Suspension

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Suspension Vehicle Dynamics
WPI Motorsports Club Presents
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The Basics

• Purpose
• Control vehicle to road surface interaction ?
  grip
• Translate Driver inputs ? response
• Critical Topics
• Vehicle Dynamics What the car does and why
• Forces Acting on the body
• Physics and Kinematics of Suspension
• Components of the System

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The Basics

• Only 2 Parameters matter
• Grip (Mechanical)
• Holding to the road
• Mechanical Chemical interaction of tire and
  surface
• Response
• Lag between input and output ?Hysteresis
• Multi-Order partial diff EQ Control problem in
  space-time

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Wait Say What?

• What Is Suspension?!

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Is it Suspension?
Double Wishbone (Pontiac Solstice)
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Is it Suspension?
F1 Springless Wheel w/Heave Spring a T-bar ARB
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Is it Suspension?
Monoshock (Shuttle Style) Dallara
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Is it Suspension?
Mono Leaf
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Is it Suspension?
Live Axle
Watts Linkage
DeDion Tube
DeDion Tube
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Is it Suspension?
Race Kart Suspension?
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Load Transfer
Start With a FBD!

• Forces act on CG
• Reactions from suspension
• Bumps produce force ?displacement
• Roll/Weight Transfer produces moments

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Load Transfer

• Lateral Load Transfer (Roll)
• Governed by relative Height of C.G.
• Wider track less transfer
• Impact suspension dynamic behavior
• Longitudinal
• Longer wheelbase less transfer
• Weight Distribution
• Why 50/50 understeers (load inside/outside and
  front to rear)
• Suspension kinematics can impact magnitude

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Load Transfer

• FBD - Front View

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Kinematics and Alignment

• (Insert Picture of Alignment setup here)

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Camber
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Camber

• Angle of the tire (from Vertical)
• Influences tire contact patch in cornering
  conditions
• Keeps the rubber on the road F. Puhn
• Impact on temperature, wear and steering
• Best compromise
• Creates Camber Thrust
• Force perpendicular to the direction of travel
• A tire leaned on edge wants to turn i.e.
  Motorcycles

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Tire Wear and Failure
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Toe

• Angle defined by the wheel and longitudinal axis
  of the vehicle

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Toe

• Impacts turning and vehicle stability
• Positive Toe (in) Increases stability,
  decreases steering response, counteracts camber
  thrust from positive camber (Inverse for Toe out)

• Front
• Too much in Wandering under braking, refusal to
  turn in or hard turn in followed by understeer
• Too much out Instability during braking
  Straight-line instability, understeer

• Rear
• Too much in Slow transient, midcorner
  understeer, instability at turn in
• Too much out Violent throttle on, off and trail
  braking rotation/oversteer

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Caster, KPI, Scrub, Trail
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Caster, KPI, Scrub, Trail

• Caster and KPI angle will cause the tire to
  camber as it is turned
• ? Camber KPI (COS(Steering Angle)) Caster
  (SIN(Steering Angle))
• Caster and mechanical trail produces a self
  centering torque to the tire
• High caster increases steering effort
• Caster adds damping (hysteresis), Trail adds
  feel
• Mechanical Trail and Scrub radius determine
  forces on steering linkages
• Too large Heavy Stering
• Too little no feedback

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Roll Centers
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Roll Centers

• Instant Center
• Point about which a wheel pivots in
  jounce/rebound
• Roll Center
• The axis about which the car rolls
• Kinematic vs Force Based
• Distance from the RC to the COG creates rolling
  moment
• Determines load transfer
• RC Migration
• Roll center will move during roll
• NEVER CROSS THE GROUND PLANE!

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Tires
TIRES ARE THE MOST IMPORTANT PAR OF ANY
SUSPENSION!
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Tires The Basics

• Cross Ply (Slicks) or Radial (Modern Street/Wets)
• Many Compounds for both kinds
• 2 Components
• Tread and Sidewall
• Inflation
• Higher Pressures Stiffer, smaller contact patch,
  Less rolling resistance, less mechanical grip,
  better turn in response, lower temps

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Tires - Sizing

• Sizing
• Wheels
• Big enough for tire size to clear brakes
• Bigger More rotating mass (Weight Inertia)
• Tires
• Width Contact patch
• Aspect Ratio (Sidewall) ? smaller stiffer
• Less flex but twitchy , less give ? Harsh
  Ride

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Tires Contact Patch

• Tire deforms to road Surface
• Impacted by
• Tire Pressure-Higher pressure smaller contact
  patch
• Vehicle Weight/Load- Higher load larger contact
  patch
• Tire Construction
• Bigger not always better
• Suspension Design, Sidewall, Camber Sensitivity,
  etc

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Slip Angle

• Angle between the direction of the tire and the
  direction of travel
• Caused by deformation in tire carcass and tread
  and Steering Geometry (Ackerman-different
  steering angle inner vs. outer tire)

• Results in a force perpendicular to the wheel's
  direction of travel Cornering Force
• Graph on Next Slide
• Occurs away from contact patch center
• Pneumatic Trail

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Slip Angle

• Note Curve changes as load changes!
• Tires are load sensitive!!
• Not Linear Doesnt follow classic friction
  theory
• Max cornering Vertical Load 0.7 to 0.9

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Slip Angle Why?

• Ratio Effect (Front/Rear)
• gt11 Understeer
• lt11 Oversteer
• Instantaneous Slip Angle depends on a multitude
  of factors
• Suspension design can promote certain dynamic
  characteristics
• Alter Relative Roll Couple varying weight
  transfer front vs rear
• Adjusting Roll Centers
• Roll Stiffness (spring or ARB)
• Steering Geometry
• Ackerman (pro or anti) or Parallel
• Toe

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The Parts

• Main Components are
• Springs
• Wheel and Anti Roll Bars
• Holds the car up
• Dampers
• Control the springs (oscillations/bouncing)
• Combine with springs to change handling
• Linkages
• Holds it all together
• Mind your compliance

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Springs

• Main contributor to stiffness
• Stiffer Better Response
• Can achive lower ride height and COG with out
  bottoming
• Softer Better Grip
• Spring rate determined by natural frequency
• Basis for all calulations and physic
• All cars are the same stiffness for the same for
  the same frequency regardless of differences

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Springs
If you dont watch your units, you cant have
any pudding! Kg or lbm N/m or lbf/in! (refer
to optimum G website)
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Roll

• Controlled by anti-roll bars (sway bars) and Ride
  Springs
• Roll rate defined at Deg/g
• 0.2-0.7 Aero Cars, 1.0-1.8 Sedans
• Stiffer Quicker transient response, less grip
• Front/Rear Distribution impacts balance
• Typically
• Front roll stiffness 5gt Weight Distribution
• i.e. 40/60 weight will have roll split 45/55

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Roll
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Dampers

• Controls Spring
• Damping Force dependent on input Force and
  velocity
• Basically just a pump for fluid
• Valves determine rate of fluid flow ? Damping

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Review and Compare
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Review and Compare
Questions??
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What does it all mean?

• Suspension is complex
• No perfect system ? Best Compromise
• Systems can be broken down into much simpler
  systems using basic mechanical principles
• ANYONE can learn, design and work on these
  systems!
• Its Just physics