Preventing Brake Fade: Maximizing How Friction Materials Handle Heat – UnderhoodService

Preventing Brake Fade: Maximizing How Friction Materials Handle Heat

Brake fade is something nobody wants to experience. When you step down on the brake pedal with a certain amount of force, the vehicle should slow predictably. But when the brakes get too hot and start to fade, it takes more and more pedal effort to get the same amount of braking force. Eventually the point is reached where the vehicle won’t stop no matter how hard you stand on the brakes — and the results can be disastrous.

The brakes on most vehicles that are driven normally in everyday traffic never get hot enough to experience brake fade. But when driving down a steep mountain, prolonged braking may cause the brakes to overheat and fade (which is why drivers should never “ride” the brakes and should pull over so the brakes can cool if they start to feel any significant fade).

The brakes may also overheat and fade if the pads or shoes are dragging because of a mechanical or hydraulic problem in the brake system. The constant friction builds up heat and can increase the risk of brake fade when the driver steps on the pedal.

Brake fade also occurs on the racetrack. The combination of high speeds and heavy braking can push brake temperatures to the limit. If a racecar is equipped with pads that can’t take the heat, or rotors that can’t provide adequate cooling, the driver will probably find himself in serious trouble long before he ever sees the checkered flag.

The basic purpose of any brake system, whether it’s on a Kia, a minivan or a NASCAR racecar, is to slow the vehicle by means of friction. When the brakes are applied, friction created by the pads rubbing against the rotors slows the wheels and generates heat. Kinetic energy (the energy of motion) is converted into thermal energy (heat).

On an ordinary rear-wheel-drive car or truck, repeated high-speed braking can push the front brake temperatures to 300° F or higher. On a front-wheel- drive car, the front pads and rotors, which work even harder, may see 600 degrees or more with repeated braking. But on a racetrack, especially a short track where the driver is constantly on and off the brakes, brake temperatures of 800 to 1,100 degrees are not unusual. On a road course, the rotors are always hot and can sometimes reach temperatures as high as 1,400° F! Under these conditions, the rotors will glow bright orange and give off sparks when the brake pads make contact.

Unless the brakes can quickly dissipate all this heat, two things can happen. The heat builds up faster than the pads and rotors can get rid of it. Once this happens, the linings can no longer generate enough friction to stop the car and the brakes fade. Or, the brake fluid in the calipers can boil, causing a loss of brake pedal. Either way, the guy behind the wheel is in big trouble unless the brakes have time to cool and recover.

The secret to preventing brake fade is managing heat. It makes no difference how hot the brakes get as long as the system is designed to handle the heat so the brakes can bring the vehicle to a safe and controlled stop. Heat is managed by using friction materials that are designed to operate within certain temperature ranges, and by using rotors that have sufficient cooling capacity to keep the pads within their temperature design limits.

Vented rotors have fins between the rotor surfaces to pull air through the hub and rotor. The OEM rotors are designed to work with the brakes on a specific vehicle, so it’s important to make sure replacement rotors are a similar design and provide the same (or better) cooling.

Vehicle manufacturers use a wide variety of different rib configurations in their rotors to optimize cooling for different vehicle applications. So even though the brakes may appear to be identical on two different models, one may require increased cooling because the vehicle is heavier, has a more powerful engine or has less air flow to the brakes.

Most of the aftermarket brake engineers we’ve spoken with have told us they prefer to follow the OEM lead on rib design and use the same configuration, rather than change the design to reduce the number of SKUs in their product lines. The OEMs currently use almost 70 different rib configurations in their rotors. Differences include the number of ribs, the size and thickness of the ribs, the type of ribs (straight, curved or segmented), and the orientation and direction of the ribs. Some rotors are directional while some are not.

Many quality rotors use non-traditional rib patterns to maximize cooling and reduce harmonics that can cause brake squeal. Radial venting, where all the ribs are neatly aligned next to each other and evenly spaced, is “old technology” as far as the OEMs are concerned. But radial venting is still common in many aftermarket and economy rotors.

Changing the rib design changes the air flow, cooling and noise characteristics of the rotor — which may make things better or worse depending on the application. That’s why some aftermarket rotor manufacturers use the same design as the original unless there is a valid reason to change it.

One brake manufacturer showed us a cutaway of an offshore rotor for a particular vehicle that had 32 cooling ribs. The OEM rotor, by comparison, had 37 ribs and provided up to 15% better cooling when tested in the laboratory. Could such a difference have any real world impact on braking safety? It might, under hard use if the brakes got hot enough to fade.

What about rotors that are drilled or slotted to improve cooling? Won’t they help a vehicle resist brake fade? In theory, yes. Racers used to use cross-drilled rotors to increase cooling. But, at high temperatures, spider cracks can form and spread outward from drilled holes, increasing the risk of rotor failure (chamfering the holes reduces the risk of cracking). Drilled replacement rotors are available for street cars, but most serious racers today use rotors that have shallow surface grooves or slots rather than holes. Why? Because grooves are less apt to propagate cracks. Grooves also help clean the pads and allow hot gases that build up under the pads to escape for less fade at high temperature.

Friction materials are formulated to provide certain friction, noise and wear characteristics. For ordinary street-driven vehicles, the most important criteria are usually good stopping ability at normal brake temperatures, quiet operation (no noise!), predictable pedal feel whether the linings are hot or cold, and reasonable service life. Most friction materials designed for everyday driving have a high coefficient of friction and are fairly stable up to about 600 or 700° F. Beyond that, fade and wear become serious issues.

For performance-oriented vehicles and racecars, the most important criteria is the ability to brake hard with minimal heat fade at high temperature. Noise and wear are not a concern, provided the linings do their job. Friction materials designed for high-temperature use typically have a lower coefficient of friction at room temperature. But the coefficient of friction goes up as the brakes get hot. These types of friction materials are often good for temperatures up to 1,200° F before they start to fade. But, for everyday driving, they may be unsuitable because of their poor cold-braking performance. That’s why “race-only” pads should not be used on the street (severe rotor wear can be an issue, too!).

The loss of friction that occurs at high temperature happens when the pad material begins to melt. The phenolic resin that binds the filler materials together in the pads can oxidize and give off gases that act like a lubricant between the pads and rotors. And if the linings get hot enough, they may “glaze” over, leaving a permanent smear of partially melted material on the surface. Glazed pads cannot provide the same friction and braking as normal pads, and can also squeal. For these reasons, glazed pads should be replaced.

A condition known as “green fade” can also occur with newly installed brake linings. Regardless of whether the linings are “fully cured” or not, all linings undergo a short break-in period after they are first installed. When the resin gets hot, the linings outgas a small amount of vapor. This can create a friction-reducing layer between the pads and rotor that increases braking effort until the pads are broken-in. If the brakes are not broken in gradually by doing a series of controlled stops (a couple dozen gradual stops from about 30 mph, with about 30 seconds between stops for the brakes to cool), there is a chance the pads may fade or glaze if the vehicle is subjected to a sudden panic stop or hard braking.

Ordinary nonasbestos organic (NAO) friction materials are usually quiet and provide predictable braking at low brake temperatures. But as the temperature goes up, they soon start to fade. And the hotter the linings run, the faster they wear.

Semi-metallic friction materials, by comparison, are formulated for higher-temperature operation. The addition of chopped steel and other metal fibers to the material allows the lining to withstand higher temperature and also conduct heat away from the rotors — unlike NAO compounds which conduct heat poorly and act like insulators to keep the heat in the rotors.

Semi-metallic friction materials typically require a little more pedal effort when they are cold and work best when the brakes are warm or hot. Semi-metallic linings are also harder than NAO linings and typically experience much less wear at high brake temperatures. But the trade-off may be increased noise and rotor wear.

Ceramic friction materials, by comparison, use various types of ceramic fibers and other ingredients. The main advantages of linings with a high ceramic content are quieter operation (no squeal), reduced rotor wear (depends on the formula), cleaner braking (less dust) and good wear on most vehicles at normal braking temperatures.

Unfortunately, the term “ceramic” has been badly abused and misused. There are literally hundreds of different “ceramic” friction formulas in use today, so it’s hard to make sweeping generalizations about ceramic linings. Some pads have a high ceramic fiber content, others are essentially NAO compounds with a little ceramic added in, and still others are actually low-metallic compounds with some ceramic content. The exact recipes are all proprietary secrets, so there is no way to know for sure how much ceramic or what type of ceramic is actually in a set of pads.

Some brake suppliers have more than 20 different ceramic formulas in their product line, while others use only one. The problem with “one-size-fits-all” ceramic formulas is that they typically suffer from high “Mu Variability” — that’s engineering lingo for a lot of variation in hot and cold friction coefficients as the pads heat up. This can increase pedal effort and reduce stopping power when the brakes get hot.

Ceramic linings are often sold as an upgrade for vehicles that were not originally equipped with ceramics. As a general rule, you should always replace same with same (or better) to maintain the same feel, stopping power and braking performance as the OEM linings. This goes for OEM ceramic linings as well as semi-metallic linings.

Semi-metallic linings are designed for high-temperature applications and to resist fade when the brakes get hot. For larger SUVs and trucks where the brakes tend to run hot, semi-metallic linings will usually provide the best fade resistance at high temperature.

Finally, don’t confuse heat-related pad fade with heat-related fluid fade. If the brakes get hot enough to boil the fluid inside the calipers, the formation of bubbles may increase pedal travel to the point where the pedal goes all the way to the floor before the brakes can grab and stop the vehicle.

The risk of fluid boil is higher in older vehicles because the brake fluid is often saturated with moisture. Glycol-based DOT 3 and DOT 4 brake fluids are both “hygroscopic,” which means they attract moisture. Over time, moisture lowers the boiling temperature of the fluid, increases its viscosity and promotes internal corrosion. That’s why various chemical additives are put into the fluid to help it fight corrosion and oxidation. But the additives can’t counter the effects of moisture on the boiling temperature.

All brake fluid must meet minimum performance standards established by the U.S. Department of Transportation (DOT). For DOT 3 fluid, the minimum “dry” (contains no water) boiling temperature is 401° F and the minimum “wet” (saturated with water) boiling temperature is 284° F. For DOT 4 fluid, the minimum dry boiling temperature is 446° F and the wet boiling temperature is 311° F. Most fluids exceed these minimum specifications and provide an increased margin of safety. But after years of service (and neglect), the fluid is often saturated with moisture.

Only one percent moisture can lower the boiling point of some DOT 3 fluids down to 369 degrees. Two percent water can push the boiling point down to 320 degrees, and three percent can drag it all the way down to 293 degrees — which is getting dangerously close to the minimum DOT requirements, and temperatures that may be encountered under severe braking conditions.

The type of friction linings on the vehicle can also be a factor. Semi-metallic linings will conduct more heat away from the rotors to the calipers. One way to reduce the risk of fluid boil is to use a heat-insulating shim or coating on the backs of the pads.

Another factor in fluid boil is the type of pistons in the calipers. Aluminum and steel pistons conduct more heat from the pads to the fluid than phenolic pistons. Most OEM brake systems are designed with enough thermal capacity so fluid boil and fade is not an issue with normal braking. But with abnormally heavy braking or racing, all bets are off.

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