Suspension

Outline

• Introduction• Suspension

components• Suspension type

examples– Solid axle– Double Wishbone– MacPherson Strut

• Introduce basic geometry

Suspension Purpose

• Isolate passengers and cargo from vibration and shock

• Improve mobility

• Improve vehicle control

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

Basic Terminology• Unsprung Mass

– Mass of components that move when suspension is displaced

– Minimizing the unsprung mass allows for more optimal suspension operation

Basic Terminology

• Bump– Vertical displacement of entire sprung mass

Basic Terminology

• Roll– Front View angular

rotation of the sprung vehicle mass

Basic Terminology

• Pitch– Side View angular rotation of the sprung

vehicle mass

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

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

Basic Terminology

Camber

Front View tilt of the tire.

Leaning the top of the tire inboard adds negative camber

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

Basic Terminology

Steering Axis

Axis about which the wheel/Tire rotate about during steering inputs

Also known as “King Pin Axis”

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

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

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

Basic Terminology

Steering Arm

Line between the steering axis and there steering linkage “tie rod”

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

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).

Trophy truck video

• Front Independent double wishbone Suspension

•Rear Solid axle

•High horse power vehicle

Baja Buggy

•Front Independent double wishbone Suspension

•Rear Independent double wishbone Suspension

•Light weigh low horsepower vehicle

Extreme mobility

•Control in extreme maneuvers

Simplified Quarter Car Model

• Two Degree-of-Freedom System – However the longitudinal

and lateral stiffness of most suspension cannot be totally disregarded

Simplified Half Car Model

• Two Degree-of-Freedom System – However the longitudinal and lateral stiffness of most

suspension cannot be totally disregarded

Components of Suspension

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

Mass

Tire

• Acts as both a spring and a damper

• These rates are affected by air pressure and tire construction.

Pivot Joints• Spherical & Roller

bearings– High performance

applications– Minimal Compliance– High Precision– Low Stiction/Friction

• Bushings– Low Cost– Offers compliance

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

Spring Types

• Torsion Bars

– Little to no internal damping

– Low cost– Often difficult to

package

Spring Types

• Coil Springs– Little to no internal

damping– Low cost– Compact Size– Used in many

Suspension types

Solid Axle Suspensions

Applications:

-Pick Up Trucks (Rear)

-Stock Cars (Rear)

-Rock Crawlers (Front & Rear)

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

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

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

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

Truck, NASCAR applications

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)

-Most Modern Sports Cars

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

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

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.

Formula 1 Application

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

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.

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.

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.

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.

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