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.
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.
Fade in drum brakes
High sensitivity of drum brakes due 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 1950s by self adjusting brakes. Maladjustment with wear is still a factor in trucks that use drum air brakes. 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)'
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. Their disk surfaces also have no gas venting features.
Railroads have been using disk brakes on passenger cars for more than 60 years, but coupled with a Rolokron anti-lock system to avoid the creation of flat spots (or “square wheels”) when wheels locks and skid on the rail surface (audible as rhythmic bang-bang-bang noise as a train goes by). Usually, brake disks are installed in the center of the axle, but in some applications (such as Bombardier Bi Level commuter cars), only one disk is used, mounted on the axle end outside the truck frame. High speed trains (such as the TGV) may use four disks per axle.
Freight cars (and some passenger cars like multiple-unit cars whose traction motors do not yield room on axles to allow the placement of disk brakes) are equipped with clasp brakes which directly grab the rolling surface of the wheels (much like the old horse buggy brakes of yesteryear). Such brakes are far less susceptible to locking than disk brakes so freight cars are not equipped with anti-lock systems.
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 be used as gently as possible for the first 100 miles or so to "break them in" and eliminate green fade.
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 standard 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 discs/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.
The "gaseous boundary layer" is an hot rod mechanics explanation for failing self servo effect of drum brakes because it felt like a brick under the brake pedal when it occurred. To counter this effect, brake shoes were drilled and slotted to vent gas. In spite of that, drum brakes were abandoned for their self servo effect. Disks do not have that because application force is applied at right angles to the resulting braking force. There is no interaction.
Adherents of gas emission have carried that belief to motorcycles, bicycles and "sports" cars, while all other disk brake users from the same automotive companies have no holes through the faces of their discs, although internal radial air passages are used. Vents to release gas have not been found on railway, aircraft and passenger car brakes because there is no gas to vent. Meanwhile heavy trucks still use drum brakes because they offer more heat dissipation than disks that would fit in the same space. Railways have never used internal expanding drum brakes because they cause skidding, causing expensive flat spots on steel wheels.
Both disc and drum brakes can be improved by any technique that removes heat from the braking surfaces.
Drum brake fade can be reduced and overall performance enhanced somewhat by an old "hot rodder" technique of drum drilling. A carefully chosen pattern of holes is drilled through the drum working section; drum rotation centrifugally pumps a small amount air through the shoe to drum gap, removing heat; fade caused by water-wet brakes is reduced since the water is centrifugally driven out; and some brake-material dust exits the holes. Brake drum drilling requires careful detailed knowledge of brake drum physics and is an advanced technique probably best left to professionals. There are performance-brake shops that will make the necessary modifications safely.
Brake fade caused by overheating brake fluid can also be reduced through the use of thermal barriers that are placed between the brake pad and the brake caliper piston, thus reducing the transfer of heat from the pad to the caliper and hydraulic brake fluid. Some high-performance racing calipers already include such brake heat shields made from titanium or ceramic materials. It is also possible to purchase aftermarket titanium brake heat shields that will fit your existing brake system to provide protection from brake heat.
References and sources
- Tire Rack article accessed 2006-10-23
- Friction Material Standards Institute
- Titanium Brake Heat Shield Technical Info
- Times Online article accessed 2006-10-23