If you are installing new brake linings, don't expect to be satisfied with the results if all you do is a quickie "hang and turn" brake job with economy grade replacement linings. This kind of brake work may be good enough if you are only concern is saving money. But if you are more concerned about safety and performance, and want to restore the brakes to "like-new" condition, then go with "application specific" replacement linings. These are linings that are designed to closely match the OEM linings on the vehicle in brake performance, feel and sound.

The term "application specific" means a friction material is formulated for a specific type of vehicle application. The application may be a single model of vehicle or an entire family of vehicles that have similar brake systems and braking requirements.

Application specific is the opposite of tying to use only a few basic friction formulas to cover a broad range of vehicle applications, which historically was the norm for aftermarket friction lines, but no more. Today, most quality aftermarket suppliers offer friction lines that include application specific linings that more closely match OEM brake performance.

The term application specific was coined in 1994 by Ferodo America Inc. with the introduction of their "SpectraOne Application Specific" brake line. Other suppliers have also used application specific in a generic sense or similar phases such as "application engineered" to describe friction lines that include multiple friction compounds for different types of vehicle applications.

The need for application specific replacement linings in the aftermarket came about because of changes that have been taking place in new vehicles. The introduction of front-wheel drive created a demand for semi-metallic replacement linings, and the phase-out of asbestos created a similar demand for nonasbestos organic (NAO) shoes and pads.

According to one brake supplier, 15 years ago the typical OEM used only two basic friction compounds for most of their vehicles (an asbestos compound for drum brakes and asbestos or semi-metallic for disc brakes). Seven years ago, that number had increased to 15 different compounds (different varieties of NAO and semi-metallic). Today that number has jumped to 30 or more different compounds, with new ones being added with each new model introduction!

The proliferation of so many different compounds combined with a proliferation of brake systems and vehicle types has made it harder to harder to provide broad coverage with only a few basic compounds.

"Over the last 15 to 20 years, brake systems have gotten smaller and lighter," said one supplier, "but vehicle weights have not decreased nearly as rapidly. This means rotors and pads are being asked to do the job of larger systems, which has resulted in higher heat generation and higher pad wear rates. Plus, today there are a multiplicity of vehicles on the road from subcompact cars to large pickup trucks all with unique braking characteristics. One single friction formulation cannot meet the braking requirements of all these different vehicles. So application specific friction that incorporates multiple formulations based on heat generation variances specific to each type of vehicle was introduced."

The same supplier said they discovered a consistent ratio between three primary vehicle characteristics (vehicle weight, disc pad area and configuration, and rotor weight and design) that determined how hot the brakes would get and how quickly heat could be dissipated. With this knowledge, they were able to develop an "energy index" rating system that could be used to determine the optimum friction requirements for a given vehicle application.

"We found that certain energy index ranges were common to large numbers of vehicles. Based on that, we developed multiple friction compounds tuned specifically to those ranges. We also considered OEM pad configurations to determine whether to make each pad semi-metallic or NAO, which was different than the way friction lines had been marketed in the past."

Another supplier said the need for different friction compounds was not so important when all vehicles were rear-wheel drive and most brake systems provided adequate cooling. But it became a concern when front-wheel drive, downsized brake systems and higher operating temperatures became the norm. This same supplier said they now use six different semi-metallic compounds in their product line where they used to offer only one. Another supplier said they now have twelve different friction compounds in their line, including semi-metallics, low-metallics and NAOs).

A semi-metallic compound is usually one that contains more than 30% by weight iron or steel. A low-metallic is one with less than 30% metal. A nonasbestos organic contains little or no iron or steel. The exact formulation of any given friction compound is a closely guarded secret, but generally includes fibers, fillers, binders and resins. NAO compounds typically contain several times as many ingredients as semi-metallic compounds, are are more complex to engineer and manufacture.

NO SURPRISES

The last thing anybody wants when they step on the brake pedal, especially in a panic stop situation, is a surprise. Brakes should be predictable. If the feel and/or stopping characteristics of the brakes have changed as a result of installing replacement linings that differ significantly from the original, it may create an unsafe condition especially on a wet or slick road surface.

A lot of variables can affect stopping distance. One of these is the point at which the brakes lock up and skid. A vehicle will stop in the shortest distance possible when tire slippage is kept to a minimum. Skidding breaks traction and increases the stopping distance. It causes a dangerous loss of steering control and directional stability.

If the front wheels lock up during a hard stop, the driver loses all ability to steer and avoid obstacles. If the rear wheels lock up, the vehicle may spin out of control. Antilock brakes minimize such risks by preventing wheel lockup. But all ABS systems are calibrated to work with the friction characteristics and front-to-rear brake bias of the original brake system and linings. Replacing the original front or rear linings with ones that behave differently can upset this balance as well as the operation of the ABS system. And on vehicles that do not have ABS, it may increase the risk of wheel lockup especially on wet or slick surfaces.

BRAKE PERFORMANCE

The brake performance of a vehicle depends on many things. One is the weight of the vehicle itself. Bigger, heavier vehicles need more stopping power than smaller, lighter ones. The layout of the drivetrain is also a factor. Front-wheel drive shifts more of the braking effort to the front brakes. Most important, though, is the design of the brake system itself. This includes the configuration of the system (disc/drum or disc/disc), the size of the rotors and drums, the type of rotors (vented or unvented), the type of calipers (floating or fixed, single or multi-piston), the piston area of the calipers and wheel cylinders, the size of the pads and shoes, the hot and cold friction characteristics of the brake linings, how the hydraulics are split (front-to-rear or diagonally), the pedal ratio and amount of assist provided by the brake booster, and how brake pressure is split front-to-rear by the proportioning valve. All these variables are an integral part of the brake system design and have a bearing on the operating temperature and performance of the brake linings.

Other variables that also play a role in brake performance and may change with age or wear include various properties of the brake linings (which can change as a result of exposure to heat, wear, contaminants and environmental factors), the condition of the tires, tire inflation pressure and traction, the condition of suspension components such as the springs, shocks and struts, and the condition of the brake fluid.

Other variables that can change anytime a vehicle is driven include vehicle loading (especially in pickup trucks, vans and sport utility vehicles, which is why many of these vehicle have load sensing rear proportioning valves) as well as road conditions.

Given all of these variables, it should be obvious that different vehicles have different friction requirements. The best way to assure safe braking and to restore and maintain the original brake feel and performance is to not only replace same with same (semi-metallic pads with semi-metallic, and asbestos or nonasbestos organic pads or shoes with NAO), but to also choose replacement linings that closely match the stopping power, fade resistance and pedal effort of the OEM linings. In other words, you want "application specific" replacement linings.

MANY DIFFERENCES

The brake system on a Corvette is obviously a lot different than that on a Geo Metro or a Chevy Suburban. Consequently, the type of friction materials specified by the OEM engineers for all these vehicles will differ significantly from one another. The requirements of a front disc brake are also different from that of a rear disc brake, so this will require differences in compounding, too.

When an OEM engineer designs the brake system for a new vehicle, he usually does not have to take an off the shelf friction material and make it work on his new car or truck. He can ask his supplier to develop a new friction compound that meets specific performance criteria for stopping distance, fade resistance, pedal effort, wear and noise, which has created the proliferation of new friction compounds we see today.

On some applications, the OEMs have switched from semi-metallic pads back to NAO (while also redesigning the brakes to increase heat dissipation) to reduce noise and roughness. NAO pads generally do not wear as well as semi-metallics and cannot handle heat as well, but are usually quieter and easier on rotors. So friction suppliers have to keep up to date with the latest changes and try to duplicate the current OEM friction materials as best as possible.

REPLACEMENT LININGS

What happens if a set of replacement linings are not application specific? It depends. Some vehicles are more sensitive than others depending on the design of the brakes and operating conditions. Changes in friction have less of an effect when braking on dry pavement than on wet or slick surfaces.

If a given set of replacement linings are more aggressive than the original, the brakes may feel grabby or upset front-to-rear brake balance increasing the risk of the front or rear brakes locking up on wet or slick roads especially if the more aggressive linings end up on the back brakes. On the other hand, if the replacement linings are less aggressive than the original linings or have less fade resistance, the vehicle may require increased pedal effort and take longer to stop than before. And if these linings end up on the front brakes, they may increase the risk of rear wheel lockup, too.

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 at one point in time with Brand A linings, and the rear brakes are relined at a later time by you or somebody else with Brand B linings, the vehicle may end up with a less than ideal combination of friction materials.

For this reason, some brake experts recommend relining both the front and rear brakes at the same time if either are worn. This reduces the risk of a mismatch in friction materials and helps assure proper brake balance. Many would say this is "unnecessary" based on brake wear alone, and they have a point. But the issue here is maintaining the same brake balance as before as well as overall brake performance, which means using linings on the front and rear brakes that are ideally the same brand and grade of product (but not necessarily the same type of friction material because that will vary depending on the application).

For example, if you install Brand A premium grade semi-metallic pads on the front brakes of a car, you should probably install Brand A premium grade NAO shoes on the rear brakes when the rear brakes are relined at a later date. But in the real world this is not always possible for a variety of reasons. One is that somebody else may reline the other set of brakes. Another is that if one set of wheels have already been done, you may not be able to determine the brand or grade of linings that were installed. What is more, you probably have your own brand preferences and may not want to match the brand of linings that are already on the vehicle, especially if you believe the other brand is an inferior product. Some experts say it is best to do all four wheels at once. Then you know the linings are properly matched not only to the application but to each other.

The same precaution applies to "loaded" calipers that come assembled with pads ready to install. If you are replacing both calipers, it is no problem. But if you are replacing only one caliper, make sure both calipers 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.

What all of this means to you is that you should install a quality brand of replacement linings from a supplier who is tune with the braking requirements of today's vehicles. Price is important, but should only be a secondary consideration to performance. The goal, after all, is to have safe brakes. And the best way to accomplish that is to restore the brakes to "like-new" condition: the same feel, the same stopping power, and the same (or better) degree of noise control and wear resistance. In most cases, that means application specific replacement linings.