With all the time and money we put into getting our rides to go faster, we sometimes overlook the importance of slowing them down, too. A car that won’t start is an inconvenience – one that won’t stop is a disaster. When you step on the wide pedal, you’re pushing hydraulic fluid to the calipers, which squeeze the brake pads against the rotor, causing friction, heat, brake dust, and with any luck, a decrease in vehicle speed. Those pads provide an easy and relatively inexpensive way to tune the braking performance of your vehicle, whether it’s a street cruiser with OEM calipers and rotors, a dedicated race car, or anything in between.
The friction that brake pads generate against the rotors, their operating temperature range, and their ability to resist fade are all important, and the balance of these qualities will change depending on how you drive. A pad that provides excellent “bite” when cold might be the right choice for autocross, but leave you in the gravel trap when subjected to extended periods of high heat in an endurance racing environment. Pads that are quiet and low-dust are great on the street, but those qualities might be at odds with the demands of racing.
When you’re in the market for a new set of brake pads, the research (if done right) can get a little overwhelming. We needed an education, so we sat down with Todd Gartshore of Baer Brakes, and Jerry DeMarino of Hawk Performance to clear up the mysteries and help you in selecting the right pads for your steed. It didn’t take long to figure out there is a lot of science and a little art involved in producing brake pads.
Friction compounds of the brake pads are the “tuneable” component in brake system performance, controlling:
• Initial Bite
• Total Friction
• Wear rates of both pad and rotor
• Noise, Vibration, Harshness (NVH)
There are a myriad of materials used to produce brake pads. Manufacturers blend a mix of ingredients, like metals, resins, and fillers to produce a specific pad. All pads are a mix of components and the size and weight of the ingredients used in the friction formula can create very different performance levels and characteristics. What ingredients are used, how they are blended and how the ingredients are manufactured are what makes up the brake pad’s key braking characteristics. According to Baer’s Gartshore, “If there’s no dust, there is no stopping power. If the pad is not wearing out, it’s not working.” The pad has to wear, which produces dust in the process of generating the friction and stopping power.
Some of the brake pad terms to be familiar with:
• Mu: Pronounced as “myoo”, Μ or μ (a letter in the Greek alphabet). As an engineering term, it refers to Coefficient of Friction, or total friction of the pad and rotor. For our braking discussion, a number between 0.0 and 1.0 is used to represent friction, where a value of zero would mean no friction at all, and 1.0 represents no slippage or a total weld of the pads and rotor. Coefficient of friction is an empirical measurement; in other words, it has to be measured experimentally, and cannot be found through calculations. Per Gartshore, “An aggressive domestic pad is rated .36 to .40, while German sport sedans are about .42 to .44. Baer’s Decela pads are about .45 μ and our Extreme compound is .5”.
• Onset: The initial “bite” of the pad to the rotor at the first hint of pressure on the brake. Street pads are designed to have an effective co-efficient of friction at cold to medium range temperatures (up to 600 degrees F). A race pad will have a peak coefficient of friction at higher rubbing speeds and elevated temperatures (up to 1600 F). The key is to design a street or race pad for the vehicle’s temperature range. A race pad must withstand constant braking and constant high deceleration rates, but the tradeoff is usually low friction until the pad and rotor have sufficient heat in them.
• Rising or trailing friction: This refers to how that pad applies friction to the rotor. Typically, you want a flat torque curve for optimum stopping power. “Rising friction is when the coefficient of friction increases throughout a brake engagement,” explains Hawk’s DeMarino. “The more it rises, the quicker the brake stops.” This type of pad is used mainly in racing; as the driver eases off the brake going into the apex of a corner, the pad maintains the same braking power while the driver is reducing brake pressure. DeMarino continued, “Trailing friction occurs when there is a decrease in the coefficient of friction. This phenomenon normally occurs when brakes become very hot and fade occurs, in that the harder you press on the brakes the less stopping power you get – usually not a good thing! For example, once the MOT (Maximum Operating Temperature) is achieved from a brake friction material, the compound begins to overheat and loses its effectiveness.”
• Transfer Film: In an automotive brake system there are three key frictional elements; the brake pad, the rotor surface, and the transfer film. The transfer film is a micro-thin layer of material generated by two interacting surfaces while in relative motion. All three elements are distinctly different in terms of chemical composition and physical structure. What is important to know is that the transfer film directly impacts coefficient of friction (stopping power), brake pad life and rotor life. A smooth, even transfer film provides optimal brake performance. It will appear “glassy” and darker than the rotor surface and should cover the entire contact area evenly. Note the emphasis on the word ‘even’, as uneven or spotty deposits on the rotor face are the number one (and almost exclusive) cause of brake judder or vibration. DeMarino reiterated, “The thickness of the transfer film is also important. Transfer films that are too thin cause excessive rotor wear, grooving, heat checking or warpage. Transfer films that are too thick produce excessive brake dust and typically display a poor coefficient of friction.”
• Firepath: The “Firepath” of the pad is considered the area on the rotor swept by the pads.
• Noise, Vibration, Harshness: (also NVH) Lower NVH means less noise, less vibration fed back through the pedal, and less squeal. Ford’s OEM standard is an additional 10db or less when the brakes are applied. There are a number of reasons you can have brake noise. Sometimes brake squeal can be an indication of a problem. Maintenance is required if you experience brake squeal for any of the following reasons:
• Lack of friction material (pads need to be replaced)
• Loose-fitting brake in the caliper
• Loose-fitting or missing brake hardware (shims, anti-rattle clips)
• Loose lug nuts or caliper hardware
• Debris caught between the rotor and the surface of the brake pad
• Heat-cracked or worn rotors
• Uneven finish on resurfaced rotors
• Improper brake component installation or maintenance (caliper, rotor, pads, etc.)
• Poor brake component lubrication
• Poor aftermarket caliper or rotor design
• Any major change to the original vehicle design (suspension, brakes, tires/rims, weight, etc.)
Sometimes you may experience brake noise when no maintenance is required. Brake squeal is typically caused by vibration between the brake pads, rotors, and brake calipers. Some brands of pads are more likely to experience brake squeal due to material composition, transfer film quality, level of coefficient of friction produced or physical design.
• Green-fade: A type of “fade” of braking power caused by the binding agents coming out of the pad too quickly. These binding agents may create a gas layer between the pads and rotors which does not have a high coefficient of friction.
Brake Pad Types/Materials
Asbestos, a set of six naturally occurring silicate minerals, was used almost exclusively in brake pads up until a 1989 ban by the EPA. Since that time, friction material companies have made major advancements in non-asbestos brake pad technology, further improving the overall performance and effectiveness of today’s brake systems. The four basic classifications of brake pad materials are Metallic, Semi-Metallic, Organic and Exotic.
• Metallic: (Also known as Sinter Metallic) Brake pad formulations with over 75% metallic content. The materials are brought to a temperature where copper or brass literally fuse together forming a structural matrix. Metallic pads are very aggressive, have a high thermal conductivity, and tend to be very noisy in automotive applications. In terms of coefficient of friction, metallic pads have a natural propensity to perform well against steel, but poorly against grey cast iron. Today, metallic brake pads are virtually non-existent in the automotive industry where vented grey cast iron rotors are the standard. Conversely, metallic pads still dominate the motorcycle, ATV, snowmobile, and general aviation markets where solid steel rotors are still utilized.
• Semi-Metallic: Brake pad formulations with less than 75% but more than 15% metallic content. Semi-metallics are a blend of organic, synthetic and metallic materials that are held together in a thermal-set resin matrix or binder system. Typically, the raw materials are fibers and particles that vary in size, shape, and chemical makeup. They typically produce a high coefficient of friction, good pad and rotor wear, and are less susceptible to fade at elevated temperatures when compared to organics. Because of the dramatic effects of additives such as carbon fiber, Kevlar, and ceramics, many new subcategories under semi-metallic have been established.
Per DeMarino, the HPS compound from Hawk is a “carbon semi-metallic friction formulation/compound. It will improve stopping power, prolong life, reduce dust, and be more fade-resistant than the OEM pads. Its MOT (Maximum Operating Temperature) reaches upwards of 700 degrees F, versus an OEM pad reaching 450 F. The price, compared to OEM, is usually a little lower and approximately 10-30% higher than the popular branded general automotive pads on the market.”
• Organic: Brake pad formulations with less than 15% metallic content. Basically, these pads have the same composition as semi-metallic, but with a much lower metallic content. Organics tend to produce dust that is less likely to oxidize, and will offer improved rotor life and minimized brake noise. Conversely, organic brake pads will wear out faster, produce a lower coefficient of friction, and will fade at elevated temperatures. Just like semi-metallic brake pads, the performance attributes of any given organic formulation can vary greatly.
• Exotics: Unique brake materials that are not used in the general automotive market due to cost, application effectiveness, or compliance with existing brake system design. There are basically two relevant technologies that fall under this category; carbon-carbon and ceramic composite brakes. Both technologies require the user to install a pad and rotor that are constructed from the same material, along with major modifications to the brake system. Due to cost and the inability to operate effectively in a standard automotive brake application, these technologies have been restricted to limited production exotic sports cars and professional motorsports.
What are ‘ceramic’ pads and why are they so popular?
“Ceramic” has become a major buzz word in the automotive industry. The fact is, ceramic pads are simply organic or semi-metallic brake pads with typically 1% to 7% ceramic content. A “ceramic” brake pad should provide a linear coefficient of friction, ultra low dust output, good pad and rotor wear and minimal NVH. Note that general automotive “ceramic” brake pads should never be confused with the ceramic composite brake system (CCB) that requires special brake components.
Choosing the proper brake pad compound that will provide the best performance for your application can be made easier by following some guidelines, provided courtesy of Hawk:
• Determine what compounds and styles of brake pads are available for your calipers and type of driving.
• Determine what qualities are most important to you and rank them – Ultimate braking power, low NVH, brake dust, and of course, cost. This is important, since compromises have to be made between these areas because some are mutually exclusive. “A street-type pad can move to the track or a track-type can move to the street, but there are usually too many compromises involved for the customer to be totally happy,” Gartshore explained.
• How do you know you’ve made the wrong choice? When a pad is used in a substantially higher temperature range than it was intended -generally an overheated brake pad will continue to provide a hard pedal but require more pedal effort to achieve even marginal performance. Continued use can result in complete brake pad failure. As the pad begins to lose performance effectiveness, the driver may try to compensate by pushing harder and longer on the pedal. In extreme cases, delaminating between the friction material and the pad’s backing plate can occur. DeMarino told us, “True track pads are developed to be run on a true race car and NEVER on the street. Why? Track pads are made with materials that are not suitable for low temperature and low pressure use. If you run a track pad on the street you will end up replacing rotors quickly and probably ruin wheels that get caked with debris. Track pads are engineered to withstand high temperatures (up to 1600 F) and actually become softer and work better at temperatures above 300 F. If the temperatures are not elevated, then you will have a very bad experience and will not be pleased with the performance of the pad. The reverse is true with running street pads at the track. Street pads do not have the same chemical properties as race pads and do not require extreme temperatures to work. The hotter you get street pads, the more apt they are to fade and not work effectively.” Hawk HP-Plus pads are specifically designed for autocross and light track days and are safe to drive to and from the race track. But Hawk recommends changing to a street pad for every-day driving.
Bedding the Pads
The most important item, after correct installation, is breaking in your new pads. It causes the face of the pad and rotor to mate, allowing the new pads’ resins to burn off (preventing green-fade), and allowing the pad and rotor to create a third element, the transfer film, on the rotor. A proper transfer-film takes 10-50 stops to form, depending on the manufacture of the pad. Also note that pre-burnished or scorched pads need to be bedded in to generate the necessary transfer film. Properly bedding and transfer-film application increases the optimum stopping performance of the brake pad and can reduce pad and rotor wear.
As an example of the process, Baer Claw systems feature Baer’s ceramic-based SPORT TOURING “D-compound” brake pads. If both the rotor and pad are new and the rotor surfaces are unplated, it is important to run the pads through normal commuting-type driving for at least 150-200-miles before using them aggressively. If the new rotor surface finish is plated or the rotor is used with a compound other than the SPORT-TOURING ceramic-based pad, increase the commuter type driving with no hard use to a total of 250-300-miles to accomplish the blending of the pad surface to the rotor surface, and never drag the brakes.
Sample Brake Pad Bedding Procedure:
1. Perform four repeated light to medium stops, from 65 to 10 mph, to bring the rotors to temperature.
2. Perform three light stops in succession. Perform eight heavy stops, back to back, at a point just pending wheel lock, from 65 mph to about 5 mph.
3. Drive for ten minutes to create cooling airflow, without using the brakes if at all possible.
4. Perform three light stops in succession. Perform eight heavy stops, back to back, at a point just pending wheel lock, from 65 mph to about 5 mph.
5. Drive for ten minutes to create cooling airflow, without using the brakes if at all possible.
Braking Points to Ponder
• Switching from carbon metallic pads to semi-metallic brake pads (not recommended) – When switching from carbon metallic pads to semi-metallic brake pads, the new pads will need to wear through the layer of carbon that the old pads have deposited in the rotor surface. The new pads won’t grip well at all until this layer of carbon is removed. Whenever switching from one manufacturer’s pad to another’s, be sure to turn the rotor to remove the transfer film. If not, brake dust could be heavy, they may brake poorly, and possibly create excessive vibration.
• Road Race/Track Day: Racers should “bed in” a few sets of pads at a time. In the event you need to change brake pads during a race, you MUST use a set of “bedded” pads. Racing on non-bedded pads leads to “green fade”. Always allow a substantial coast down zone when bedding pads that will allow you to safely drive the car to a stop in the event of fade. Use pads with no shims. The shim is not attached with high temperature glue and can dislodge and rub against the hub of the rotor like a blade. Hawks HP-Plus pads have no shims.
• Race Pads on Street Wheels: Besides the performance disadvantages to using race-compound pads on street-driven cars, there’s also the fact that the dust generated by their compounds may actually damage the finish on street wheels. DeMarino explained, “It’s simple physics: when the 1000 degree F particles that are released melt into the clear coat of the wheel, it does bad stuff. If you’re going to be on a track regularly, use different rims!”
• Four at a Time: If you’re going to change to race pads, do it on both ends of car to preserve the correct brake bias built into the car. Warped rotors, boiling brake fluid and galling rotors can occur because the fronts are doing all of the work if you don’t change all four sets of pads.