Suspension? - Tuned Design

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 11 months ago)

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Presented by:

Brian Barnhill

Suspension &




Vehicle Dynamics

WPI Motorsports Club Presents:

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


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 …

Wait… Say What?

What Is Suspension?!


Is it Suspension?

Double Wishbone (Pontiac Solstice)

Is it Suspension?

F1 “Springless Wheel” w/Heave Spring a T
-
bar ARB

Is it Suspension?

Monoshock (Shuttle Style)


Dallara

Is it Suspension?

Mono Leaf

Is it Suspension?

Watts Linkage

DeDion Tube

Live Axle

DeDion Tube

Is it Suspension?

Race Kart


Suspension?

Load Transfer

Start With a FBD!


Forces act on CG


Reactions from suspension


Bumps produce force

displacement


Roll/Weight Transfer
produces moments

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


FBD
-

Front View

Load Transfer

(
Insert Picture of Alignment setup here)

Kinematics and Alignment

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



Camber

Tire Wear and Failure

Angle defined by the wheel and longitudinal axis of the vehicle

Toe

Impacts turning and vehicle stability


Positive Toe (in)


Increases stability, decreases steering response,
counteracts camber thrust from positive camber (Inverse for Toe out)

Toe

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

Caster, KPI, Scrub, & Trail

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

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!

Roll Centers

Tires

TIRES ARE THE MOST IMPORTANT PAR OF ANY SUSPENSION!


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

Tires


The Basics


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



Tires
-

Sizing




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…

Tires


Contact Patch




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)

Slip Angle


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




Slip Angle


Note: Curve changes as load
changes!



Tires are load sensitive!!


Not Linear


Doesn’t
follow classic friction
theory


Max cornering ~ Vertical
Load ^0.7 to 0.9




Ratio Effect (Front/Rear)


>1:1


Understeer


<1:1


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*

Slip Angle


Why?


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


The Parts


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

Springs

Springs

“If you don’t watch your units, you can’t have any pudding!” Kg or lbm & N/m or lbf/in!

(refer to optimum G website)


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 5%> Weight Distribution


i.e.: 40%/60% weight will have roll split 45%/55%

Roll

Roll

Dampers


Controls Spring


Damping Force dependent
on input Force and velocity


Basically just a pump for fluid


Valves determine rate of
fluid flow


Damping

Review and Compare

(From
Suspension Geometry and Design
, John Heimbecher,
DaimlerChrysler Corporation)

Review and Compare

Questions
??

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!


It’s Just physics