Brake fade

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Vehicle braking system fade, or brake fade is the reduction in stopping power that can occur after repeated application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any vehicle that utilizes a friction braking system including automobiles, trucks, motorcycles, airplanes, even bicycles.

Brake fade is caused by a buildup of heat in the braking surfaces and the subsequent changes and reactions in the brake system components and can be experienced with both drum brakes and disk brakes. Loss of stopping power, or fade, can be caused by friction fade, mechanical fade, or fluid fade. Brake fade can be significantly reduced by appropriate equipment and materials design and selection.

Brake fade occurs most often during high performance driving or when going down a long, steep hill. Owing to their configuration this is more prevalent in drum brakes. Disk brakes are much more resistant to brake fade and have come to be a standard feature in front brakes for most vehicles.

1 Causes of brake fade
2 Fade in drum brakes
3 Controlling fade through driving technique
4 Brake modification to reduce fade
5 References and sources

[edit] Causes of brake fade

The reduction of friction termed brake fade is caused when the temperature reaches the "kneepoint" on the temperature-friction curve. [All brake lining is cured under mechanical pressure following a heating & cooling curve, heating the friction material up to 450°F to "cure" (cross-link) the phenolic resin thermoset polymers: There is no melting of the binding resins, because phenolic resins are thermoset, not thermoplastic] In this form of fade, the brake pedal feels firm but there is reduced stopping ability. Fade can also be caused by the brake fluid boiling, with attendant release of compressible gases. In this type of fade, the brake pedal feels "spongy". This condition is worsened when there are contaminants in the fluid, such as water, which most types of brake fluids are prone to absorbing to varying degrees. For this reason brake fluid replacement is standard maintenance.

[edit] Fade in drum brakes

Truck air brakes can fade due to expansion if improperly adjusted

High sensitivity of Drum brakes to small changes in friction coefficient can severely affect their performance. Fade at high temperatures, caused by dissipating kinetic energy while slowing a vehicle can cause loss of braking while an increase can cause lock-up. Because fade may not occur simultaneously on all wheels, differential braking can cause a vehicle to swerve. As fade increases, typically while descending long steep grades, brakes on all wheels may fail and cause a runaway. High sensitivity to changes in friction coefficient is inherent to expanding-shoe drum brakes, the type formerly used on most cars and still used on many trucks, especially for rear axles and trailers.

The inherent design problem of drum brakes, from which their sensitivity to small changes in friction coefficient arises, is self energizing servo by which brake shoes, when pressed against the drum, push themselves into contact by their own friction, giving responses from lock-up to complete failure. For example, on damp mornings, drum brakes can lock on first application, skidding to a stop even after the brake pedal has been released. This occurs because servo or positive feedback magnifies small changes in friction coefficient. In contrast, with disk brakes, that have no servo effect, these small changes in friction coefficient are hardly noticeable.

Brake failure was also caused by brake drum thermal expansion in which brake shoe clearance had become excessive from wear. This was remedied in the 1950's by self adjusting brakes.^[1] Maladjustment with wear is still a factor in trucks that use drum air brakes. ^[2] Another explanation for brake failure was that heated brake shoes evaporated to generate gas that separate them from the drum. When experiencing brake fade one could readily imagine such effects even though they are physically impossible, considering the volume of gas required for such an effect, the gas bearing needing replenishment as fast as the disk moves, it having no gas on its surface when entering the interface. After cooling, faded brakes perform as well as before with no visible change to brake shoes. In contrast, disk brakes, using much the same materials, operate well even with glowing hot disks (color vs. temperature)'^{[citation needed]}

Long dual-tire skid marks on highways, made by trucks with drum brakes, are visible examples of non-linearity between brake response and pedal pressure. Large trucks still use drum brakes because they are economical and fit easily where an equivalent disk brake would not. More recently disk brakes for trucks have been promoted listing features such as no fade, made possible in the absence of brake servo.^[3] Their disk surfaces also have no gas venting features. Although railroads use disk brakes, they do not use drum brakes because steel wheels skid too easily and cause flat spots, an effect that can be be heard occasionally as a thumping sound on passing trains.

[edit] Controlling fade through driving technique

Brake fade and rotor warping can be reduced through proper braking technique; When running down a long downgrade that would require braking simply select a lower gear (for automatic transmissions this may necessitate a brief application of the throttle after selecting the gear). Also, periodic, rather than continuous application of the brakes will allow them to cool between applications. Continuous light application of the brakes can be particularly destructive in both wear and adding heat to the brake system. Finally, new brakes should used as gently as possible for the first 100 miles or so to "break them in" and eliminate green fade.

[edit] Brake modification to reduce fade

High performance brake components provide enhanced stopping power by improving friction while reducing brake fade. Improved friction is provided by lining materials that have a higher coefficient of friction than stock brake pads while brake fade is reduced through the use of more expensive binding resins with a higher melting point along with slotted, drilled, or dimpled rotors that reduce the gaseous boundary layer in addition to providing enhanced heat dissipation. Heat buildup in brakes can be further addressed by body modifications that direct cold air to the brakes.