Fade Resistance in Car Brakes: Meaning, Calculation & More
Fade resistance refers to the ability of the brakes to resist the loss of friction and stopping power over multiple, sustained applications [1]. It is an important measure of brake performance and safety. When brakes fade, they can take longer to slow or stop the vehicle. Fade resistance helps ensure the brakes continue working effectively even after repeated hard stops. This prevents potentially dangerous situations where the brakes overheat and lose stopping power. Higher fade resistance allows drivers to brake hard multiple times in succession without losing braking performance. This is especially critical for high performance driving and race conditions. Maximizing fade resistance ensures consistent, reliable braking ability.
How Brakes Work
Brakes in cars work through friction between the brake pads and the rotors attached to the wheels. When the driver steps on the brake pedal, this pushes brake fluid through hydraulic lines to the brake calipers which clamp down on the rotors. The friction between the pads and rotors slows the wheels and stops the car. Most modern cars use disc brakes which have pads that squeeze rotors on both sides. Older cars often used drum brakes where curved pads squeezed the inside of a drum attached to the wheel [1][2].
The hydraulics in the brake system allow the driver to stop a heavy car with light pedal pressure. The pedal push gets multiplied by hydraulic pressure in the lines and calipers, allowing for strong clamping force on the rotors. The incompressible brake fluid transmits the pedal movement instantly to all four wheels, bringing the car to a stop. As the pads clamp the rotating rotors, friction converts the kinetic energy of the car into heat, slowing it down.
What is Brake Fade?
Brake fade refers to the reduction in braking power and performance of the brakes when they overheat from repeated or extended braking. As the brakes get hotter, their ability to slow down the vehicle decreases (Tires Plus, 2019).
Brake fade occurs when the heat generated from friction cannot dissipate quickly enough. The main causes of brake fade include:
- Excessive braking over a short period of time, like repetitive braking in stop-and-go traffic (Delphi Auto Parts, n.d.).
- Braking when going downhill for extended periods of time (Delphi Auto Parts, n.d.).
- Braking under high loads, like when towing a heavy trailer (Delphi Auto Parts, n.d.).
As the brake components overheat, the brake fluid can boil, causing a soft or spongy brake pedal. The overheated pads and rotors are also less effective at creating friction to stop the wheels. All these effects reduce braking power and lead to brake fade (Tires Plus, 2019).
Measuring Fade Resistance
Fade resistance is commonly measured through standardized test procedures that quantify changes in brake performance during repeated stops from high speeds. Some key ways to quantify fade resistance include:
Fade tests like the SAE J2521 procedure involve making a series of stops from 60 mph to 0 mph at a set time interval, then measuring changes in stopping distance, pedal force, and rotor temperature after each stop. The brake temperature where fade begins and the magnitude of pedal force or stopping distance increase are used to evaluate fade resistance (Dixcel).
Another metric is the friction coefficient, or the ratio between braking force and normal force on the brake pads. Measuring how little the friction coefficient changes from beginning to end of a fade test provides a quantification of fade resistance (Crash Forensics).
Comparing the number of repeated stops a brake system can make from a set speed until totally faded is another established testing method. For example, Car and Driver tested various vehicles to find the maximum number of 60-0 mph stops they could make before fading (Car and Driver).
Factors Affecting Fade
There are several factors that can contribute to brake fade:
Brake pad material plays a significant role. Organic pads are more prone to fade compared to semi-metallic and ceramic pads. Organic pads can’t withstand high temperatures as well. Semi-metallic and ceramic pads are formulated for improved heat dissipation.
Brake fluid is also a key factor. As brake fluid absorbs moisture over time, its boiling point is lowered. This causes it to vaporize under high braking temperatures, resulting in a loss of hydraulic pressure. Using high quality, fresh brake fluid is important for maintaining fade resistance.
Rotors must be able to dissipate heat effectively. Cracked, warped, or contaminated rotors are more susceptible to overheating and fade. Proper bedding-in of new rotors is essential for optimal performance.
Environmental conditions like high ambient temperatures and frequent mountain driving can accelerate brake fade. More heat gets transferred to the brakes, pushing them past their limits quicker.https://www.medmal-law.com/blog/2022/february/what-factors-cause-brakes-to-fade-or-fail-/
Improving Fade Resistance
There are several ways to improve the fade resistance of brakes and reduce the chances of brake fade occurring:
High Heat Capacity Pads
Upgrading to high performance brake pads that are designed for high temperature applications can help improve fade resistance. Heat-resistant compounds like carbon-ceramic pads can withstand much higher temperatures than standard pads before fading occurs (Source). The tradeoff is these pads tend to be noisier and produce more dust.
Slotted/Drilled Rotors
Slotted or drilled rotors provide more surface area for heat dissipation, allowing the brakes to stay cooler under heavy braking. The slots and holes allow hot gases and air to escape rather than becoming trapped near the rotor surface. This improves airflow and reduces heat buildup (Source).
Brake Cooling
Additional cooling measures like brake ducts or fans can be added to force cool air over the rotors and calipers. Cooling the brakes prevents excessive heat buildup during repeated heavy braking. Race cars and high performance vehicles often utilize brake ducts specifically for cooling (Source). Keeping the brakes cooler overall increases fade resistance.
Fade Resistance Standards
There are some key standards that are used to measure fade resistance for brakes:
Government Standards
Many governments have standards in place to regulate minimum levels of fade resistance in vehicle brakes. For example, in the United States, Federal Motor Vehicle Safety Standard (FMVSS) No. 135 specifies minimum fade resistance requirements for hydraulic brake systems on vehicles under 10,000 lbs gross vehicle weight rating (GVWR) [1]. The standard requires brakes to undergo a series of stopping tests from high speeds, with limited rest periods in between to induce brake fade. Braking effectiveness must stay above certain prescribed minimums throughout the test.
Manufacturer Standards
Most automotive manufacturers also have their own internal standards and test procedures focused on verifying fade resistance that meets or exceeds government regulations. For example, General Motors has a brake system validation procedure where vehicles undergo simulated mountain descent driving on a dynamometer test rig to ensure adequate fade resistance [2].
Aftermarket brake component manufacturers similarly test and validate the fade resistance of their products to published standards so that customers can be assured of a certain level of performance.
Testing Brake Fade
There are two main methods for testing brake fade:
Track Testing
Track testing involves driving the vehicle on a closed circuit or test track under controlled conditions. The vehicle is subjected to repeated hard braking from high speeds to simulate real-world fade conditions. Instrumentation and data logging equipment can precisely measure brake temperature, pedal pressure, stopping distance, and other metrics to evaluate fade resistance over many cycles of brake use. Track testing provides the most realistic assessment of brake fade in a safe, controlled environment [1].
Simulated Testing
Simulated testing uses machines like brake dynamometers to repeatedly apply the brakes in a laboratory setting. The brake assembly is mounted to the machine and cycled through heat buildup and hard stops to induce fade. This allows fade characteristics to be evaluated without the risks of testing on the road. Simulated testing provides very consistent and repeatable results, but does not replicate real-world conditions as precisely as track testing. Overall, a combination of track and simulated testing delivers the most complete brake fade data.
Symptoms of Brake Fade
Two of the most noticeable symptoms of brake fade include:
- Spongy pedal – With brake fade, you may notice the brake pedal feels soft and spongy when you press on it. This is due to a loss of hydraulic pressure in the brake fluid as the fluid heats up and expands.
- Loss of braking power – As the brakes overheat, you’ll experience a loss of braking power and torque. You’ll have to push the pedal farther to get the same slowing effect. In extreme cases, the brakes may fail completely for a short time until they can cool down again.
According to Autonation, other symptoms can include pulling to one side when braking, brake noise or vibration, and brake odor from overheated components.
Conclusion
In summary, brake fade is an important issue for drivers to understand and manage properly. It occurs when brake system components overheat from repeated or heavy braking, which reduces braking effectiveness. Proper maintenance, brake pad/rotor selection, and avoiding over-use of brakes can help minimize brake fade. While modern brakes are designed to resist fade, it can still occur under extreme conditions. Being aware of the symptoms of brake fade is critical so that drivers can take steps to allow brakes to cool when needed. Understanding the causes and physics behind brake fade also allows drivers and mechanics to make informed choices to improve fade resistance through proper equipment and driving habits. Overall, managing brake fade is vital for safe vehicle operation and reducing the risk of brake failure in critical situations.