2 PurposeSuspension to be used on a small (lightweight) formula style racecar.Car is intended to navigate tight road coursesSurface conditions are expected to be relatively smooth
3 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 suspensionSpecific vehicle characteristics will be considered.
4 ConsiderationsInitially 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.
6 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-upThe arm only has to provide a restoring force to the moment generated by the tire on the lower ball joint
7 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 loadIn this case study the lower A-arm will carry a larger rod end to compensate for the larger forces seen by this component.
8 ComponentsUprightThe upright serves several purposes in the suspensionConnects the upper A-arm, lower A-arm, steering arm, and the tireCarries the spindle and bearing assemblyHolds the brake caliper in correct orientation with the rotorProvides a means for camber and castor adjustment
9 Components Spindle Spindle can come in two basic configurations Live spindleFixed spindleIn the live spindle configuration the whole spindle assembly rotates and carries the tire and wheelThe fixed spindle configuration carries a hub assembly which rotates about the spindleBoth configurations carry the brake rotor
10 Live Vs. Fixed Spindle Advantages and Disadvantages Live Spindle :Less partsLighter weight if designed correctlyMore wheel offsetBearing concernsRetention inside of the upright assemblyFixed spindleSimple constructionHub sub-assemblySpindle put in considerable bendingMore components, and heavier
11 ComponentsPush rodThe push rod carries the load from the lower A-arm to the inboard coil over shockThe major concern with this component is the buckling force induced in the tube
12 Components Toe rod (steering link) The toe rod serves as a like between the steering rack inboard on the vehicleThe location of the ends of this like are extremely critical to bump steer and Ackermann of the steering systemThis link is also used to adjust the amount of toe-out of the wheels
13 ComponentsBellcrankThis is a common racing description of the lever pivot that translates to motion of the push rod into the coil over shockThe geometry of this pivot can be designed to enable the suspension to have a progressive or digressive natureThis component also offers the designer the ability to include a motion ratio in the suspension
14 Components Coil-over Shock Absorber This component carries the vehicle corner weightIt is composed of a coil spring and the damperThis component can be used to adjust ride height, dampening, spring rate, and wheel rate
15 Components Anti-Roll bar This component is an additional spring in the suspensionPurpose: resist body rollIt accomplishes this by coupling the left and right corners of the vehicleWhen 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 springThis 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
16 Beginning the Design Process Initially the suspension should be laid out from a 2-D front viewStatic and dynamic camber should be defined during this step
17 CamberThe main consideration at this step is the camber change throughout the suspension travel.
18 Camber Static Camber Dynamic Camber Describes the camber angle with loaded vehicle not in motionDynamic CamberDescribes the camber angle of a corner at any instant during a maneuver i.e.: cornering, launching, braking
19 Contact PatchTread area in contact with the road at any instant in time
20 CamberCamber is used to offset lateral tire deflection and maximize the tire contact patch area while cornering.
21 Camber Negative Camber angles good for lateral acceleration, cornering bad for longitudinal acceleration, launching/brakingThis is because the direction of the tire deflection is obviously not the same for these two situations
22 Camber Cornering Situation Maximum lateral grip is needed during cornering situations.In a cornering situation the car will be rolled to some degreeMeaning the suspension will not be a static positionFor this reason static suspension position is much less relevant than the dynamic
23 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 degreeSuspension will not be in a static position
24 CompromiseIt is apparent that the suspension is likely to be at the same position for some cornering maneuvers as it is during launching/braking maneuversFor this reason we must compromise between too little and too much negative camberThis can be approximated with tire data and often refined during testing
25 Defining CamberOnce we set our static camber we must adjust our dynamic camber curvesThis is done by adjusting the lengths of the upper and lower A-arms and the position of the inboard and out board pivotsThese lengths and locations are often driven by packaging constraints
26 Instant CenterThe instant center is a dynamic point which the wheel will pivot about and any instant during the suspension travelFor a double wishbone configuration this point moves as the suspension travelsCHASSISInstant Center
27 Mild Camber Change Design -Suspension arms are close to parallel -Wide instant center locations
28 Mild Camber Change Design 0.4° of Neg. Camber Gain Per inch of Bump
29 Aggressive Camber Change Design -Suspension arms are far from parallel -Instant center locations are inside the track width
30 More Aggressive Camber Change Design 1. 4° of Neg More Aggressive Camber Change Design 1.4° of Neg. Camber Gain Per inch of Bump
31 Jacking forcesIt is important to consider the Instant Center Position, because when it moves vertically off the ground plane Jacking forces are introduced
32 Jacking forces Caused during cornering by a moment Force: lateral traction force of tireMoment arm: Instant Center heightMoment pivot: Instant centerCHASSISInstant CenterI.C. HeightLateral ForceGround
33 Jacking ForcesCaused by geometrical binding of the upper and lower A-armsThese forces are transferred from the tire to the chassis by the A-arms, and reduce the amount of force seen by the springJacking ForcesCHASSISI. C.I.C. HeightLateral Force
34 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 lineVehicle CenterLineI. C.Roll Center
35 Roll CenterFor a parallel-Iink Situation the Roll Center is found on the ground planeVehicle CenterLineRoll Center
36 Significance of the Roll Center Required Roll stiffness of the suspension is determine by the roll moment. Which is dependant on Roll center heightSprung Mass C.G.Roll Center
37 Roll MomentPresent during lateral acceleration (the cause of body roll)Moment Arm:B = Sprung mass C.G. height – Roll center heightForce:F = (Sprung Mass) x (Lateral Acceleration)Sprung Mass C.G.BR. C.
38 Roll Axis To consider the total vehicle you must look at the roll axis Sprung Mass C.G.Roll AxisRear Roll CenterFront Roll Center
39 Side ViewThe next step will be to consider the response of the suspension geometry to pitch situationFor this we will move to a 2-D side-viewInboard A-arm pivot pointsCHASSISGroundFrontRear
40 Anti-FeaturesBy angling the A-arms from the side jacking forces are createdThese forces can be used in the design to provide pitch resistanceAnti-LiftAnti-DiveCHASSISGroundRearFront
41 Anti-FeaturesRacecars 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
42 Pitch CenterThe pitch center can be identified from this 2-D side viewFound at the intersection lines drawn for the Instant center to the contact patch center pointPitch Center
43 Pitch CenterThe pitch center can be identified from this 2-D side viewFound at the intersection lines drawn for the Instant center to the contact patch center pointPitch Center
44 Pitch Moment Present during longitudinal acceleration Moment Arm: B = Sprung mass C.G. height – Roll center heightForce:F = (Sprung Mass) x (Longitudinal Acceleration)FBPitch Center