Fundamental Principles For Vehicle Brake System
The most important safety feature of an automobile is its brake system. The ability if a braking system to provide safe, repeatable stopping is the key to safe motoring. A clear understanding of the brake system is essential for anyone involved in servicing vehicles.
The basic principle of brake operation id the conversion of energy. Energy is the ability to do work. The most familiar forms of energy in automotive use are: chemical, electrical and mechanical. For example starting an engine involves several conversions. Chemical energy in the battery is converted to electrical energy in the starter. Electrical energy is converted to mechanical energy in the starter as it cranks the engine.
Cycle of Heat Energy:
Burning hydrocarbons and oxygen in the engine creates heat energy. Nothing can destroy energy once it id released, it can only be converted into another form of energy. Heat energy is converted into kinetic energy as the vehicle is put into motion. Kinetic energy is a fundamental form of mechanical energy; it is the energy of a mass in motion. Kinetic energy increases in direct proportion to weight increase and increases by four times for speed increases.
Coefficient of Friction:
Fiction is the resistant to movement between two objects in contact with each other. It also converts energy of motion to heat. If we allow the vehicle to coast in neutral on a level surface, eventually the kinetic energy would be converted to heat in the wheel bearings, drive-train bearings, and at the tire and road surface to bring the vehicle to a complete stop. The brake system provides the means of converting kinetic energy through stationary brake shoes or pads which press against a rotating surface, generating friction and heat.
The amount of friction produced is proportional to the pressure between the two objects, composition of surface condition. The greater the pressure applied to the object, the more friction and heat is produced. The more heat produced by friction the sooner the vehicle is brought to a stop which i\results in stopping control.
The coefficient of friction is a measurement of the friction between two objects in contact with each other. Force is the effort required to slide one surface across the other. It is determined by dividing the force required to move an object by the weight of an object..
The following example illustrates how the type of friction surface can influence the coefficient of friction(COF):
100 pounds of ice pulled across a concrete floor may require 5 pounds of force to move.
5/100 = 0.05
COF = 0.05
However 100pounds of rubber pulled across a concrete floor may require 45 pounds of force to move.
45/100 = 0.45
COF = 0.45
The coefficient of friction varies in the two examples above based on materials used. The same is true in a brake system, the coefficient of friction varies on the type of lining used and the condition of the drum or rotor surface.
Basic Brake System:
The most widely utilized brake systems at present are the foot operated main brake and manual type parking brake. The main brake actuates the brake assemblies at each wheel simultaneously using hydraulic pressure. Fluid pressure created at the master cylinder is transmitted to each of the wheel cylinders through brake tubing. The wheel cylinders force the shoes and pads into contact with a drum or rotor spinning with the wheels generating friction and converting kinetic energy to heat energy. Large amounts of heat is created resulting in short distance stopping and vehicle control. The converted heat is absorbed primarily by the brake drums and dissipated to the surrounding air.
Brake drums and rotors are forced to absorb a significant amount of heat during braking. Brake fade describes a condition where heats is generated at a faster rate than they are capable of dissipating heat into the surrounding air. For example, during a hard stop the temperature of drums or rotors may increase more than 100 degrees F in just seconds. It may take 30 seconds to cool these components to the temperature prior to braking. During repeated hard stops, overheating may occur and a loss of brake effectiveness or even failure many result.
There are primarily two types of brake fading caused by heat;
Mechanical fade occurs when the brake drum overheats and expands away from the brake lining resulting in increased brake pedal travel. Rapidly pumping the pedal will help to keep linings in contact with the drum.
Lining fade affects both drum and disc brakes and occurs when the friction material overheats to the point where the coefficient of friction drops off. When the coefficient of friction drops off, friction is reduced and the brake assemblies ability to convert added heat is reduced.
Brake fade is the primary reason for weight limits for towing and trailer brake requirement for vehicles above a given trailer weight. The added kinetic energy resulting from increased vehicle mass requires added heat conversion capacity when the brake are applied.