Simulating and Prototyping a Formula SAE Race Car Suspension System

Transcription

1 Simulating and Prototyping a Formula SAE Race Car Suspension System Mark Holveck 01, Rodolphe Poussot 00, Harris Yong 00 Progress Report January 6, 2000 MAE 339/439 Advisor: Prof. Bogdonoff

2 Role of a Race Car Suspension System Transfers forces from the tire contact patch to accelerate a car: : relative motion between the ground, tire/wheel and car body governs manner of force transfer concerned with geometry : forces between the tires and the car behavior of the car concerned with rates

3 Assumptions Sprung and unsprung masses Front/rear mass distribution Center of gravity height Rigid frame Assumed maximum accelerations: 1.5 G cornering 1.2 G braking < 1 G acceleration

4 Major Components Control arms Rigid suspension links Upright Interface between control arms and wheels Spring and damper (shock absorber)

5 Basic Design Independent double A-arms Flexibility in choosing parameters Mostly axial loading Common race car design Outboard springs and dampers Reduced complexity Sufficient adjustability

6 Suspension Bottom line: Maximize tire contact patch utilization Correct geometry between tire and ground

7 Camber Affects tire s ability to generate lateral (cornering) forces

8 Camber Camber needs to change with wheel travel because car rolls to the side during cornering

9 Camber Gain Different for front and rear suspension 6 Camber Curves Camber Angle Wheel Displacement (Bump Positive) Camber required to keep tires flat Front wheel camber Rear wheel camber

10 Caster Caster centers steered front wheels Also introduces camber change on steered front wheels

11 Caster and Camber Camber Curves 6 Camber Angle Wheel Displacement (Bump Positive) Camber required to keep tires flat Front w heel camber Rear w heel camber

12 Roll Center Front and rear roll centers define roll axis of vehicle Determines amount of body roll and load transfer distribution Jacking effects

13 Jacking

14 Anti Effects Reduce pitching during accelerating and braking Anti-dive: 12% Anti-lift: 5% Anti-squat: 12%

15 Compromises Roll center and camber objectives often conflict Other parameters to optimize: Tire scrub Scrub radius Kingpin inclination Trail Bump steer Many others!

16 Reynard Free evaluation software from Reynard Motorsport Parametric kinematics

17 Suspension Behavior of the car undergoing accelerations Bottom line: Choose spring, damper, and other rates to optimize among a set of compromises

18 Reduce Body Roll Especially important for tight Formula SAE courses Body roll slows transient response Shorten distance between roll center and center of gravity Results in high roll center and jacking effect

19 Reduce Load Transfer Tire coefficient of friction decreases with vertical load Different from elementary physics Net grip is best when tires share the total vertical load evenly Minimize load transfer from one tire to another

20 Reducing Load Transfer Widen track, wheelbase Lower center of gravity

21 Cornering Behavior Understeer Turning radius larger than intended car plows Stable Too much load (transfer) on front tires Oversteer Turning radius smaller than intended car spins out Unstable Too much load (transfer) on rear tires

22 Cornering Behavior Neutral steer Car stays on track Unlimited cornering capability Requires fine balance of load distribution

23 Adjusting Cornering Behavior Axle that resists roll the most usually has less cornering ability than the other axle Vary front/rear spring and damper rates Also reduces body roll Anti-roll bar Couples left and right wheels together to resist opposite motion

24 Calculations Used Microsoft Excel to determine rates B C F G H I J K L M N Description Units Design Intent (FR) Design Intent (RR) With Worst Case RC (FR) With Worst Case RC (RR) With 15% lower ride frequency (FR) With 15% lower ride frequency (RR) With 10% Greater Sprung Weight (FR) With 10% Greater Sprung Weight (FR) With Sprung Mass Distrib Basic Ve hicle Len gths an d CG Vertical L ocation CG he ig ht White cells in wheelb ase are fo r da ta mm wheelb ase entry. in tra ck mm Gray ce lls are tra ck in calcul ated or aver age t rack in dervied values aver age t rack mm Spring and Damper Mount ing Orientation chassis t o sprin g mou nting p oint mm chassis t o lower b all joint mm linkage ratio perpendicular - not used not used not used not used not used not used not used not used not used spring/damper mounting angle from perpendicular deg not used not used not used not used not used not used not used not used not used spring/damper mounting angle from perpendicular rad not used not used not used not used not used not used not used not used not used motion ratio (net calculated approximation) - not used not used not used not used not used not used not used not used not used motion ratio (according to Reynard ) Vehicle Weight s and Weig ht Distribut io n 1 axle sprung weight lb axle sprung weight lb axle unsprung weight lb axle sprung weight lb a xle total weigh t lb a xle total weigh t lb spr ung m ass CG in spr ung m ass distr ib ution % over all mass distributio n % Derive d Rates ride frequency Hz ride frequency cpm ride freq uency r atio ride rate lb/in tire rate lb/in tire static load ed ra dius in wheel ce nter rate lb/in spr in g ra te lb/in Roll Geom etry and Rate s ro l ce nter height mm ro l ce nter height in roling moment lever arm in ro ling mom ent p er g la teral a ccelera tio n lb-f t/g a xle spring roll ra te lb-f t/deg a xle spring roll ra te lb-f t/deg ro l g radien t with spr in gs alone deg /g Anti-Roll Bar G eomet ry ARB she ar m odulus ARB dime nsions psi ar e 1.0 0E E E E E E E E E+ 07 ARB inn er r adius on approximate in and ARB ou ter r adius are used only t in o ARB ar ea m ome nt of in ertia gen era te ad ditional in^ E E E E E E E E E-04 ARB leve r ar m leng th roll sti ffness values in chassis t o ARB att achme nt poin t fo r b alancin g mm load ARB linkag e ra tio dist rib uti on ARB len gth mm Anti-Roll Bar Con tribut io n ARB twist r ate lb-f t/deg 2.3 6E E E E E E E E E- 01 ARB r ate lb-f t/deg ARB roll rate lb-ft/deg a xle ARB ro l r ate lb-f t/deg Net Roll Cha racter istics axle r oll rate lb-f t/deg a xle roll ra te lb-f t/deg

25 CarSim Educational Simulates vehicle behavior Can help to analyze sensitivity of parameters Deviations from design intent Complement design with road testing

26 Importance of completing all the dynamic events Ability to engineer next iteration based on successes and failures Structural strength to maintain intended kinematics and dynamics

27 A-arm Load Analyses 1.5G cornering and 1.2G braking Maximum tensile stress: 57 MPa under cornering Maximum compressive stress: 42 MPa front suspension under braking All loads under 650 MPa yield strength of 4130 chromoly steel

28 Loads on Front Upright A-arm loads resolved into loads on upright No severe stresses Modeling is not representative of braking forces Constrained hub carrier and applied previous loads Hub carrier not yet fully designed

29 Upright: CNC machined from 6061-T6 aluminum Control arms Welded 4130 chromoly steel tubing Mounting brackets Welded 4130 chromoly rectangular tubing Purchased items: Wheels Dampers Various hardware

30 Analyzed suspension design in context of Formula SAE requirements Compromised among parameters for best first year car Combine with testing Next semester: Complete suspension construction Minor changes to suspension Brakes Steering

31 Questions