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Replacement Brake Linings

According to a recent Brake & Front End survey, more than 90 percent of our readers say they prefer "application-specific" replacement linings.

In recent years, application specific has become a marketing buzzword. What exactly does it mean? Generally speaking, it means replacement linings are engineered to closely match the friction requirements of different vehicle types and platforms. This usually means a brake supplier has to use a number of different friction compounds within a given product line to optimize brake performance across a wide spectrum of applications.

One leading brake supplier told us they now use about 40 different friction compounds in their various product lines to achieve application-specific coverage. Another supplier said they currently have 25 different compounds in their line, and will be adding more as needed to keep pace with new models and changes at the OE level.

Other suppliers we interviewed wouldn't quote a specific number, but said they use a variety of different compounds to provide application-specific coverage.

The braking requirements of a 2-1/2-ton SUV are obviously different than those of a small front-wheel drive subcompact. 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 a Lincoln Navigator may not be the best choice for a Toyota Echo, and vice versa. Different types of vehicles have different braking characteristics and friction requirements. Thus, application-specific compounds have come about to satisfy the diverse requirements of today's vehicle mix.

Back in the 1970s when suppliers talked about application-specific linings, they were usually referring to replacing semi-metallic linings with the same, and asbestos or nonasbestos organic (NAO) linings with the same.

Most friction suppliers at that time manufactured only a single grade of semi-metallic linings, and only one or two grades of NAO (one for drum brake shoes and another for disc brake pads). But as time went on, the number of different friction compounds began to proliferate. The rapid growth of front-wheel drive in the 1980s, followed by the introduction of ABS and record sales of light trucks and SUVs in the 1990s, fueled the development of more and more new friction compounds. The industry went through successive generations of semi-metallics and NAOs, and today there are probably several hundred different compounds being produced by friction suppliers worldwide.

It's difficult to nail down an exact definition for application specific because of the way in which the term is used to market different brands of brake linings. The term application specific was first used in 1994 by Ferodo with the introduction of their "SpectraOne Application Specific" brake line. Since then, it has taken on a generic quality and has been become "application engineered" and "engineered for specific applications," and so on.

Some aftermarket brake suppliers use the term to imply their replacement linings closely match the friction and performance characteristics of the OEM linings. They back up their claims with testing that "certifies" their linings equal or exceed the performance of the OEM brake linings.

Other suppliers say application specific means they use different friction compounds for different vehicle applications, but that their replacement linings are not necessarily equivalent to the OEM linings. They may use different base materials than the OEM linings and substitute low-metallic or non-asbestos non-metallic (NANM) compounds for OEM semi-metallic compounds. This, they say, provides additional benefits such as improved noise reduction, longer pad life, shorter stopping distance, better pedal feel, etc.

Other brake suppliers use the term even more loosely and say application specific means they've tried and tested a variety of different friction materials to find the ones that work best on certain vehicle platforms or types. This may mean the supplier is using only a few different compounds within their product line. Their argument is: If a given friction material has been proven to work, why add additional compounds that aren't really necessary?

In reality, no aftermarket supplier is 100 percent application specific. It would be cost-prohibitive to develop a unique friction compound for each and every different vehicle application. Many friction compounds can and do provide excellent braking characteristics in a fairly broad range of applications. So unique compounds tend to be used only on those applications that require something different to provide optimum brake performance.

The rear brakes on a pickup truck with four-wheel disc brakes, for example, can be very sensitive to overly aggressive linings. If the replacement pads for the rear brakes have too high a coefficient of friction, they can upset brake balance and make the rear brakes grabby. This can cause premature wheel lockup (especially on wet or slick roads) and trigger the ABS system.

The only way for a brake supplier to know whether or not their replacement linings will work satisfactorily in a given application is to test them. Historically, this has meant many miles of field testing, fleet testing and customer feedback. In recent years, efforts have also been made to develop new laboratory test procedures that more closely simulate real world braking conditions.

One of the challenges of testing friction compounds in a laboratory is that it isn't the real world. Therefore, a laboratory test has to employ various means to simulate as best as it can vehicle weight, momentum and braking dynamics on a mockup of the vehicle's brake system. This is done with computerized controls on a special brake dyno that reproduces the loading and torque effects on the brakes. A "dual mass" dyno is needed to duplicate the interaction between the front and rear brakes in the lab.

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 a very detailed test that takes about 15 hours to complete. It does not have pass/fail standards, but is designed to reveal how a given set of linings compare to the FMVSS 135 requirements for new brakes.

Data generated by the J2430 test shows where a friction material falls within the limits of a performance window. This gives a much more accurate picture of how well a friction compound will actually perform on a vehicle than a simple laboratory "chase" test which only measures hot and cold friction characteristics.

FMVSS 135 is a minimum brake performance standard issued by the National Highway Traffic Safety Administration (NHTSA). It 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 to new vehicles only - which means currently there is no government safety standard or minimum performance requirements for aftermarket brake linings.

The Brake Manufacturers Council would rather see a voluntary self-certification program for the aftermarket rather than government-imposed standards. If NHTSA does decide to regulate aftermarket brake linings at some point in the future (which some say may occur as a result of the recent tire recalls and consumer concerns about safety), the J25430 test can be used as a means of certifying compliance.

What happens if a set of aftermarket replacement brake linings are not application specific or are not well matched to the vehicle they fit? Linings that are more aggressive than the originals may make the brakes feel grabby or upset front-to-rear brake balance. This can increase the risk of the rear brakes locking up on wet or slick roads, especially if the more aggressive linings end up on the back brakes. If the replacement linings are less aggressive than the originals or have less fade resistance, the vehicle may require increased pedal effort and take longer to stop.

The risk of a mismatch in friction material increases even more when different brands of aftermarket replacement linings are installed on the front and rear brakes. One manufacturer's replacement linings may be a close match to the OEM linings, but another manufacturer's linings might not. If the front brakes are relined with Brand A linings, and the rear brakes are relined at a later time with Brand B linings, it could create an imbalance that might cause braking problems and result in a comeback.

Nearly 90 percent of all new vehicles today are equipped with anti-lock brakes, and many also have traction control. Some new vehicles also have "stability control" systems that automatically apply the brakes without the driver's intervention or knowledge to help the vehicle corner better and maintain control when making quick steering maneuvers. These high-tech ABS systems are closely calibrated to the friction characteristics of the OEM brake linings so it's important that aftermarket replacement linings closely match the OEM linings to maintain the same operating characteristics of these systems.

For this reason, some brake experts now recommend relining both the front and rear brakes at the same time with application-specific linings from the same supplier. This approach, though more costly, can minimize the risk of mismatched brakes if the rear brakes are not relined until later.

The same precaution applies to "loaded" calipers that come assembled with pads ready to install. If you're replacing both calipers, it's no problem. But if you're replacing only one caliper, many experts say both calipers should have the same type and grade of friction material. A friction mismatch side-to-side may create a brake imbalance that causes a brake pull as well as uneven wear.

Though few consumers can tell much about the quality of a brake job, one thing they will notice right away is any brake noise, vibration or harshness (NVH) after their brakes have been relined. Noise complaints are the number one cause of customer dissatisfaction and unwanted comebacks, so minimizing NVH has become the primary goal of most brake suppliers today.

Many suppliers have introduced product lines that are directly targeted at this issue. Quiet brake pads result in happy customers, so quiet operation is a key feature of many premium pads today.

In the past, reformulating a friction compound to reduce noise often meant sacrificing pad life or stopping power (or both!). No brake manufacturer will sacrifice safety in order to achieve any of these goals, but more often than not, durability suffered when a friction compound was reformulated to reduce noise. Not so anymore.

Noise is caused by vibration. Hard, high metallic content pads tend to be noisy because you're rubbing steel against cast iron. Reducing the metallic content of the compound and/or substituting other nonferrous ingredients, such as brass and copper, for the chopped steel wool fibers can reduce noise. The use of various carbon compounds and ceramic fibers are also ways to make linings quieter without affecting stopping power or pad life.

Additional reductions in NVH can also be achieved by using integrally-molded insulators in disc brake pads, using various surface coatings (titanium dioxide, copper, graphite, etc.), or 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 reduce the surface area of the brakes slightly, 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. Built-in insulator shims can also provide a dampening layer to absorb and dissipate vibrations before they can cause noise.

The point here is that each brake supplier uses their own unique combination of friction materials and design features to provide what they believe is the best possible combination of noise control, brake performance and durability within a given product line. And as a rule, you'll always get the best performance with premium-grade linings.

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 cars 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 are also 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 carbon, 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 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. Some use special ingredients such as ceramic fibers, carbon or even platinum, while others have special coatings such as copper or titanium dioxide to help seat the pads on the rotors, dampen noise and/or eliminate the need for special break-in procedures.

Some premium pads are also "preburnished" to eliminate many of the problems that can occur if the pads are not broken in properly. 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. 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'll 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 ways toward reducing comebacks and improving customer satisfaction. Because of this, more and more technicians are opting to install premium pads only - and to encourage their customers to have all four brakes relined at the same time instead of doing just the front brakes.

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