Choosing Brake Friction Materials

Choosing Brake Friction Materials

The possibilities seem almost endless...

When you’re doing a brake job on a customer’s vehicle and replacing the brake linings, you have to decide what kind of linings to install. Obviously, you want them to perform the same or better than the original linings in terms of stopping power, fade resistance and pedal feel. You also want linings that don’t squeal or groan so your customer doesn’t come back with a noise complaint. And you want to install linings that wear reasonably well and provide good value for the money. So what’s your choice? OEM linings? Premium aftermarket linings? Standard or economy grade aftermarket linings?

Most of our readers tend to go with OEM linings or premium grade aftermarket linings. Why? Because these linings typically meet all of the above criteria. Most owners of import vehicles want the best and are not as price-sensitive about what kind of brake linings are installed on their vehicles, as opposed to owners of older makes. Your typical Audi, BMW, Infiniti, Lexus and Mercedes-Benz owner will want premium-grade replacement linings, even if they cost a few bucks more than standard aftermarket linings.

On the other hand, if you’re doing a brake job on a 12-year-old Honda or Toyota that belongs to a second, third or fourth owner, price may be more of an issue. It’s probably a second car or belongs to a young person who can’t afford the best. For these kinds of jobs, standard-grade aftermarket linings are perfectly fine and will do an adequate job. Economy grade pads are more for do-it-yourselfers and generally don’t provide the longevity or performance of standard- or premium-grade replacement linings.

The decisions you make regarding which kind of linings to use is an important one because the wrong choice can result in a dissatisfied customer – which is something you don’t want. Noise complaints are the number one reason for brake repair comebacks, and most noise complaints are associated with semi-metallic disc brake pads.

Noise problems aren’t necessarily the fault of the pads, though semi-metallic friction compounds are inherently noisier than other materials because of their composition. They contain chopped steel fibers in a hard resin matrix that tends to magnify noise-producing vibrations rather than dampen them. If there’s play between the pads and caliper, or the caliper and knuckle, the brakes may squeal or groan every time they are applied. Some motorists don’t object to this kind of noise and consider it normal – especially if their original OEM pads were always noisy. But others won’t tolerate any noise from the brakes and will insist that you use a friction material that’s quiet. So what are your options?

One way to minimize brake noise is to switch from hard semi-metallic pads to softer and quieter ceramic or non-asbestos organic pads. There are many different friction compounds in use today, so keep reading and we’ll bring you up to speed on what’s available.

It seems that almost every major aftermarket and original equipment brake supplier these days uses some type of ceramic-based friction material in their products. Ceramic-based friction linings have been used on Japanese and domestic vehicles for more than 15 years and, in recent years, the percentage of makes that use these kind of linings has exploded.

One brake supplier estimates that brake linings containing ceramic ingredients are now used on 50 to 60 percent of all new vehicles. If you count only those applications where ceramic fibers are the primary ingredient, the figure drops to about 40 percent but is still a significant portion of the new vehicle fleet.

European automakers have mostly used low-metallic friction materials to date. Low metallic formulas offer good braking performance but tend to be noisy and wear quickly. Low metallic compounds can also leave a black grimy residue on alloy wheels that makes them look dirty. A recent J.D. Powers survey in Germany revealed that many European car owners are not that happy with their brakes and would prefer quieter, cleaner, longer-lasting brake linings. The same probably holds true for owners of European luxury makes here, too. That’s why ceramic linings are growing in popularity. Ceramics provide good braking performance, quiet operation and low dusting. They’re kinder to rotors than semi-metallic pads, too.

Ceramic fibers are a good choice for brake linings because they have stable and predictable friction characteristics, more so than most semi-metallic materials. Ceramics provide a consistent pedal feel that’s the same whether the pads are hot or cold because the coefficient of friction doesn’t drop off as quickly as semi-metallics. NVH (noise, vibration and harshness) is also less with ceramics, so the brakes are significantly quieter.

Ceramic compounds can be very complex and may use 18 to 20 different ingredients in a formula, including various fillers and lubricants that are added to help dampen vibrations and noise. A typical semi-metallic compound, by comparison, might contain only eight or nine ingredients.

Though semi-metallic linings generally provide better wear at higher temperatures, ceramics perform just as well, if not better, at lower temperatures for the average driver. Consequently, pad life is often improved.

Low dust is another desirable characteristic of the material. The color of most ceramic materials is a light gray, so it’s less visible on wheels (unlike some NAO pad materials that produce a dark brown or black dust that clings to wheels).

One very important point to keep in mind about ceramic-based friction materials is that they are not all the same. In other words, “ceramic” is not a generic term for a type of friction material. It’s a marketing buzzword that covers a wide spectrum of different friction products. The only thing they have in common is that they all contain some type of ceramic as an ingredient.

Every brake manufacturer uses their own ceramic-based or ceramic-enhanced compounds. The type of ceramic used, the particle size, distribution, hardness and other ingredients that go into a ceramic type of friction material can all vary – and even from one vehicle application to another. One supplier uses more than 20 different ceramic formulas to cover all the different vehicle applications in its product line (more than 260 SKUs currently!). Others may use only a couple of different formulas.

Some people disagree over what should and should not be called a ceramic. Ceramic materials include a variety of substances including potassium titanate fibers, as well as clay fillers. Some brake manufacturers use clay filler in certain friction linings but do not call these products ceramic linings. Others do. Consequently, the type of ceramic compounds used in a brake supplier’s ceramic product line may vary significantly from those used by another brake supplier – along with the performance characteristics of their linings.

The bottom line is that the actual amount of ceramic that’s used in a friction material can vary a great deal from one brake supplier to another. It’s like how much pepperoni one pizza maker puts on his pizzas compared to the guy down the street. None of the brake lining manufacturers we interviewed for this article would revel the exact ceramic content of their linings. But several did make it very clear that the ceramic content can vary from a few percent to a significant percentage. What really counts, though, is braking performance and noise, which depends on the combination of ingredients in the friction material as well as the design of the pads themselves.

Most ceramic-based linings perform well in a wide variety of applications but, for some applications, other materials work just as well or better. It depends on the vehicle platform and the type of driving. That’s why some brake suppliers use a “best fit” philosophy when choosing a particular friction compound for a given vehicle application.

The trend today among many brake suppliers is to closely match whatever type of friction material the OEM chose for the vehicle. If an OEM application is ceramic, the replacement pads will likely be ceramic – though you can’t always tell by looking at the box. Some brake suppliers make a big deal out of touting their ceramic linings, while others don’t.

One reason why the aftermarket is following the OEMs so closely these days is because of the new FMVSS135 standards that 2001 model year cars and 2003 model year trucks have to meet. This applies to new vehicles only. There are no FMVSS standards for aftermarket replacement linings. Even so, aftermarket brake suppliers realize that their products should be as safe as the OEM brake linings. Consequently, their testing procedures have improved along with their products.

FMVSS135 is a minimum braking performance standard set by the National Highway Traffic & Safety Administration. It’s a tougher standard than the former FMVSS105 rules, which has forced automakers to upgrade the brakes on some vehicle models so they meet the new stopping requirements.

One part of the new rule requires vehicles to stop within the same distance under the old FMVSS105 rules but with 25 percent less pedal effort. To meet this specification, some manufacturers have had to switch to more aggressive linings. This, in turn, means the aftermarket has to offer replacement linings that equal the performance of the OEM linings.

According to a recent Babcox Research survey, more than 90 percent of our readers say they prefer to install “application-specific” brake linings. In this case, “application specific” means the friction material has been carefully chosen to match the braking requirements of a particular vehicle platform. Consequently, a brake supplier’s product may contain various ceramic compounds, various semi-metallic, low-metallic and non-asbestos organic compounds all under the same brand name. In this case, you don’t have to decide which type of material is the best to use because the supplier’s engineers have already done their homework and figured out the answer for you.

Regardless of what type of friction material is used in a set of pads or shoes, the brakes must provide safe, reliable stopping power. Ideally, a set of aftermarket replacement linings will perform the same as, or better than, the OEM linings. Most do, and the brake manufacturers do extensive laboratory and field testing to ensure that they do.

Jim Lawrence of the Brake Manufacturers Council (BMC) said their “Brake Effectiveness Evaluation Procedure” (BEEP) is one way brake suppliers are making sure that their aftermarket products meet essentially the same requirements as the OEM linings. The BEEP test uses a single-end brake dynamometer to measure the friction characteristics, fade resistance and recovery curve of brake linings or specific vehicle platforms.

Currently, the test procedure covers about 30 to 40 percent of the vehicle applications that are on the road. The goal is to develop test procedures for about 70 percent of the vehicle platforms – a process that takes a great deal of time and engineering effort to measure and calibrate specific vehicle applications.

In recent years, most aftermarket brake suppliers have also introduced new or upgraded premium or even “ultra-premium” product lines that feature their best lining materials. Some of these new lines are targeted at SUV/LT market, which can be very demanding on brakes.

Premium friction materials typically provide the best combination of stopping power, fade resistance, noise and wear. They also command a higher price, which more consumers are willing to pay on late-model, luxury-laden cars and light trucks.

The bottom line is that premium grade pads are almost always the best choice for your customers’ vehicles. Premium pads usually provide the best combination of braking performance, noise control and wear, are application-engineered for specific vehicle applications and provide the best all-round value for the money. That means more satisfied customers and fewer comebacks. What could be better?

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