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| ], '''fade''', or '''brake fade''' is the reduction in stopping power (i.e. the reduction of friction between the brake pad and the brake rotor or drum) often experienced after repeated application of the brakes, especially in high load or high speed conditions. Brake fade can be a factor in any ] that utilizes a ] braking system including ]s, ]s, ]s, ]s, even ]s. | |||
| '''Brake fade''' in automotive ]s, refers to a reduction in | |||
| braking power caused by heating. Brake shoes (friction material), as | |||
| most materials, change friction coefficient when heated, in spite of | |||
| being designed to not do so. beyond that, drum brakes, the brakes | |||
| that suffer from fade, are highly sensitive to the friction | |||
| coefficient of their brake shoes. Because fade may not occur on all | |||
| wheels at the same time, it can cause the vehicle to swerve. If | |||
| significant fade occurs, typically while descending long steep grades, | |||
| brakes on all wheels may fade significantly and cause a runaway. An | |||
| inherent design feature of the ], formerly used in most | |||
| automobiles and still used on trucks causes fade. | |||
| 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. Brake fade can be reduced by appropriate equipment and materials design and selection. | |||
| Fade was commonly attributed to brake drum expansion from heat, that | |||
| reduced brake shoe contact with the drum. This ignores how little | |||
| thermal expansion a hot drum experiences and that the drum is formed | |||
| into an oval by brake shoe pressure. The more obscure reasoning was | |||
| that a gas bearing caused by vaporising brake shoes to separate brake | |||
| shoe and drum. When experiencing brake fade one could easily imagine | |||
| such effects even though they are not physically possible, considering | |||
| the gas volume required. Additionally, after cooling, faded brakes | |||
| perform as well as before with no visible change to their brake shoes. | |||
| In contrast, disk brakes, using the same materials operate well even | |||
| with glowing hot disks. | |||
| 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 ]s. ]s are much more resistant to brake fade and have come to be a standard feature in front brakes for most vehicles, although the brake rotors can become warped due to excessive heating{{fact}}. | |||
| The inherent problem that causes fade is the self energizing feature | |||
| ] of drum brakes that causes them to fade or become grabby. | |||
| This is evident when, on occasion on damp mornings, brakes lock solid | |||
| upon first application, skidding to a stop even after the brake pedal | |||
| is released. The servo effect of ] multiplies | |||
| small changes in friction coefficient to cause large changes in | |||
| braking. The opposite occurs when the friction coefficient decreases | |||
| as brakes become hot. In contrast, disk brakes have no servo effect, | |||
| making changes in friction coefficient hardly noticeable. | |||
| == Causes of brake fade == | |||
| Long dual-tire skid marks on highways made by trucks using drum brakes | |||
| are a visible example of non-linear brake response to brake pedal | |||
| pressure. Large truck drum brakes are used because they are | |||
| economical and fit easily where an equivalent disk brake would not. | |||
| Although railroads use disk brakes, they never 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. | |||
| The reduction of friction termed brake fade can be caused by the gasification of organic materials in the brake pad causing a gas cushion to form between the pad and disk<ref name="Times">http://driving.timesonline.co.uk/article/0,,22750-1560640,00.html Times Online article accessed 2006-10-23</ref>, or by the melting of the binding resins in the pad that then act as a lubricant<ref name="Times"/>. The former is most common on new pads and is termed "green fade"; brake manufacturers typically bake new brake linings to minimize green fade<ref name="Times"/>. In both of these forms of fade, the brake pedal feels firm but there is reduced stopping ability. Fade can also be caused by the ] 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 ] is standard maintenance. | |||
| The fading drum brake was replaced by the disk brake that has no servo | |||
| effect and therefore no fade and has an effect proportional to pedal | |||
| pressure (linearity), an effect that is achieved by having application | |||
| force and braking force at right angles to one another, thereby being | |||
| completely decoupled so that there can be no servo effect. | |||
| == Fade in drum brakes == | |||
| '''Reference:''' | |||
| In the case of ]s the change in dimension of components in response to heat; the curvature of the brake shoes then failing to match the curvature of the brake drum{{fact}}. | |||
| [http://digital.lib.msu.edu/collections/sesquicentennial/se/se-38.pdf?CFID=1834\ | |||
| 320&CFTOKEN=86763474|''Spartan Engineering 1959''] search: "fade". | |||
| == 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. | |||
| == 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. | |||
| == References and sources == | |||
| <references/> | |||
| http://www.tirerack.com/brakes/tech/techpage.jsp?techid=78¤tpage=87 Tire Rack article accessed 2006-10-23 | |||
| ==See also== | |||
| *] | |||
| *] | |||
| ] | |||
Revision as of 19:13, 20 February 2007
Braking system, fade, or brake fade is the reduction in stopping power (i.e. the reduction of friction between the brake pad and the brake rotor or drum) often experienced 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. Brake fade can be 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, although the brake rotors can become warped due to excessive heating.
Causes of brake fade
The reduction of friction termed brake fade can be caused by the gasification of organic materials in the brake pad causing a gas cushion to form between the pad and disk, or by the melting of the binding resins in the pad that then act as a lubricant. The former is most common on new pads and is termed "green fade"; brake manufacturers typically bake new brake linings to minimize green fade. In both of these forms 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
In the case of drum brakes the change in dimension of components in response to heat; the curvature of the brake shoes then failing to match the curvature of the brake drum.
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.
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.
References and sources
- ^ http://driving.timesonline.co.uk/article/0,,22750-1560640,00.html Times Online article accessed 2006-10-23
http://www.tirerack.com/brakes/tech/techpage.jsp?techid=78¤tpage=87 Tire Rack article accessed 2006-10-23