Remember the law of conservation of energy? High school physics at its best! The law basically states that energy can neither be created nor destroyed. Instead, it is transferred from one form to another.
Now, consider how a vehicle is set into motion: it converts potential energy (in fuel or battery cells) into kinetic energy (through engines or electric motors). In order to stop the vehicle, the kinetic energy needs to be converted into another form of energy. This other form of energy is “thermal” energy.
Brake systems work by mainly converting kinetic energy into thermal energy (or heat). The friction provided by the pads interfacing with the rotor’s surface creates heat and that heat is then dissipated into the atmosphere through the brake rotors, the airflow over the brake system, the brake pads themselves, and the brake calipers.
It is possible to have too much energy converted by the brake system. When this happens, the system can’t transfer the built-up heat to the surrounding environment. This raises the temperature of the brake system to a point over what the system has been designed to handle. At that point, like most materials, the brake pads will start to break down (or melt). As that happens, the binding resins/components begin to give off gas in what’s called outgassing.
When outgassing occurs, a thin layer of gas forms between the pad surface area and the rotor surface area. This layer acts as a slippery barrier that prevents full contact of the two aforementioned surfaces. The hotter the system gets, the more gas is let out, and the less effective your brake system is. This is the mechanism behind brake fade.
When you’re most likely to experience Brake Fade
All brakes can be expected to fade at one point or another if you push your brakes hard enough. This most often happens during spirited driving or during towing applications where weight increases significantly.
For those preferring to get a little more theoretical, remember that the equation for kinetic energy is ½ x [mass] x [velocity]2. So, we can see that the two variables that effect energy are mass and velocity. As mass increases, energy will also increase linearly assuming velocity remains the same. However, when velocity (speed) increases then there is an exponential change in energy. This is the energy that will need to be transferred to thermal energy. The more kinetic energy built up, the more energy you’ll have to convert into thermal energy.
The below graph illustrates energy build up at different speeds and different weights. The energy built up is the energy that will need to be converted into thermal energy in order to stop the vehicle. You can see how energy increases significantly as speed increases for a given vehicle weight. Doubling the weight of a vehicle will double the energy whereas doubling the speed of the vehicle will quadruple the energy!
How to Mitigate Brake Fade?
Understand that brake fade basically happens when the rate of thermal energy build-up is greater than the rate of heat dissipation of the brake system. When this happens, the system will over-heat and the brake pad compound will break down resulting in outgassing which hinders performance levels of the brake pad.
So, what should you do if you are experiencing brake fade?
- The First Step: Make sure your brake pads are suitable for your specific driving style. Choose a brake pad that has been designed for street performance, performance, towing, or whatever you need it for. These more focused brake pads will have higher Maximum Operating Temperatures and will most likely offer you better control over your brakes and vehicle. Here is an article to help you decide what type would suit you.
- The Next Step: Use rotors that help with airflow and gas management. Slotted, cross-drilled, or slotted and cross-drilled rotors provide escape paths for gasses during high-temperature operation. These escape paths will help in maintaining contact between the pad surface and the rotor surface. Rotors with air vanes will help in circulating the air throughout the system which in turn increases the rate of heat dissipation. Features such as the cross-drilled holes also have the added benefit of increasing airflow through the rotors which will help in managing heat build-up in the system. A guide for rotor selection can be found here.
- The Extra Step: Use two-piece rotors to help with heat dissipation. Two-piece rotors generally use aluminum alloy hats and have much better airflow capabilities. For the majority of street and track enthusiasts, a standard one-piece brake rotor will suffice. However, if you find you need extra heat management capabilities (along with better handling) then two-piece rotors will be the next option.
Should you ever find that the above solutions don’t work for you, then odds are you are in an area of extreme driving conditions and a complete upgrade of your brake system may be the best solution. In this case, we suggest you consult directly with your service shop or with a brake system manufacturer to get a better idea of what options are available to you.