Automotive Braking Systems

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Automotive Braking Systems
By Shane Dunlevy
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Overview

• Brakes convert kinetic energy into heat by
  creating friction
• System must have very high reliability
• Types of brakes
• - Disk
• - Drum

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System Basics

• Hydraulic actuation allows multiplication of
  pedal force.
• In this system, a 10lb force on the pedal
  produces 360 lbs of force at the friction surface.

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Braking Dynamics

• Dynamic weight transfer caused by braking shifts
  the weight balance towards the front of the car
• Dynamic weight for the front and rear can be
  found using Newtons Law

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Braking Dynamics

• Because the maximum braking force is equal to the
  coefficient of friction times the normal force,
  the front wheels will have an increased capacity
  to provide braking force
• Also, the system must be designed to distribute
  the application of force between front and rear

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Disk Brakes

• Caliper squeezes pads to create force on the
  surfaces of the rotor.
• Used in most automotive applications.
• Benefits
• Simple design
• Self adjusting
• Rotor venting allows faster heat dissipation

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2 Types of disk brake calipers

• Floating caliper
• Piston pushes pad against the inner rotor
  surface, reaction force causes the caliper to
  slide and contact the outer surface

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2 Types of disk brake calipers

• Fixed caliper has at least one piston on each
  side
• Each piston drives its corresponding pad into
  contact with the rotor

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Drum Brakes

• Expanding shoes create force on the inner surface
  of the drum
• Used on the rear of some trucks and SUVs
• Self-energizing design requires less activation
  force
• Require periodic adjustment

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Tandem Master Cylinder
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Master Cylinder

• Provides a reservoir for brake fluid and contains
  the driving pistons in the hydraulic circuit
• 2 Types
• Front - Rear split
• -One piston for front brakes and one for rear
• -If a leak occurs you could lose front brakes
• Diagonally split
• -One piston drives one front wheel and one rear
  wheel
• -Diagonal layout allows you to maintain
  directional control if a leak occurs

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Diagonally Split System
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Power Assistance

• Power booster reduces pedal force required to
  stop vehicle
• Engine vacuum is used to actuate a large
  diaphragm
• When the pedal is depressed, vacuum is applied to
  only one side of the diaphragm and force is
  appled to the master cylinder push rod
• Booster stores sufficient vacuum for several
  brake applications if the engine fails.

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Power Booster in closed position
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Anti-Lock Brakes

• A locked (sliding) wheel offers less braking
  force than a decelerating rolling wheel
• The locked wheel also produces little lateral
  force, preventing steering control
• Anti-Lock systems (ABS) monitor wheel lock-up and
  modulate brake pressure to provide controlled
  braking under most circumstances

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Anti-Lock Brakes

• System can have 2, 3 or 4 channels
• Trucks typically use 3 channel with only one
  sensor for the rear axle
• Most modern cars use 4 channel system
• Wheel speed sensors monitor each wheel speed
• ABS controller and high-pressure pump increase or
  reduce pressure to wheels in order to maintain
  consistent wheel speeds

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Typical 3 channel system
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Valves and Switches

• Pressure Differential Switch
• -Senses a pressure differential between the two
  master cylinder stages and warns the driver that
  the system has a leak
• Metering Valve
• -Prevents the front brakes from engaging until
  the rear drums have enough pressure to overcome
  the shoe return springs (Front disk-Rear drum
  systems only)
• Proportioning Valve
• -Reduces the pressure to the rear wheels to
  balance the effect of weight transfer to the
  front wheels.