Spend any time at the track and you’re bound to hear about–or even experience–brake fade. When 3500-pound machines get into triple-digit speeds, repeatedly turning all that kinetic energy into heat is going to have some side effects. Knowing how to spot the various heat-related failure modes–and how to correct them–is key to success in braking zones.
Pad Fade: Hard Pedal Yet Not Enough Stopping
Pad fade occurs when the excess temps reduce the friction between the brake pads and the rotor surface. Typically, the extreme heat causes an outgassing of the binding material used to make a brake pad.
That gas layer reduces the friction at the point of contact between the pad and the rotor, resulting in reduced stopping force. Other possible pad failure modes from too much heat: The pad material can also liquify a microscopic surface layer or simply crumble to a fine dust.
Typically, pad fade is associated with a firm brake pedal yet insufficient stopping force–basically, brake pedal travel remains unchanged because the calipers are actuating normally, yet the pad can no longer deliver the desired stopping power. Pad fade frequently results when the pads are inappropriate for the level of heat they’re dealing with.
The wrong brake pads–the incorrect compound or just ones that have glazed over–can cause fade. Photography Credit: David S. Wallens
Pad fade can also lead to second-order negative effects on the braking system. One of the most common is usually referred to as “glazing” of the rotor. In these cases, the extreme temperatures produced by the pad and rotor interface can cause the crystalline structure of the iron rotor to change: Carbon atoms become excited enough to bond to iron atoms, producing a carbide compound called cementite.
If you’ve ever been too aggressive with too dull of a drill bit going into unlubricated material, you’re familiar with the concept of work hardening. Well, that’s basically what’s going on here. These cementite deposits are extremely hard but extremely brittle. They’re also poor heat conductors and have a much different coefficient of friction than the rest of the rotor.
So, if you’ve experienced heat-related pad fade failure and are left with a pulsating brake pedal–what many people incorrectly call a warped rotor–you’ve likely deposited these super-hardened patches of material on the rotor face and are now feeling various frictions with every rotation.
Fluid Fade: Soft Pedal Yet Not Enough Stopping
Fluid fade can also be a problem. That’s when too much heat has transferred from the pad/rotor interface to the hydraulic fluid found in the calipers and lines. That heat reduces the compression properties of the fluid.
Brake systems use hydraulic fluid because it’s incompressible, providing a fluid-mechanical link between the pedal and the calipers–the things that actually squeeze the pads against the rotors. When that compressibility is altered, the results are bad for your brakes.
The wrong brake fluid–contaminated by moisture and/or just not engineered to withstand the temperatures of the task–can also lead to brake fade. (Hot tip: Grab one of these inexpensive testers.) Photography Credit: David S. Wallens
Fluid fade manifests as a long pedal with little to no initial stopping force and is caused by a compressible pocket that has developed in the fluid system due to boiling. This boiling can come from a few sources: the fluid itself, fluid that has had its boiling point reduced by the emulsification of water molecules, or water that has intruded into the fluid lines.
In any of these cases, this boiling produces an air pocket–which is definitely compressible–and pumping the pedal just compresses that pocket. If you’re lucky, a few pumps of the pedal will compress this pocket to the point where it starts pushing fluid again. Pray this happens before the wall.
Fixing That Fade
Now, how can you rectify these failures? We’ll start with pad fade since it’s typically easiest to fix.
In this case, the answer is simply to use an appropriate set of pads for your application. A great deal of pad fade can be traced to OEM or cheap parts-store pads simply not being up to the elevated temperatures seen on track (although most OEM pads these days are much better than they were just a decade ago).
Any reputable brake company will publish ideal temperature ranges for a given set of pads and provide detailed descriptions of the best use cases for each compound. That’s not just marketing speak, even if a lot of it is wrapped up in flowery language. Those descriptions cover the physical characteristic of each compound so customers aren’t surprised when entering that first corner.
Proper bedding of new pads and rotors can also ensure that those pads enter their proper operating temperatures protected from uneven heating. Bedding is the process of bringing pads and rotors up to temperature in a steady, controlled fashion. With new components, this transfers a layer of pad material to the rotor in an even fashion, ensuring even friction across the surface.
Skipping this step, or overly aggressive bedding, can leave uneven deposits across the rotor surface. This can lead to spot hardening, which can lead to failure and cracking, which can lead to sadness–and nobody wants that.
Brake companies usually supply their own bedding instructions, but typically the operation consists of several hard-but-not-threshold brake applications that slow but don’t fully stop the car. Once everything is hot enough, allow the brakes to cool progressively and evenly, usually while still in motion to avoid hot spots with parked pads. Again, read the instructions.
The proper solution for many of us? Both pads and freshly bled fluid that can handle the task at hand–and, when necessary, appropriate ducting. Photography Credit: Chris Tropea
Preventing fluid fade is mostly a matter of ensuring that your fluid is up to the task and free of contamination and moisture. For the former, most track-capable brake fluids list the dry boiling temperature right on the container–in many cases it’s even part of the name.
We recommend fluid with a bare minimum of a 500°F dry boiling point, but for heavier cars or more serious track use, something even more heavy-duty will provide additional headroom.
Remember that brake fluid can only resist boiling to the maximum temperature of its components with the lowest-boiling-point component. Put more simply, contaminated brake fluid boils easier than fresh stuff, particularly if that fluid is contaminated with water–which is more likely than you may realize.
Brake fluids are rated for both dry and wet boiling points, with the latter denoting the fluid’s performance once contaminated with just 3.7% water–theoretically, that comes from just two years of normal street service.
Remember our hypothetical minimal fluid carrying a dry rating of 500°F? Adding 3.7% water pulls its wet boiling point down toward the 300° mark. A motorsports-grade fluid carrying a dry boiling point close to 600° would only be good for about 400° once “wet.”
The repeated heating and cooling of brake systems is actually a pretty good moisture magnet, and it’s worth testing your brake fluid for moisture about as often as you check your oil. A $15 moisture tester can be an easy-to-use, instant-check device.
Brake fluid showing any signs of moisture should be flushed and replaced. If you live in a hot, humid environment, this probably means after every few track events–if not even more often. At a bare minimum, cars that see occasional track use should get annual brake fluid changes, particularly if they’re driven home from a hard weekend of track use and then parked for weeks at a time.
Also, remember that whenever brake fluid boils, its physical properties are altered such that it’s far less able to resist boiling again. Any brake fluid that boils, dry or wet, should be flushed before the next session.
We’ve said this too many times to count, but brakes are the most powerful speed-altering device you have at your disposal. They’re also the only thing keeping you out of the gravel (or the tire wall) beyond the next braking area.
Learn to recognize the signs they give you when they’re working beyond their capacity, and they’ll keep treating you well lap after lap.