Suspension

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Suspension
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Outline

• Introduction
• Suspension components
• Suspension type examples
• Solid axle
• Double Wishbone
• MacPherson Strut
• Introduce basic geometry

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Suspension Purpose

• Isolate passengers and cargo from vibration and
  shock
• Improve mobility
• Improve vehicle control

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Basic Terminology

• Sprung Mass
• Mass of all components that do not move much when
  suspension is displaced. (given the frame as a
  fixed reference)
• (Frame, engine, passengers, etc,)
• Some suspension components are actually partially
  sprung mass

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Basic Terminology

• Unsprung Mass
• Mass of components that move when suspension is
  displaced
• Minimizing the unsprung mass allows for more
  optimal suspension operation

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Basic Terminology

• Bump
• Vertical displacement of entire sprung mass

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Basic Terminology

• Roll
• Front View angular rotation of the sprung vehicle
  mass

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Basic Terminology

• Pitch
• Side View angular rotation of the sprung vehicle
  mass

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Basic Terminology

• Roll Center
• Center at which the sprung mass pivots about
  during a roll situation (lateral acceleration)
• This is a dynamic point moves around throughout
  suspension travel

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Basic Terminology

• Pitch Center
• Center at which the sprung mass pivots about
  during a Pitch situation (fore/aft acceleration)
• This is a dynamic point moves around throughout
  suspension travel

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Basic Terminology
Camber Front View tilt of the tire. Leaning the
top of the tire inboard adds negative camber
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Basic Terminology
Toe Top view angle of the tire in a static
situation Turning the front of the tire in is
referred to as adding toe in Important for both
front and rear tires
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Basic Terminology
Steering Axis Axis about which the wheel/Tire
rotate about during steering inputs Also known as
King Pin Axis
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Basic Terminology
Caster Angle Side view tilt of the steering
axis. Creates camber change with steering
input Creates a restoring force for centering
steering wheel
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Basic Terminology
Caster Trail Side view distance from the steering
axis ground plain intersection and the contact
patch center point Creates a restoring force for
centering steering wheel
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Basic Terminology
Scrub Radius Distance From which the ground plain
intersection of the Steering axis and the center
of the tire contact patch Large effect on drivers
feel and steering effort
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Basic Terminology
Steering Arm Line between the steering axis and
there steering linkage tie rod
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Basic Terminology
Bump Travel Vertical distance wheel is able to
move up from static position, with reference to
vehicles sprung mass
Droop Travel Vertical distance wheel is able to
move down from static position with reference to
vehicles sprung mass
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Passenger Comfort

• The perception of vehicle comfort is very
  subjective. Much depends on the cabin
  conditions. The main objective of the designer
  is to minimize the rate of change of acceleration
  (jerk).

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Trophy truck video

• Front Independent double wishbone Suspension
• Rear Solid axle
• High horse power vehicle

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Baja Buggy

• Front Independent double wishbone Suspension
• Rear Independent double wishbone Suspension
• Light weigh low horsepower vehicle

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Extreme mobility

• Control in extreme maneuvers

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Simplified Quarter Car Model

• Two Degree-of-Freedom System
• However the longitudinal and lateral stiffness of
  most suspension cannot be totally disregarded

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Simplified Half Car Model

• Two Degree-of-Freedom System
• However the longitudinal and lateral stiffness of
  most suspension cannot be totally disregarded

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Components of Suspension

• Tire
• Linkage
• Bearings, Bushings
• Springs
• Dampers
• Sprung / Unsprung Mass

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Tire

• Acts as both a spring and a damper
• These rates are affected by air pressure and tire
  construction.

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Pivot Joints

• Spherical Roller bearings
• High performance applications
• Minimal Compliance
• High Precision
• Low Stiction/Friction
• Bushings
• Low Cost
• Offers compliance

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Spring Types

• Leaf Springs
• Used in many early applications
• Internal friction provides damping
• Provide Fore/Aft/Lateral location for the axle
• Heavy
• Prone to weaken over time

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Spring Types

• Torsion Bars
• Little to no internal damping
• Low cost
• Often difficult to package

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Spring Types

• Coil Springs
• Little to no internal damping
• Low cost
• Compact Size
• Used in many Suspension types

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Solid Axle Suspensions
Applications -Pick Up Trucks (Rear) -Stock Cars
(Rear) -Rock Crawlers (Front Rear)
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Advantages

• Fewer Individual Components
• Easier to cheaply manufacture and assemble
• Simplified drivetrain layout
• High Load Capacity
• Axle Components are protected
• Can use leaf or coil springs
• Also can accept many different types of linkages
  to gain desired geometry
• Solid wheel attachment
• Minimal alignment eminence

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Disadvantages

• Disadvantages
• Higher Unsprung Weight
• Can lead to wheel hop
• Axle wrap when in a leaf spring configuration
• High Roll Center Height
• Not an Independent Design
• Corners are coupled
• Fixed Camber Angles

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Camber Change

• The diagram below shows how the camber is
  statically fixed, and does not change in rebound
• The middle diagram shows how the camber of the
  two wheels are linked to one another

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Solid axle adjustability

• As far as the static characteristics of the
  suspension the camber and castor are preset in
  the manufacturing of the axle housing
• However the dynamic characteristics of the
  suspension are highly adjustable with various
  forms or bar linkages
• There are many different linkage designs for a
  solid axle ranging from leaf springs to
  multi-link suspension systems

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Truck, NASCAR applications
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Double Wishbone Suspension
With Unequal Length Upper and Lower Arms
Found On -Stock Cars (Front) -Corvettes (C5 C6)
(Front and Rear) -Honda Civics (88-00)
(Front) -Most Modern Sports Cars
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Advantages

• Arguably the best handling suspension design
• Wheel gains negative camber in bump
• Low Unsprung Weight
• Packaging does not compromise styling
• Low Height
• Many different geometry characteristics possible
• Designer can design suspension with minimal
  compromises
• Infinite adjustability, with the most ease
• Vehicles roll centers can be placed almost
  anywhere

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Disadvantages

• More expensive
• More components to make and assemble
• Alignment and fitment are critical to vehicle
  performance, large area of adjustment
• Tolerance of parts must be smaller
• Requires constant alignment checks for optimum
  performance
• More complex
• Design often becomes more complex because all
  suspension parameters are variable
• Frame has to be able to pick up a-arm inboard
  points
• Tire scrub occurs with vertical wheel
  displacement
• However this can be minimized during design

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Double Wishbone Tuning

• The double wishbones complexity enables it to be
  adjusted quite readily
• The suspension geometry can be adjusted in two
  distinct ways
• Move the location of the inner Chassis attachment
  points
• Adjust the inclination of the upright and the
  pick-up locations on the upright
• The camber, castor, roll center, etc. can all be
  individually adjusted on this type of suspension
  relatively easily.

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Formula 1 Application
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Suspension Types MacPherson Strut

• Invented by Earl S. MacPherson
• First used on the 1951 Ford Consul
• Ford held the patent for the Macpherson strut
  system by many rival companies invented similar
  systems to avoid Ford royalties

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Suspension Types MacPherson Strut

• Advantages
• Low production costs
• Stamped construction
• Preassembled
• Strut body carries spring assembly
• Compact
• Simple mounting and no need for an upper control
  arm
• Simplicity
• Reduction in fasteners and alignment of vehicle
  suspension components.

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Suspension Types McPherson Strut

• Disadvantages
• Large camber variation
• Body roll and wheel movement contribute to camber
  attitude
• Vertically tall mounting position
• This compromises vehicle styling
• Rough ride
• Some ride comfort may be lost, as it is hard to
  move smoothly because of bending input force
• Dangerous replacement
• The spring must be compressed and assembled on
  the strut body, this causes the handling of a
  charged spring.

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Associated forces in the strut

• Since the strut serves as the upper control arm
  as well as the damper it is required to provide
  the force to hold the wheel at the desired camber
  attitude.

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Adjustment of MacPherson Strut

• Since the strut governs the King pin axis as well
  as the camber of the tire, these two parameters
  are linked
• The adjustability of the MacPherson strut is
  limited, due to its simplicity
• Most adjustability of the suspension is achieved
  by modifying the location of the upper strut
  mounting location.

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Suspension Types MacPherson Strut

• MacPherson Struts are widely used on a variety of
  cars today from the everyday road car to world
  class race cars