SAE 1: Mini-Baja Four Wheel Steering Team: Clyde Baker Ken Brown Alex Cherukara Kevin Eady Sponsor: Dr. Patrick Hollis Problem Statement • Reduce Turning Radius • Applicable to Formula SAE Car • Budget of $500 Needs Assessment • As defined by group: – Turning radius of 7’ or less – Strong enough to withstand forces experienced during competition – Must not interfere with other components – Must remain stable – Least weight possible Concepts • – – – Steering Configuration Opposite Wheel Steering Parallel Wheel Steering Combination Concepts • Front Wheel Angle – 35 degrees: Toggling becomes a concern – 20 degrees: Must have high front to rear wheel angle ratio to satisfy turning radius – 30 degrees: Standard for most production cars. Good compromise Concepts • Wheel Angle Ratio – 1:1 : Greatly reduces turning radius, unstable during high speed turns. – 2:1 : Good compromise. Will meet turning radius requirement, more stable than 1:1. – 3:1 : Cannot satisfy turning radius requirement Steering Ratio Concepts • How to achieve front to rear ratio? – Geometry: Easier to construct, more reliable, less components. – Gearing: Easier to design, identical geometry and components for subsystems. Final Concept • The complete concept is a combination of all other concepts. – Opposite wheel steering – 30 degree front wheel angle – 2:1 front to rear wheel angle ratio – Gearing to achieve ratio System Configuration • Layout/Operation of the system: – Two subsystems: Rack and pinion for front and rear, identical geometry and components. – Steering column fitted with 2” bevel gear. Meshes with 2” bevel gear on shaft to front system and 4” gear on shaft to rear system. – As steering wheel is turned the shafts turn. Front rack will travel 5” total, the rear 2.5”. Front Steering Assembly Center Gearbox Rear Steering Assembly Force Analysis • Performed by hand initially, vector analysis of statically loaded system. • Equations written in Mathcad equation solver software, angle of wheels and impact changed. • Given stresses through components and design strengths of components based on factor of safety. Vector Analysis • Vector analysis performed in following way: • Performed for each individual component: Steering arm, tie rod, tie rod ends, bolts and rack Initial Force • Initially force was taken as the force with the largest expected driver and a 30 – 0 mph velocity with an elapsed time of .7 seconds. • Following more research the initial velocity was lowered to 20 mph and the E.T to .55 seconds. Force Breakdown • Forces in steering arm as wheel travels from 0-30 degrees TOTAL STRESS (MPa) STEERING ARM TOTAL STRESS VS. WHEEL ANGLE 500 400 300 200 100 0 0 10 20 WHEEL ANGLE 30 Force Breakdown • Forces on bolt as wheel travels from 0-30 degrees TOTAL STRESS (MPa) TIE ROD BOLT TOTAL STRESS VS. WHEEL ANGLE 80 70 60 50 40 30 20 10 0 0 10 20 WHEEL ANGLE 30 Force Breakdown • Forces on tie rod as wheel travels from 0-30 degrees TOTAL STRESS (MPa) TIE ROD TOTAL STRESS VS. WHEEL ANGLE 250 200 150 100 50 0 0 10 20 WHEEL ANGLE 30 Force Breakdown • Forces in tie rod end as wheel travels from 0-30 degrees TIE ROD END TOTAL STRESS (MPa) TOTAL STRESS VS. WHEEL ANGLE 500 400 300 200 100 0 0 10 20 WHEEL ANGLE 30 Force Breakdown • Forces on rack as wheel travels from 0-30 degrees TOTAL STRESS (MPa) RACK TOTAL STRESS VS. WHEEL ANGLE 500 400 300 200 100 0 0 10 20 WHEEL ANGLE 30 Stress Reduction • Initially the design strengths of the components were too high. The stress and therefore design strength of the components were lowered by increasing their sizes. Maximum Stress (MPa) Comparison of Stresses Pre/Post Reduction 1000 900 800 700 600 500 400 300 200 100 0 Pre Stress Reduction Post Stress Reduction Steering Arm Tie Rod Bolt Tie Rod Component Tie RodEnd Rack Design Strength • The final design strengths of components COMPONENT STRENGTH VS. SELECTED MATERIAL STRENGTH 1200 STRENGTH (MPa) 1000 800 COMPONENT DESIGN STRENGTH 600 400 SELECTED MATERIAL STRENGTH 200 0 STEERING ARM TIE RO D BO LTS TIE RO D CO MPO NENT TIE RO D END RACK Modeling • All parts and assemblies were modeled in ProEngineer • ADAMS was inoperable during the first half of the semester, therefore work is still being done to confirm the hand calculated force analysis numbers. • Predict our force analysis values are 50% high. Modeling Modeling Modeling Component Selection • Components selected based on force analysis results. Picked to match the design strength of the components. – Tie Rod Ends: ¾” Chrome-moly ends – Rack: 11” steel rack with 5” total travel – Gears: 2” Steel bevel pinion(2), 4” Steel bevel gear – All other components will be fabricated by team Final Car Specs Turning Radius 6.5’ Front Wheel Angle 30 degrees Rear Wheel Angle 15 degrees Cost $475 (est.) Car Weight 800 lbs. (estimated) Current Work • Complete ADAMS modeling. • Reselect components if needed • Order Components Spring Agenda • • • • • Test current steering system. Construct system. Test new steering system. Analyze test results. Rework system if necessary. Downhill From Here