standard specifies maximum stopping distances, criteria for fade resistance and other aspects of brake performance for all 2000 model year and newer cars and light trucks. It supersedes FMVSS 105 that was the former standard for 1999 and earlier vehicles.
FMVSS 135 is a tougher standard than FMVSS 105 and, like the earlier standard, applies only to new vehicles, which means there are currently NO government safety standards or minimum performance requirements for aftermarket brake linings. The Brake Manufacturers Council would like to keep it that way and prefers a voluntary self-certification program such as their new Brake Effectiveness Evaluation Procedure (BEEP), which identifies approved linings with a BEEP seal.
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