Download - Suspension system
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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 TerminologyBump 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.
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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 SuspensionWith Unequal Length Upper and Lower Arms
Found On:
-Stock Cars (Front)
-Corvettes (C5 &C6) (Front and Rear)
-Honda Civics (‘88-’00) (Front)
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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