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Suspension System and It's Components in a Car - DIYguru
The document discusses suspension systems and components. It provides three key objectives of suspension systems: 1) To provide good ride and handling performance by ensuring wheels follow the road profile with minimal tire load fluctuation. 2) To ensure steering control is maintained during maneuvers by keeping wheels in the proper position. 3) To ensure the vehicle responds favorably to braking, accelerating and cornering forces from tires by resisting body movement. It then discusses various suspension types, kinematic analysis methods, force analysis, and concepts like roll center analysis and anti-dive/anti-squat characteristics.
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Introduction to suspension systems and components.
Key objectives include ride performance, steering control, vehicle response to forces, and vibration isolation.
Description of a right-hand orthogonal axis system in vehicles, including axes and rotations.
Definitions of camber angle and swivel pin inclination and offset in tire mechanics.
Definitions of caster angle and toe-in/toe-out related to wheel positioning.
Visual representation of suspension mechanisms.
Discussion on 3D mechanisms, compliant bushes, and the requirements for wheel movement.
Insights into degrees of freedom in suspension and motion guidance.
Explanation of dependent, semi-dependent, and their characteristics in vehicles.
Description of independent suspension mechanisms and their advantages.
Objectives and graphical methods for analyzing suspension mechanisms.
Calculations related to camber angle and suspension ratio in a double wishbone system.
Further exploration of kinematic analysis and results.
Definitions and limitations of roll centre analysis in vehicle dynamics.
Methods for determining roll centre and its implications on handling.
Analysis of spring and wheel rates in suspension systems, addressing non-linear relationships.
Overview of force analysis in double wishbone and other suspension systems.
Analysis of dynamic loading conditions on suspension during cornering and bumps.
Mechanics of anti-squat and anti-dive during vehicle braking and acceleration.
Diagrammatic representation of anti-squat and anti-dive scenarios in suspension.
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Suspension System and It's Components in a Car - DIYguru
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- 2. 2 of 42 Objectives •To provide good ride and handling performance – – vertical compliance providing chassis isolation – ensuring that the wheels follow the road profile – very little tire load fluctuation • To ensure that steering control is maintained during maneuvering – – wheels to be maintained in the proper position wrt road surface • To ensure that the vehicle responds favorably to control forces produced by the tires during – longitudinal braking – accelerating forces, – lateral cornering forces and – braking and accelerating torques – this requires the suspension geometry to be designed to resist squat, dive and roll of the vehicle body • To provide isolation from high frequency vibration from tire excitation – requires appropriate isolation in the suspension joints – Prevent transmission of ‘road noise’ to the vehicle body
- 3. 3 of 42 VehicleAxis system • Un-sprung mass • Right-hand orthogonal axis system fixed in a vehicle • x-axis is substantially horizontal, points forward, and is in the longitudinal plane of symmetry. • y-axis points to driver's right and • z-axis points downward. • Rotations: – A yaw rotation about z-axis. – A pitch rotation about y-axis. – A roll rotation about x-axis SAE vehicle axes Figure from Gillespie,1992
- 4. 4 of 42 TireTerminology - basic • Camber angle – angle between the wheel plane and the vertical – taken to be positive when the wheel leans outwards from the vehicle • Swivel pin (kingpin) inclination – angle between the swivel pin axis and the vertical • Swivel pin (kingpin) offset – distance between the centre of the tire contact patch and – intersection of the swivel pin axis and the ground plane Figure from Smith,2002
- 5. 5 of 42 TireTerminology - basic • Castor angle – inclination of the swivel pin axis projected into the fore–aft plane through the wheel centre – positive in the direction shown. – provides a self-aligning torque for non-driven wheels. • Toe-in and Toe-out – difference between the front and rear distances separating the centre plane of a pair of wheels, – quoted at static ride height – toe-in is when the wheel centre planes converge towards the front of the vehicle Figure from Smith,2002
- 6. 6 of 42 Themobility of suspension mechanisms Figure from Smith,2002
- 7. 7 of 42 Analysisof Suspension Mechanisms • 3D mechanisms • Compliant bushes create variable link lengths • 2D approximations used for analysis • Requirement – Guide the wheel along a vertical path – Without change in camber • Suspension mechanism has various SDOF mechanisms
- 8. 8 of 42 Themobility of suspension mechanisms • Guide motion of each wheel along (unique) vertical path relative to the vehicle body without significant change in camber. • Mobility (DOF) analysis is useful for checking for the appropriate number of degrees of freedom, • Does not help in synthesis to provide the desired motion M = 3(n – 1) – jh – 2jl Two-dimensional kinematics of common suspension mechanisms Figure from Smith,2002
- 9. 9 of 42 SuspensionTypes -Dependent • Motion of a wheel on one side of the vehicle is dependent on the motion of its partner on the other side • Rarely used in modern passenger cars – Can not give good ride – Can not control high braking and accelerating torques • Used in commercial and off-highway vehicles
- 10. 10 of 42 HotchkissDrive • Axle is mounted on longitudinal leaf springs, which are compliant vertically and stiff horizontally • The springs are pin- connected to the chassis at one end and to a pivoted link at the other. • This enables the change of length of the spring to be accommodated due to loading Hotchkiss Drive Figure from Smith,2002
- 11. 11 of 42 Semi-dependentSuspension • the rigid connection between pairs of wheels is replaced by a compliant link. • a beam which can bend and flex providing both positional control of the wheels as well as compliance. • tend to be simple in construction but lack scope for design flexibility • Additional compliance can be provided by rubber or hydro- elastic springs. • Wheel camber is, in this case, the same as body roll Trailing twist axle suspension
- 12. 12 of 42 SuspensionTypes - Independent • motion of wheel pairs is independent, so that a disturbance at one wheel is not directly transmitted to its partner • Better ride and handling Macpherson Strut Double wishbone Trailing arm Swing axle Semi trailing arm Multi-link
- 13. 13 of 42 KinematicAnalysis -Graphical Graphical Analysis • Objective – The suspension ratio R (the rate of change of vertical movement at D as a function of spring compression) – The bump to scrub rate for the given position of the mechanism. Figure from Smith,2002
- 14. 14 of 42 KinematicAnalysis -Graphical • Draw suspension mechanism to scale, assume chassis is fixed • Construct the velocity diagram VB = ωBArBA Figure from Smith,2002
- 15. 15 of 42 KinematicAnalysis – Sample calculation • Double wish bone • The objectives are – Determine camber angle α, and suspension ratio R (as defined in the previous example) – For suspension movement described by “q” varying from 80° to 100° – Given that in the static laden position “q” = 90°. Simplified suspension model Figure from Smith,2002, Google search
- 16. 16 of 42 KinematicAnalysis – Sample calculation • Positions are provided • Two non-linear equations solved for positions described interval 1
- 17. 17 of 42 KinematicAnalysis
- 18. 18 of 42 KinematicAnalysis The second part of the solution begins by expressing the length of the suspension spring in terms of the primary variable and then proceeds to determine the velocity coefficients
- 19. 19 of 42 KinematicAnalysis - Results Figure from Smith,2002
- 20. 20 of 42 Rollcentre analysis Two Definitions • SAE : a point in the transverse plane through any pair of wheels at which a transverse force may be applied to the sprung mass without causing it to roll • Kinematics : the roll centre is the point about which the body can roll without any lateral movement at either of the wheel contact areas Figure from Smith,2002
- 21. 21 of 42 Limitationsof Roll Centre Analysis • As roll of the sprung mass takes place, the suspension geometry changes, symmetry of the suspension across the vehicle is lost and the definition of roll centre becomes invalid. – It relates to the non-rolled vehicle condition and can therefore only be used for approximations involving small angles of roll – Assumes no change in vehicle track as a result of small angles of roll.
- 22. 22 of 42 Roll-centredetermination • Aronhold–Kennedy theorem of three centers : when three bodies move relative to one another they have three instantaneous centers all of which lie on the same straight line • Iwb can be varied by angling the upper and lower wishbones to different positions, thereby altering the load transfer between inner and outer wheels in a cornering maneuver. • This gives the suspension designer some control over the handling capabilities of a vehicle For a double wishbone Figure from Smith,2002
- 23. 23 of 42 Roll-centredetermination • In the case of the MacPherson strut suspension the upper line defining Iwb is perpendicular to the strut axis. • Swing axle roll center is located above the “virtual” joint of the axle. Macpherson strut Swing Axle Figure from Smith,2002
- 24. 24 of 42 Roll-centredetermination Roll centre for a four link rigid axle suspension Roll centre location for semi-trailing arm suspension Roll centre location for a Hotchkiss suspension Figure from Smith,2002
- 25. 25 of 42 ForceAnalysis - spring and wheel rates • Relationship between spring deflections and wheel displacements in suspensions is non- linear • Desired wheel-rate (related to suspension natural frequency) has to be interpreted into a spring-rate W and S are the wheel and spring forces respectively v and u are the corresponding deflections Notation for analyzing spring and wheel rates in a double wishbone suspension
- 26. 26 of 42 Springand wheel rates From principle of virtual work Wheel rate
- 27. 27 of 42 Springand wheel rates Combined Equation is Similarly can be derived for other suspension geometries
- 28. 28 of 42 Wheel-ratefor constant natural frequency with variable payload Simplest representation of undamped vibration kw – wheel rate ms – proportion of un-sprung mass Change in wheel rate required for change in payload. Static displacement To maintain wn constant, the static deflection needs to be constant. Combining both equations
- 29. 29 of 42 Wheel-ratefor constant natural frequency with variable payload Integrating the equation and substituting with initial conditions provides the following expression Substituting back , we obtain
- 30. 30 of 42 Wheel-ratefor constant natural frequency with variable payload Wheel load v. wheel deflection Wheel rate v. wheel deflection Typical wheel load and wheel rate as functions of wheel displacement Figure from Smith,2002 R and dR/dv are known from geometric analysis gives ks if kw is known from the above graphs ks can be obtained as a function of v and u so as to get constant frequency
- 31. 31 of 42 Forcesin suspension members - Basics • Mass of the members is negligible compared to that of the applied loading. • Friction and compliance at the joints assumed negligible and the spring or wheel rate needs to be known • Familiar with the use of free-body diagrams for determining internal forces in structures • Conditions for equilibrium Equilibrium of two and three force members, (a) Requirements for equilibrium of a two force member (b) Requirements for equilibrium of a three-force member Figure from Smith,2002
- 32. 32 of 42 Verticalloading Force analysis of a double wishbone suspension (a) Diagram showing applied forces (b) FBD of wheel and triangle of forces (c) FBD of link CD and triangle of forces Figure from Smith,2002
- 33. 33 of 42 Verticalloading • Assume FW is the wheel load and FS the force exerted by the spring on the suspension mechanism • AB and CD are respectively two-force and three force members • FB and FC can be determined from concurrent forces • Similar analysis possible for other types also.
- 34. 34 of 42 Verticalloading- Macpherson Force analysis of a MacPherson strut, (a) Wheel loading, (b) Forces acting on the strut Figure from Smith,2002
- 35. 35 of 42 OtherLoadings • Lateral Loadings due to cornering effects • Longitudinal loadings arise from – braking, – drag forces on the vehicle and – shock loading due to the wheels striking bumps and pot-holes. • Same method as before used to analyze these loading conditions
- 36. 36 of 42 Forcesin suspension members – Dynamic factors for Shock loading
- 37. 37 of 42 AntiSquat / Anti-dive • During braking there is a tendency for the sprung mass to “dive” (nose down) and • During acceleration the reverse occurs, with the nose lifting and the rear end “squatting” Free body diagram of a vehicle during braking Figure from Smith,2002
- 38. 38 of 42 Anti-squat/ Anti-dive • During braking there is a tendency for the sprung mass to “dive” (nose down) and • During acceleration the reverse occurs, with the nose lifting and the rear end “squatting” Free body diagram of a vehicle during braking Figure from Smith,2002
- 39. 39 of 42 Wheelloads during braking • Assume fixed braking ratio – k=Bf/(Bf+Br) • Add D’Alembert force (- ma) • Moment about the rear tyre gives – NfL-mah-mgc=0 – Nf=mah/L+mgc/L – Nr=mgb/L-mah/L Effective load increases in the front and decreases at the rear wheel
- 40. 40 of 42 •Consider side view and wheel pivoting at Of • Sf – suspension force • Sf = Sf + Sf (static + change) • Moments about Of give – Nfe-Sfe-Bff=0 0 ( ) tan f f mahe B f L B mak f h e kL Condition for no dive %age of anti dive given by tan ( )*100 tan '
- 41. 41 of 42 Anti-squat/ Anti-dive • For rear suspension • If these conditions are met – zero deflection in front / rear tires • If the pivots lie below the locus less than 100% anti-dive will be obtained. • In practice anti-dive does not exceed 50% : – Subjectively zero pitch braking is undesirable – There needs to be a compromise between full anti-dive and anti-squat conditions – Full anti-dive can cause large castor angle changes (because all the braking torque is reacted through the suspension links) – resulting in heavy steering during braking. tan (1 ) f h e L k
- 42. 42 of 42 Anti-squat/Anti-dive Figure from Smith,2002