Glossary of braking terms

Racing Pad terms

“Bite” describes the response time or initial reaction the pad’s have when first engaged. This characteristic is designed into the PFC pads to maximize deceleration in the first ½ second of the stopping event while both aero and dynamic grip-traction of the racecar are at their optimum.

“Torque” is the amount of deceleration during the braking event for the given amount of driver effort. An important area of improvement for PFC is how smooth the torque delivery is for the given driver’s input.

“Release” or “modulation” is the ability to control the release or time response of the decrease in braking power to prevent overwhelming the available mechanical grip of the tire’s traction with the track surface allowing the driver to optimize tire adhesion without locking wheels. This is where PFC stands out from other manufacturers, in that our superior release characteristics give the driver unrivaled control with the highest torque. Release characteristics are ultimately important in technical corners and high-speed finesse braking.

“Feel” is how dynamically the bite, torque, pedal travel, and release interact with both the brake system components and the vehicle’s grip-traction level. This interaction, and the subsequent effects and their value are subject to debate from driver to driver. Certainly, a moving target for all racing brake manufacturers.

“Mu” (µ) is the value of the coefficient of friction between the rubbing surfaces. Like tires, many factors affect this value.

“Race ready” is a PFC trademark that refers to in its processing of brake pads regardless if they are race or street compounds. Essentially, the PFC brake pads are processed so that the “green effectiveness” is minimized, and very little “bedding or burnishing” is required. New PFC pads may be used on new PFC discs with no issues (See procedures for bedding the brakes).

“Pedal travel” is the amount of travel for a given force input or torque output.

“Pre-bedded or Dyno-bedded or Dyno-burnished” – although not essential for PFC products, we offer pre-bedding to help minimize the perceived track time lost or to help isolate the driver’s influence. The pads and/or discs are dyno-burnished in a controlled environment, with computer control plans in place to ensure peak performance.

“Transfer layer” is the layer of friction material that is transferred to the disc’s wear surface.

“Oxidation layer” this refers the pad material that has been oxidized by heat, which causes discoloration. In PFC’s case, there is a patented graphite enhanced high-temperature powder coating that reduces the effects of oxidation and will have a brownish tint to it. This can be a very good visual aid to determine if the brake pads have been overheated. See technical section.

“Hard, Medium and Soft” brake pads, within the Performance Friction Carbon Metallic patented processes there is very little difference in the pad’s density. These terms come from earlier pad technologies where race tire terms were used to describe the pad’s characteristics. Today’s PFC pads have different nuances designed into them with very little change to the friction material’s density.

“Brake Fade” is when the brake pedal is still firm, but both the bite and the brake torque diminishes. There are many factors that influence fade, including excessive pad temperature, fluid temperature, vehicle traction degradation, and green effectiveness.

“Green fade” occurs when either the pads or discs require an extended period of “bedding or burnishing” before they become effective. Green fade is not an issue with PFC due to the Race Ready processing for its brake pads and the machining procedures of its discs. See technical section for proper bedding procedures.

Racing Brake Component Terms

“Boiled brake fluid” There are 2 types of brake fluids made today. Glycol-ester blends and high percentage ester content blends. Most popular is the glycol based. The glycol-based fluids, normally has low compressibility but is “hygroscopic” which means it absorbs water. It is the water that gasses off and adds to the “spongy pedal” effect. This should not be confused with “brake fade”. The moment the fluid container is opened, it is absorbing water from the humidity in the air. When reviewing brake fluids, the contents label will list a dry and wet boiling point. The use of PFC caliper temperature stickers, p/n 032.0007 is highly recommended to monitor brake fluid and caliper seal condition. If the calipers exceed 430°F (210°C) for an extended period, the brake fluid and / or caliper seals will deteriorate.

High-percentage ester blends have the promise of higher dry and wet boiling points. These types of brake fluids are usually more expensive than the glycol-ester blends. The reason is esters’ less water absorption qualities. But not all high ester blends are equal, and in many cases, although less water content to boil, compressibility may be an issue at elevated temperatures, so spongy pedal effect is the same.

PFC recommends RH665 racing brake fluid because of decades of proven consistency and performance. Replacing brake fluid often and bleeding the brake system correctly (see technical section) is the cheapest insurance of a consistent brake pedal.

“Knock-back or Knock-off” – this is when the brake pad is has been forced away from the disc while the vehicle is in motion. The effect leads to soft or inconsistent initial pedal travel. If the driver taps the brake pedal and the brake pedal improves, this is knock-back. This can occur with both 2-piece and 1-piece design discs. There are several factors that influence this phenomenon.
Drivers banging curbs is the leading cause. Too much or too little lateral float with the 2-piece design disc assemblies. Other areas of interest are spindle and bearing hub deflection, the caliper’s mounting to disc concentricity at elevated temperatures, or dynamic disc symmetry to the pads (to list a few).

“Seal pull back, seal roll back” – the symptoms are the same as knock-back. The pistons in the calipers are being pulled back due to seal geometry. There are many factors that affect how the seals interact with the piston surface. Seal design, seal tension, seal groove shape, seal condition, and bore distortion are just a few areas of interest.

“Knock-back springs” are typically cone-shaped springs that sit behind the caliper’s pistons. All proper race caliper designs have provisions designed into the caliper’s well area for these springs. The goal of the springs is to help stabilize the knock-back. Excessive tension can lead to excessive brake drag and overheating.

“Recirculators or closed loop systems” refers to a hydraulic brake line system that has a series of check valves to ensure fluid fill in one direction. The theory behind these systems is that as the brake pads wear, a certain amount of fluid is displaced, localized in the caliper’s well area. Although there may be some benefits initially with protoing a promise of a firmer brake pedal, the subsequent added brake drag due to the hysterisis of the inline check valves affects release, modulation and reduces the ability for the brake pads to cool. PFC highly discourages the use of these products. These systems should not be confused with “bump cycles”, strategies used in modern ABS systems.

“Judder or brake shake” this refers to the vibration the driver feels in either the brake pedal or steering wheel, or both. In most cases it is caused by the increased thermal distortion of runout and thickness variation of the disc surfaces.

“Balance bar or dual braking systems” refers to the brake pedal controls that use 2-master cylinders for separate hydraulic circuits for the front and rear but limited to that configuration. Their pressures can be adjusted with the use of a balance bar attached to the brake pedal. Most responsible racing sanctioning bodies require these controls as it offers redundancy for safety reasons. There are number of nuances with both their configurations as well as setting these systems up. Refer to the technical section.

“Fiddle brakes” refers to a balance bar system that can not only control fore and aft brake bias but side-to-side brake bias. These systems are typically found in rally or off-road type race vehicles. Most sanctioning bodies have banned their use because of the electronic controls required to make these systems work. These systems should not be confused with modern ABS systems.

“Proportioning or prop valve” refers to a pressure limiting hydraulic valve, usually inline with the rear circuit. Racing versions of these valves are typically adjustable. They can only bend or limit peak pressure, they can not add more pressure. Mechanical proportioning valves do not release pressure at the rate of pedal release, which causes poor modulation. PFC recommends the balance bar systems over the use of these valves due to inherent hysterisis, which affects release and modulation.

“Soft or long pedal” – dynamically, there are several reasons for the brake pedal to be soft initially or go soft while the car is in motion. If initially, the area of interest is the bleeding procedures especially when a dual master cylinders when a balance bar is used. If a new brake system is installed, then ensuring the correct size master cylinder(s) is used. If the brake pedal is firm initially but drops or pumping the brakes is needed to firm up the brake pedal feel than refer to brake fluid, knock-back, judder sections as all will affect the brake pedal consistency.

“Warped rotors” – although the effects are similar to brake vibration, the leading cause is the brakes have been over-heated and leaves the discs warped. Once the discs have been warped, it is seldom that they can be refinished successfully. Please refer to the technical section about proper bedding procedures as well as the use of temperature-sensitive paints to monitor dynamic disc temperatures.

“Brake drag” is the rolling drag that brakes incur at elevated temperatures. The goal of PFC in its philosophy is to minimize this drag in all its execution of products. The drag not only slows the vehicle’s ability to accelerate; it reduces the ability for the brake pads to cool. The brake pads are the hottest component in the brake system. The drag greatly affects the pad’s ability to release and modulate.

The key to minimizing this drag is PFC disc symmetry with all of the brake components to the interface area of the brake pads. No easy task because what is required to ensure proper geometry at extreme temperatures.

“Bobbins” refers to use of a generic term for the attachment systems used to couple 2-piece disc designs with its mounting hats (AKA brake bells). There are many bobbin designs, but newer disc assemblies do not use this style of fitting.