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Tech Topic - Brake Fade

What causes it?


Let's start this topic with a definition.

Brake Fade - Loss of braking effectiveness due to excessive operating temperature of one or more brake system components.

Don't look it up in the dictionary, I just made it up. My definition excludes loss of braking effectiveness due to mechanical failure or wear conditions.

Brakes function by converting kinetic energy into heat energy. The heat is then dissipated to the atmosphere. Fade happens when we try to force the brakes to convert energy at an average rate that exceeds their heat dissipation capacity. We can do this through repeated heavy brake application without allowing adequate recovery time. The result is accumulation of heat and rising temperature culminating in brake fade.

Brakes that have high heat dissipation capacity tend to resist fade. Porsche brakes are among the most fade resistant available from any car manufacturer. They are more than adequate for aggressive street driving. However put them on a race track, push them hard and fade may become a very real and dangerous problem.

Although brake fade is always the result of heat accumulation and high temperature, there are three different ways brakes can fade. These are pad fade, fluid fade and green fade.

Pad Fade - Brake pad material has a coefficient of friction that varies with temperature. Pads designed for street use have a high coefficient at ambient temperature that remains fairly stable up to about 700F. As temperatures grow higher, street pad friction coefficients begin to decline rapidly. 

Racing brake pads have a low coefficient of friction at ambient temperature. Their coefficient climbs with temperature and reaches a plateau. The coefficient typically begins to decline above 1200F or so.

The decline in the friction coefficient is actually a result of the pad material beginning to melt. Typically it is the binder resins that melt first. The result is a lubrication effect and can be later observed as pad glazing after cooling. 

Alternatively pads can release gases at high temperatures. The gases are released between rotor and pad and create a hydroplaning condition. Although gas release was once a common problem, modern pads and binder materials are far less likely to out gas once they have been properly bedded.

Fluid fade - Get the brake fluid hot enough and it will literally boil in the calipers. The result is gas bubbles in the calipers and brake lines. The gas bubbles are compressible and cause long pedal travel with a spongy feel. Enough gas in the system can result in complete loss of braking. Racing brake fluid has a higher boiling temperature and is resistant to fluid fade.

Green Fade - New brake pads release gases the first few times they reach high temperature.  As noted in the pad fade section gases are released between the pad and rotor and create a hydroplane condition. The result is loss of friction.

Green fade can be avoided by breaking in new pads also known as "bedding in" the pads. The bedding process brings the pads up to high temperature under controlled conditions. Modern pads that are properly bedded are disinclined to out gas. Each brake pad manufacturer has its own bedding procedures that should be reviewed and followed.

Chuck Moreland - August 2002

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