Photonic
09-25-2008, 12:25 AM
For full article visit
http://www.pirate4x4.com/tech/billavista/Brakes/
(editing to fix some content check back shortly)
Brake Performance and Limits
There should be only one limit to how well your brakes work ***8211; traction. The traction between tire and road should always be the limiting factor to how quickly you can stop. Exceeding the traction limit causes the tires to lock-up and skid. If you can do this under all conditions, with reasonable and comfortable pedal effort, then your brakes are adequate. If not, some other factor is limiting their performance, and maintenance or modification will be required. The other, undesirable, limiting factors are:
Force Limit
At force limit, you push as hard as possible on the brakes but can***8217;t stop any quicker. If you could push harder, the vehicle would stop quicker. Force limit can be altered by reducing master cylinder size, using different brake pads, adding or upgrading power assist, increasing pedal ratio, increasing calliper piston area, or increasing effective rotor diameter.
Deflection Limit
At the deflection limit, the pedal hits the floor or the MC piston hits its internal stop before max brake force is generated. I.e. ***8211; the combination of designed clearance and unwanted deflection in the system exceeds the available travel. Clearance is determined by the volume of fluid required to be displaced to bring the pads into contact with the rotor, and is chiefly determined by the size of the calliper piston(s). Unwanted deflection can come from pedal and pedal-mount flex, MC mount flex, calliper bracket flex, calliper flex, hose expansion, trapped air, unevenly worn pads or rotors, and other worn components such as wheel bearings. Assuming all air is bled from the system and all components are in good working order, deflection limit can be altered by increasing MC piston diameter, decreasing calliper piston diameter, upgrading flex hoses, reducing the length of flex hose used, or using stiffer callipers, brackets, and pedals.
Wear Limit
The brakes effectiveness can be limited by worn components. If pads or rotors are worn excessively, or if any seal, tube, hose, bracket, or fitting is damaged, worn, or leaking, a reduction in performance, or worse, brake failure, can result. The likelihood of reaching the wear limit can be reduced by using different (higher quality) pads and/or rotors, using larger callipers and rotors, or increasing brake cooling. Good inspection and maintenance practices can prevent reaching the wear limit.
Temperature Limit
When brakes overheat, you reach one of the other limits (force, deflection, or wear) much more quickly. That is, hot brakes can cause lack of braking force (commonly called brake fade), increased deflection (excessive pedal travel), or rapid component wear ***8211; ranging from pad/rotor wear to boiling fluid and cooking seals. The keys to avoiding a temperature limit are:
Using sufficiently large/heavy brakes.
Sufficient brake cooling.
Selection of components designed for the temperatures encountered ***8211; including pads, rotors, callipers, and fluid.
Brake Fade
***8220;Brake fade***8221; generally refers to any loss of braking caused by overheating. In fact, there are three very distinct forms of ***8220;brake fade***8221;, and it is useful to distinguish between them, as their symptoms and solutions are entirely different.
Pad Fade.
Pad Fade is caused by the temperature of the brakes exceeding the maximum temperature limit of the brake pad friction material. When the maximum temperature limit is reached (and even before, as it is approached), brake pads can expel gases when heated, gasses that act as a lubricant between pad and rotor. When pad fade occurs, the pedal will feel ***8220;normal***8221; (high and firm) but there will be very little stopping power. Pumping the brakes will not help. Solutions to pad fade are:
To use a more temperature resistant brake pad (harder compound)
To increase the brake***8217;s thermal capacity by increasing the size/weight of the rotor
To improve cooling by using a different rotor design (vented vs. solid, or direction vs. straight vanes)
To improve brake cooling by improving cooling airflow to the brakes. In extreme circumstances, if the vehicle has air ducts for brake cooling, a fine mist of water can be sprayed into the air to cool it before it hits the rotor, improving the air***8217;s cooling ability.
To use a slotted and/or cross-drilled rotor to aid in expelling hot gasses and pad debris. Note that these designs DO NOT aid in cooling but simply allow the offending gas or particles to be removed more efficiently.
Fluid Boil
When the temperature of the calliper exceeds the boiling point of the brake fluid, tiny bubbles are formed in the brake fluid. As a result, the pedal goes soft and perhaps even to the floor as the fluid is no longer incompressible. Once this has happened, the fluid must be replaced. Over time brake fluid can absorb water vapour, and the more water vapour in the fluid, the greater its susceptibility to fluid boil. Solutions to fluid boil include: flush and fill with new brake fluid, use a brake fluid with a higher boiling point, improve cooling of the calliper, or use a calliper with an insulated piston.
Green Fade
Green fade occurs only with new brake pads if they are not ***8220;bedded-in***8221; properly after having been installed. Bedding-in brake pads is a process of breaking them in before severe use, similar to the way an engine or set of gears must be broken in. Green fade is caused by gas or liquid boiling off the organic pad lining on initial heating. The gas or liquid not only acts as a lubricant between pad and rotor, but can also ***8220;glaze***8221; the surface of the pads by cooling and re-solidifying into a hard slick surface with a very low coefficient of friction. The solution to green fade is to bed in brake pads. To do so, follow the steps outlined in "Bedding-in (http://www.pirate4x4.com/tech/billavista/Brakes/#Beddingin)".
Hard Pedal
***8220;Hard Pedal***8221; is the term used to describe brakes that require excessive pedal effort ***8211; i.e. the driver must push too hard on the pedal for the brakes to work. This condition ***8220;feels***8221; to the driver like the brake pedal is very ***8220;hard***8221;.
http://i86.photobucket.com/albums/k94/chukidori/HardPedal.jpg
Low Pedal
***8220;Low Pedal***8221; is the term used to refer to a brake pedal that requires an excessively long stroke to fully apply the brakes. What we are experiencing with low pedal is a deflection limit. In some cases, the brakes work only after being ***8220;pumped***8221; a few times. What***8217;s happening here is that the driver is able to displace more fluid more quickly than the callipers retract, and is therefore able to provide the volume required by pumping the brakes. This is usually a symptom of incorrect valving, such as lack of required residual pressure valve.
http://i86.photobucket.com/albums/k94/chukidori/LowPedal.jpg
Spongy Pedal
Often present with, but not to be confused with, low pedal; a spongy pedal starts high and doesn***8217;t travel too far ***8211; but it doesn***8217;t feel firm and solid. Instead, the brakes feel soft, squishy, and spongy when applied.
http://i86.photobucket.com/albums/k94/chukidori/SpongyPedal.jpg
Overly Sensitive
Brakes that develop enormous clamping force (usually by generating high pressure) with very little pedal effort will feel overly sensitive to the driver. They are hard to modulate, particularly for med- to high-speed work, and are generally unpleasant to drive. Exactly the point at which this occurs will be a matter of driver preference combined with the vehicle***8217;s use
http://i86.photobucket.com/albums/k94/chukidori/OverlySensitive.jpg
Maintanence Trouble Shooting
http://i86.photobucket.com/albums/k94/chukidori/Maintanence.jpg
http://240atlanta.com/forums/%5BIMG%5Dhttp://i86.photobucket.com/albums/k94/chukidori/Maintanence.jpg%5B/IMG%5D
http://www.pirate4x4.com/tech/billavista/Brakes/
(editing to fix some content check back shortly)
Brake Performance and Limits
There should be only one limit to how well your brakes work ***8211; traction. The traction between tire and road should always be the limiting factor to how quickly you can stop. Exceeding the traction limit causes the tires to lock-up and skid. If you can do this under all conditions, with reasonable and comfortable pedal effort, then your brakes are adequate. If not, some other factor is limiting their performance, and maintenance or modification will be required. The other, undesirable, limiting factors are:
Force Limit
At force limit, you push as hard as possible on the brakes but can***8217;t stop any quicker. If you could push harder, the vehicle would stop quicker. Force limit can be altered by reducing master cylinder size, using different brake pads, adding or upgrading power assist, increasing pedal ratio, increasing calliper piston area, or increasing effective rotor diameter.
Deflection Limit
At the deflection limit, the pedal hits the floor or the MC piston hits its internal stop before max brake force is generated. I.e. ***8211; the combination of designed clearance and unwanted deflection in the system exceeds the available travel. Clearance is determined by the volume of fluid required to be displaced to bring the pads into contact with the rotor, and is chiefly determined by the size of the calliper piston(s). Unwanted deflection can come from pedal and pedal-mount flex, MC mount flex, calliper bracket flex, calliper flex, hose expansion, trapped air, unevenly worn pads or rotors, and other worn components such as wheel bearings. Assuming all air is bled from the system and all components are in good working order, deflection limit can be altered by increasing MC piston diameter, decreasing calliper piston diameter, upgrading flex hoses, reducing the length of flex hose used, or using stiffer callipers, brackets, and pedals.
Wear Limit
The brakes effectiveness can be limited by worn components. If pads or rotors are worn excessively, or if any seal, tube, hose, bracket, or fitting is damaged, worn, or leaking, a reduction in performance, or worse, brake failure, can result. The likelihood of reaching the wear limit can be reduced by using different (higher quality) pads and/or rotors, using larger callipers and rotors, or increasing brake cooling. Good inspection and maintenance practices can prevent reaching the wear limit.
Temperature Limit
When brakes overheat, you reach one of the other limits (force, deflection, or wear) much more quickly. That is, hot brakes can cause lack of braking force (commonly called brake fade), increased deflection (excessive pedal travel), or rapid component wear ***8211; ranging from pad/rotor wear to boiling fluid and cooking seals. The keys to avoiding a temperature limit are:
Using sufficiently large/heavy brakes.
Sufficient brake cooling.
Selection of components designed for the temperatures encountered ***8211; including pads, rotors, callipers, and fluid.
Brake Fade
***8220;Brake fade***8221; generally refers to any loss of braking caused by overheating. In fact, there are three very distinct forms of ***8220;brake fade***8221;, and it is useful to distinguish between them, as their symptoms and solutions are entirely different.
Pad Fade.
Pad Fade is caused by the temperature of the brakes exceeding the maximum temperature limit of the brake pad friction material. When the maximum temperature limit is reached (and even before, as it is approached), brake pads can expel gases when heated, gasses that act as a lubricant between pad and rotor. When pad fade occurs, the pedal will feel ***8220;normal***8221; (high and firm) but there will be very little stopping power. Pumping the brakes will not help. Solutions to pad fade are:
To use a more temperature resistant brake pad (harder compound)
To increase the brake***8217;s thermal capacity by increasing the size/weight of the rotor
To improve cooling by using a different rotor design (vented vs. solid, or direction vs. straight vanes)
To improve brake cooling by improving cooling airflow to the brakes. In extreme circumstances, if the vehicle has air ducts for brake cooling, a fine mist of water can be sprayed into the air to cool it before it hits the rotor, improving the air***8217;s cooling ability.
To use a slotted and/or cross-drilled rotor to aid in expelling hot gasses and pad debris. Note that these designs DO NOT aid in cooling but simply allow the offending gas or particles to be removed more efficiently.
Fluid Boil
When the temperature of the calliper exceeds the boiling point of the brake fluid, tiny bubbles are formed in the brake fluid. As a result, the pedal goes soft and perhaps even to the floor as the fluid is no longer incompressible. Once this has happened, the fluid must be replaced. Over time brake fluid can absorb water vapour, and the more water vapour in the fluid, the greater its susceptibility to fluid boil. Solutions to fluid boil include: flush and fill with new brake fluid, use a brake fluid with a higher boiling point, improve cooling of the calliper, or use a calliper with an insulated piston.
Green Fade
Green fade occurs only with new brake pads if they are not ***8220;bedded-in***8221; properly after having been installed. Bedding-in brake pads is a process of breaking them in before severe use, similar to the way an engine or set of gears must be broken in. Green fade is caused by gas or liquid boiling off the organic pad lining on initial heating. The gas or liquid not only acts as a lubricant between pad and rotor, but can also ***8220;glaze***8221; the surface of the pads by cooling and re-solidifying into a hard slick surface with a very low coefficient of friction. The solution to green fade is to bed in brake pads. To do so, follow the steps outlined in "Bedding-in (http://www.pirate4x4.com/tech/billavista/Brakes/#Beddingin)".
Hard Pedal
***8220;Hard Pedal***8221; is the term used to describe brakes that require excessive pedal effort ***8211; i.e. the driver must push too hard on the pedal for the brakes to work. This condition ***8220;feels***8221; to the driver like the brake pedal is very ***8220;hard***8221;.
http://i86.photobucket.com/albums/k94/chukidori/HardPedal.jpg
Low Pedal
***8220;Low Pedal***8221; is the term used to refer to a brake pedal that requires an excessively long stroke to fully apply the brakes. What we are experiencing with low pedal is a deflection limit. In some cases, the brakes work only after being ***8220;pumped***8221; a few times. What***8217;s happening here is that the driver is able to displace more fluid more quickly than the callipers retract, and is therefore able to provide the volume required by pumping the brakes. This is usually a symptom of incorrect valving, such as lack of required residual pressure valve.
http://i86.photobucket.com/albums/k94/chukidori/LowPedal.jpg
Spongy Pedal
Often present with, but not to be confused with, low pedal; a spongy pedal starts high and doesn***8217;t travel too far ***8211; but it doesn***8217;t feel firm and solid. Instead, the brakes feel soft, squishy, and spongy when applied.
http://i86.photobucket.com/albums/k94/chukidori/SpongyPedal.jpg
Overly Sensitive
Brakes that develop enormous clamping force (usually by generating high pressure) with very little pedal effort will feel overly sensitive to the driver. They are hard to modulate, particularly for med- to high-speed work, and are generally unpleasant to drive. Exactly the point at which this occurs will be a matter of driver preference combined with the vehicle***8217;s use
http://i86.photobucket.com/albums/k94/chukidori/OverlySensitive.jpg
Maintanence Trouble Shooting
http://i86.photobucket.com/albums/k94/chukidori/Maintanence.jpg
http://240atlanta.com/forums/%5BIMG%5Dhttp://i86.photobucket.com/albums/k94/chukidori/Maintanence.jpg%5B/IMG%5D