FindItMore | Chances are that you haven’t ever experienced the phenomena of “brake fade” when driving a vehicle. Fortunately, today’s braking systems don’t “fade” often but under the right conditions, it can happen. In this article, Eastern’s Automotive Group of DC, Maryland, and Virginia, looks at what brake fade is and what you should know about it.
How it acts
Brake fade is a term used to describe the reduction or complete loss of braking power when you apply your vehicle’s brakes. It occurs when a vehicle’s brake pads and the brake rotor no longer generate sufficient friction to slow down the vehicle. The end result is ineffective braking, usually resulting in increased stopping distances.
The physics behind the scene
Brake fade is caused by overheating of the brake pad, therefore any vehicle which uses a brake pad rubbing on a brake rotor to convert the vehicle’s kinetic energy into heat has the potential to develop brake fade. This includes motorcycles, cars, and trucks. Because brake fade occurs when the brake pads are overheated, the phenomenon is only temporary and normal braking performance usually returns once the brake system has cooled down.
You should know the difference between brake fade and spongy brakes. Spongy brake is an unofficial term used in the automotive trade for “when a vehicle’s brakes seem to take up most of the braking pedal movement to stop the vehicle.” It sort of feels like one is stepping on a stiff sponge, which is where the term comes from. Spongy brakes occur when there is air in your brake hydraulics or the brake fluid has boiled.
Its all due to heat
The brake pad in any brake system is designed to work within certain operating temperatures and if used within this operating window the braking system will work fine. Get things too hot, though, and your brakes might not work. The reason? The friction compound of modern organic brake pads is a precise mix of many different materials and these individual materials perform differently under temperature. How these constituent materials respond at elevated temperatures defines the performance characteristics of that brake pad. Different formulations of brake pad can, therefore, perform very differently from each other and it is important to use the right brake pad for the right application.
Two different kinds of brake fade
“Early life brake fade” – Often called “Green Fade,” this is quite common and almost normal when new brake pads are installed on a car. It is merely a “burning in” of the new brake components and may be gone in a few brake applications. To minimize or avoid this effect it is wise to drive cautiously when your brakes are new and give yourself a little extra braking distance for the bedding in period.
It is also worth considering that brake pads are to a degree porous, hence they will absorb a small percentage of water vapor when they are new from the surrounding air. Water, of course, boils when heated and instances of green fade which are observed as a result of the emission of water content in the pad. This water vapor will quickly burn off as soon as you get some temperature into the pads.
“Stopping brake fade” – Often called “Dynamic Fade,” this is when your brakes get overheated and they don’t work as well. Sometimes it is a result of low quality brake pads. Dynamic brake fade is particularly undesirable during fast driving, since once the driver has committed to stopping their vehicle within a certain distance, there is very little they can do mid-braking to make the correction.
In the early days of drum braked vehicles, brake fade was more prevalent. Cooling of drum brakes was minimal and meant that heavily loaded or towing vehicles braking down a long descent could superheat the brake shoes, causing the surface of the friction material to vaporize inside the brake drum and leading to an almost complete loss of braking power. Traffic authorities even built vehicle run off sand traps as an emergency route for vehicles descending out of control.
Organic brake pads
Organic brake pads inherit their name from the organic phenolic resins used to bind together the different compounds used in the pads construction. There are many types of thermoset phenolic resin, but most have a maximum temperature up to which they are stable. The dominant mechanism causing brake fade is the overheating of the phenolic resins and other materials in the friction lining, which creates a film of gas at the pad-rotor interface and effectively causes the brake pad to skid off the disc. As these gasses build up between the pad-rotor, they produce backpressure which creates an opposing force to the brake caliper that is trying to hold the pads against the rotor. If there is no way for the gasses to escape, the opposing force as a result of the outgassing causes brake fade.
Special brake components
Many brake manufacturers offer slotted and dimpled brake rotors today. These high-performance brake components help sweep away the gas build up every time a slot or dimple passes over the pad surface. Cost, coupled to the fact that the vent holes can rapidly become clogged with brake dust, makes this concept unusable for standard factory brakes.
Brake fade for racers
Racecar driving is a good example of overloading braking systems. A weekend warrior takes their street-based car to a race track and drives at speeds not seen on the highway and brakes. This immediately superheats the pad and kills the friction compound, causing it to operate at conditions it was not designed for. Typically, brake temperatures shoot past 1000 degrees F and few standard brake pads will tolerate that.
If you experience brake fade on the road or on the track, consider installing an oversize brake kit with larger diameter rotors and curved internal vanes to help with heat dissipation and eliminate unwanted fade.