Automotive 
Braking System 

Friction  

• There are two basic types of friction that explain how brake systems work.
  
• • Kinetic or moving - 
  • • Friction always converts moving, or kinetic, energy into heat.
      
    • The greater the friction between two moving surfaces, the greater the amount of heat produced.
      
    • The amount of friction, or resistance to motion, depends on the:
      

    • • Type of materials the brakes are in contact with.

      • The  smoothness of their rubbing surface.

      • The pressure holding them together (often gravity or weight.)

Factors Governing Braking

• Pressure  

The amount of friction generated between moving surfaces contacting one another depends in part on the pressure exerted on the surfaces.   In automobiles, hydraulic systems provide application pressure.  Hydraulic force is used to move the brake pads or brake shoes against spinning rotors or drums mounted to the wheels.

• Coefficient of Friction    

The amount of friction between two surfaces is expressed as a coefficient of friction (COF). The coefficient of friction is determined by dividing the force required to pull an object across a surfaced by the weight of the object.

• Amount of Contact Surface

Simple put, bigger brakes stop a car more quickly then smaller brakes used on the same car.  Similarly, brakes on all four wheels slow or stop a moving vehicle faster than brakes on only two wheels.

• Heat or, more precisely, Heat Dissipation  

The braking system must be able to effectively handle the heat created by friction within the system.  The heat created during braking must be conducted away form the pad and rotor (or shoe and drum) and be absorbed by the surrounding air.

Brake Lining Friction Materials

• Brake Linings
  

• • Linings are either asbestos, non metallic organic, semi-metallic or metallic. Asbestos lining are a health hazard and have been phased out as a brake material. The materials used to replace asbestos are organic materials bonded together with a resin binder. 
    
  • For many decades, asbestos was the standard brake lining material. It offered good friction qualities, long wear and low noise.  Today, materials such as composite/organic, ceramics, and carbon fibers are being used because of the health hazards of breathing asbestos dust. In fact, the federal government has banned the use of asbestos in new vehicles and in aftermarket replacement parts.
    
    

• Semimetallic linings
  
• • Semimetallic linings are organic linings with sponge iron and steel fibers mixed into them to add strength and temperature resistance. They are fast taking heat away from the rotor and putting it into the lining. This heat transfer does not affect the service life of the lining. The hotter they get, the better they work.

• Metallic linings
  
• • Metallic linings are used in very heavy-duty and racing conditions.  They work poorly when cold. 

Principles of Hydraulic Brake Systems

• A hydraulic system uses a brake fluid to transfer pressure from the brake pedal to the pads or shoes.  Because liquids are not compressible the transfer of pressure is reliable and consistent.  The pressure applied to a liquid in a closed system is transmitted by that liquid equally to every other part of that system.  Apply a force of 5 PSI through the master cylinder and you can measure 5 PSI anywhere in the lines and at each wheel where the brakes operate.

• The force can be increased at output (that is, at the wheel) by increasing the size of the wheel’s piston.  The force at output can be decreased by decreasing the size of the wheel piston.

• Of course, the hydraulic system does not stop the car all by itself.  It really just transmits the action of the driver’s foot on the brake pedal out to the wheels.  Mechanical force is changed into hydraulic pressure, which is changed back into mechanical force by brake shoes and/or disc pads contact on the drums and rotors.

Dual Braking Systems

• The Dual System

• • The dual system differs from the single system (shown above) by employing a tandem master cylinder, which is essentially two master cylinders, each having it own with two pistons and fluid reservoirs into one cylinder bore.  
  • Each piston applies hydraulic pressure to two wheels of the vehicle.
     

•  Front/Rear Split System

• Diagonally Split System

• • Operating on the same principles as the front /rear split system, the diagonally split system uses primary and secondary master cylinders that move together to exert hydraulic pressure on each system.
  • The hydraulic brake lines on the system are, however, split front to rear (right front to left rear and left front to right rear).
  • If a system failure occurs, the remaining good system would do all the braking on one front wheel and the opposite rear wheel, this would maintain 50% of the total braking force.
    
     

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