Suspension Design casestudy
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Transcript of Suspension Design casestudy
![Page 1: Suspension Design casestudy](https://reader035.fdocuments.net/reader035/viewer/2022062316/58f9a6b3760da3da068b528b/html5/thumbnails/1.jpg)
Suspension Design Case Study
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Purpose
• Suspension to be used on a small (lightweight) formula style racecar.
• Car is intended to navigate tight road courses
• Surface conditions are expected to be relatively smooth
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Performance Design Parameters
• For this case the main objective is to optimize mechanical grip from the tire.
• This is achieved by considering as much tire information as possible while designing the suspension
• Specific vehicle characteristics will be considered.
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Considerations
• Initially the amount of suspension travel that will be necessary for this application must be considered.– One thing that is often overlooked in a four
wheeled vehicle suspension design is droop travel.
• Depending on the expected body roll the designer must allow adequate droop travel.
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Introduction
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Components• Upper A-arm
– The upper A-arm serves to carry some of the load generated on the suspension by the tire.
– This force is considerably less then the load carried by the lower A-arm in a push rod set-up
– The arm only has to provide a restoring force to the moment generated by the tire on the lower ball joint
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Components• Lower A-arm
– The lower A-arm serves the same purpose as the upper arm, except that in a pushrod configuration it is responsible for carrying the vertical load
– In this case study the lower A-arm will carry a larger rod end to compensate for the larger forces seen by this component.
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Components• Upright
– The upright serves several purposes in the suspension
• Connects the upper A-arm, lower A-arm, steering arm, and the tire
• Carries the spindle and bearing assembly
• Holds the brake caliper in correct orientation with the rotor
• Provides a means for camber and castor adjustment
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Components• Spindle
– Spindle can come in two basic configurations
• Live spindle• Fixed spindle
– In the live spindle configuration the whole spindle assembly rotates and carries the tire and wheel
– The fixed spindle configuration carries a hub assembly which rotates about the spindle
– Both configurations carry the brake rotor
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Live Vs. Fixed Spindle Advantages and Disadvantages
• Live Spindle :– Less parts– Lighter weight if designed
correctly– More wheel offset– Bearing concerns– Retention inside of the
upright assembly• Fixed spindle
– Simple construction– Hub sub-assembly– Spindle put in considerable
bending– More components, and
heavier
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Components
• Push rod– The push rod carries
the load from the lower A-arm to the inboard coil over shock
– The major concern with this component is the buckling force induced in the tube
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Components• Toe rod (steering link)
– The toe rod serves as a like between the steering rack inboard on the vehicle
– The location of the ends of this like are extremely critical to bump steer and Ackermann of the steering system
– This link is also used to adjust the amount of toe-out of the wheels
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Components• Bellcrank
– This is a common racing description of the lever pivot that translates to motion of the push rod into the coil over shock
– The geometry of this pivot can be designed to enable the suspension to have a progressive or digressive nature
– This component also offers the designer the ability to include a motion ratio in the suspension
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Components• Coil-over Shock
Absorber– This component
carries the vehicle corner weight
– It is composed of a coil spring and the damper
– This component can be used to adjust ride height, dampening, spring rate, and wheel rate
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Components• Anti-Roll bar
– This component is an additional spring in the suspension
– Purpose: resist body roll– It accomplishes this by
coupling the left and right corners of the vehicle
– When the vehicle rolls the roll bar forces the vehicle to compress the spring on that specific corner as well as some portion of the opposite corners spring
• This proportion is adjusted by changing the spring rate of the bar itself*Unclear in this picture the
Anti-Roll bar tube actually passes inside the chassis
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Beginning the Design Process
• Initially the suspension should be laid out from a 2-D front view
• Static and dynamic camber should be defined during this step
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Camber
• The main consideration at this step is the camber change throughout the suspension travel.
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Camber• Static Camber
– Describes the camber angle with loaded vehicle not in motion
• Dynamic Camber– Describes the camber angle of a corner at any
instant during a maneuver i.e.: cornering, launching, braking
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Contact Patch
•Tread area in contact with the road at any instant in time
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Camber
• Camber is used to offset lateral tire deflection and maximize the tire contact patch area while cornering.
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Camber• Negative Camber angles
– good for lateral acceleration, cornering
– bad for longitudinal acceleration, launching/braking
This is because the direction of the tire deflection is obviously not the same for these two situations
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Camber• Cornering Situation
– Maximum lateral grip is needed during cornering situations.
• In a cornering situation the car will be rolled to some degree
• Meaning the suspension will not be a static position
• For this reason static suspension position is much less relevant than the dynamic
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Camber• Launch/Braking Situation
– Maximum longitudinal grip is needed during launch/brake situations.
• In a launch/brake situation the car will be pitched to some degree
• Suspension will not be in a static position
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Compromise
• It is apparent that the suspension is likely to be at the same position for some cornering maneuvers as it is during launching/braking maneuvers
– For this reason we must compromise between too little and too much negative camber
– This can be approximated with tire data and often refined during testing
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Defining Camber
• Once we set our static camber we must adjust our dynamic camber curves
– This is done by adjusting the lengths of the upper and lower A-arms and the position of the inboard and out board pivots
– These lengths and locations are often driven by packaging constraints
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Instant Center• The instant center is a dynamic point which the
wheel will pivot about and any instant during the suspension travel– For a double wishbone configuration this point moves
as the suspension travels
CHASSIS
Instant Center
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Mild Camber Change Design
-Suspension arms are close to parallel
-Wide instant center locations
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Mild Camber Change Design
0.4° of Neg. Camber Gain Per inch of Bump
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Aggressive Camber Change Design
-Suspension arms are far from parallel
-Instant center locations are inside the track width
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More Aggressive Camber Change Design1.4° of Neg. Camber Gain Per inch of Bump
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Jacking forces
• It is important to consider the Instant Center Position, because when it moves vertically off the ground plane Jacking forces are introduced
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Jacking forces
• Caused during cornering by a moment– Force: lateral traction force of tire– Moment arm: Instant Center height– Moment pivot: Instant center
CHASSIS
Instant Center
Lateral Force Ground
I.C. Height
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Jacking Forces
CHASSIS
I. C.
Lateral Force
I.C. Height
– Caused by geometrical binding of the upper and lower A-arms
– These forces are transferred from the tire to the chassis by the A-arms, and reduce the amount of force seen by the spring
Jacking Forces
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Roll Center• The roll center can be identified from this 2-D front view
– Found at the intersection lines drawn for the Instant center to the contact patch center point, and the vehicle center line
I. C.
Roll Center
Vehi
cle
Cen
ter
Line
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Roll Center
• For a parallel-Iink Situation the Roll Center is found on the ground plane
Roll Center
Vehi
cle
Cen
ter
Line
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Significance of the Roll Center
• Required Roll stiffness of the suspension is determine by the roll moment. Which is dependant on Roll center height
Roll Center
Sprung Mass C.G.
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Roll Moment• Present during lateral acceleration (the cause of body roll)
– Moment Arm: B = Sprung mass C.G. height – Roll center height
– Force:F = (Sprung Mass) x (Lateral Acceleration)
R. C.
Sprung Mass C.G.
B
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Roll Axis
• To consider the total vehicle you must look at the roll axis
Roll AxisRear Roll Center Front Roll Center
Sprung Mass C.G.
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Side View
• The next step will be to consider the response of the suspension geometry to pitch situation– For this we will move to a 2-D side-view
Inboard A-arm pivot points
GroundFront Rear
CHASSIS
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Anti-Features
• By angling the A-arms from the side jacking forces are created– These forces can be used in the design to provide
pitch resistance
GroundFront Rear
CHASSIS
Anti-Dive Anti-Lift
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Anti-Features• Racecars rely heavily on wings and
aerodynamics for performance.– Aerodynamically efficient, high-down force
cars are very sensitive to pitch changes. – A pitch change can drastically affect the
amount of down force being produced.
• Much less important for lower speed cars
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Pitch Center
Pitch Center
• The pitch center can be identified from this 2-D side view– Found at the intersection lines drawn for the
Instant center to the contact patch center point
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Pitch Center
Pitch Center
• The pitch center can be identified from this 2-D side view– Found at the intersection lines drawn for the
Instant center to the contact patch center point
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Pitch Moment
Pitch Center
• Present during longitudinal acceleration– Moment Arm: B = Sprung mass C.G. height – Roll center height
– Force:F = (Sprung Mass) x (Longitudinal Acceleration)
B
F