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
Role of a Race Car Suspension System Intro Intro Intro • Transfers forces from the tire contact patch to accelerate a car: Kinematics Kinematics – Kinematics: • relative motion between the ground, tire/wheel Dynamics Dynamics and car body • governs manner of force transfer Reliability Reliability • concerned with geometry – Dynamics: Manufacturing Manufacturing • forces between the tires and the car • behavior of the car Summary Summary • concerned with rates
Assumptions Intro Intro Intro • Sprung and unsprung masses • Front/rear mass distribution Kinematics Kinematics • Center of gravity height • Rigid frame Dynamics Dynamics • Assumed maximum accelerations: Reliability Reliability – 1.5 G cornering – 1.2 G braking Manufacturing Manufacturing – < 1 G acceleration Summary Summary
Major Components Intro Intro Intro • Control arms – Rigid suspension links Kinematics Kinematics • Upright – Interface between control arms and wheels Dynamics Dynamics • Spring and damper (shock absorber) Reliability Reliability Manufacturing Manufacturing Summary Summary
Basic Design Intro Intro Intro • Independent double A-arms Kinematics Kinematics – Flexibility in choosing parameters Dynamics Dynamics – Mostly axial loading – Common race car Reliability Reliability design • Outboard springs Manufacturing Manufacturing and dampers – Reduced complexity Summary Summary – Sufficient adjustability
Suspension Kinematics Intro Intro • Bottom line: – Maximize tire contact patch utilization Kinematics Kinematics Kinematics – Correct geometry between tire and ground Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Camber Intro Intro • Affects tire’s ability to generate lateral (cornering) forces Kinematics Kinematics Kinematics Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Camber Intro Intro • Camber needs to change with wheel travel because car rolls to the side Kinematics Kinematics Kinematics during cornering Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Camber Gain Intro Intro • Different for front and rear suspension Camber Curves Kinematics Kinematics Kinematics 6 Dynamics Dynamics Camber Angle 0 Reliability Reliability Manufacturing Manufacturing -6 -50 0 50 Summary Summary Wheel Displacement (Bump Positive) Camber required to keep tires flat Front wheel camber Rear wheel camber
Caster Intro Intro • Caster centers steered front wheels • Also introduces camber change on Kinematics Kinematics Kinematics steered front wheels Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Caster and Camber Intro Intro Camber Curves Kinematics Kinematics Kinematics 6 Dynamics Dynamics Camber Angle 0 Reliability Reliability Manufacturing Manufacturing -6 -50 0 50 Wheel Displacement (Bump Positive) Summary Summary Camber required to keep tires flat Front wheel camber Rear wheel camber
Roll Center Intro Intro • Front and rear roll centers define roll axis of vehicle Kinematics Kinematics Kinematics – Determines amount of body roll and load transfer distribution Dynamics Dynamics – Jacking effects Reliability Reliability Manufacturing Manufacturing Summary Summary
Jacking Intro Intro Kinematics Kinematics Kinematics Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
“Anti” Effects Intro Intro • Reduce pitching during accelerating and braking Kinematics Kinematics Kinematics • Anti-dive: 12% • Anti-lift: 5% Dynamics Dynamics • Anti-squat: 12% Reliability Reliability Manufacturing Manufacturing Summary Summary
Compromises Intro Intro • Roll center and camber objectives often conflict Kinematics Kinematics Kinematics • Other parameters to optimize: – Tire scrub Dynamics Dynamics – Scrub radius – Kingpin inclination Reliability Reliability – Trail Manufacturing Manufacturing – Bump steer – Many others! Summary Summary
Reynard Kinematics Intro Intro • Free evaluation software from Reynard Motorsport Kinematics Kinematics Kinematics • Parametric kinematics Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Suspension Dynamics Intro Intro • Behavior of the car undergoing accelerations Kinematics Kinematics • Bottom line: – Choose spring, damper, and other rates to Dynamics Dynamics Dynamics optimize among a set of compromises Reliability Reliability Manufacturing Manufacturing Summary Summary
Reduce Body Roll Intro Intro • Especially important for tight Formula SAE courses Kinematics Kinematics – Body roll slows transient response • Shorten distance between roll center and Dynamics Dynamics Dynamics center of gravity – Results in high roll center and jacking effect Reliability Reliability Manufacturing Manufacturing Summary Summary
Reduce Load Transfer Intro Intro • Tire coefficient of friction decreases with vertical load Kinematics Kinematics – Different from elementary physics • Net grip is best when tires share the total Dynamics Dynamics Dynamics vertical load evenly – Minimize load transfer from one tire to Reliability Reliability another Manufacturing Manufacturing Summary Summary
Reducing Load Transfer Intro Intro • Widen track, wheelbase • Lower center of gravity Kinematics Kinematics Dynamics Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Cornering Behavior Intro Intro • Understeer – Turning radius larger than intended Kinematics Kinematics • car “plows” – Stable Dynamics Dynamics Dynamics – Too much load (transfer) on front tires • Oversteer Reliability Reliability – Turning radius smaller than intended • car “spins out” Manufacturing Manufacturing – Unstable Summary Summary – Too much load (transfer) on rear tires
Cornering Behavior Intro Intro • Neutral steer – Car stays on track Kinematics Kinematics – Unlimited cornering capability – Requires fine balance of load distribution Dynamics Dynamics Dynamics Reliability Reliability Manufacturing Manufacturing Summary Summary
Adjusting Cornering Behavior Intro Intro • Axle that resists roll the most usually has less cornering ability than the other Kinematics Kinematics axle • Vary front/rear spring and damper rates Dynamics Dynamics Dynamics – Also reduces body roll • Anti-roll bar Reliability Reliability – Couples left and right wheels together to Manufacturing Manufacturing resist opposite motion Summary Summary
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