15 BAJA Suspension

DESIGN AND DEVELOPMENT OF SUSPENSIONS OF ALL TERRAIN VEHICLE

Ravindra Deshmukh Mahindra & Mahindra Ltd. R&D Centre Nasik.

ALL-TERRAIN VEHICLE

WHAT ARE THE DEMANDS FROM •STRUCTURE •SUSPENSION SYSTEM

•STIFFNESS- DESIGN FOR MAXIMUM TORSIONAL STIFFNESS AND LEAST WEIGHT.

•PROVIDE LARGE SUSPENSION TRAVELS

•PROVIDE SUFFICIENT GROUND CLEARANCE

•USE MAXIMUM TRACK POSSIBLE.

•DESIGN TO PROVIDE TYPICAL TUNABLE FEATURES.

ROLL CAGE STIFFNESS

HOW TO EVALUATE. •HOLD REAR SHOCK ABSORBER POINTS . • APPLY TORQUE AT THE FRONT SHOCK ABSORBER ATTACHMENTS. •MEASURE DEFLECTIONS AND CALCULATE TORSIONAL STIFFNESS.

DESIGN STIFF JOINERIES AT JUNCTIONS

DESIGN STIFF ATTACHMENT POINTS

USE MAXIMUM TRACK POSSIBLE

KEEP GROUND CLEARANCE MORE THAN 200 MM

PLACE WHEELS AT FARTHEST CORNERS.

PLACE AGGREGATES LIKE FUEL TANK , POWERTRAIN AS LOW AS POSSIBLE.

PREFERRED SUSPENSION TYPES •EQUAL/UNEQUAL LENGTH WISHBONES •MACPHERSONS STRUT/ SEMISTRUTS WHAT IS MY CHOICE • EQUAL ARM DOUBLE WISHBONE BOTH FRONT AND REAR. •LARGER SUSPENSION TRAVELS POSSIBLE. •ADJUSTABLE ROLL CENTRE HEIGHTS •CONTROL OVER CAMBER CHANGE RATE •CONTROL OVER RIDE AND ROLL STEERS

WHAT SHOULD BE TUNABLE FEATURES IN THE DESIGN. •WEIGHT DISTRIBUTION. DESIGN MOUNTINGS SUCH THAT SOME EXTEND OF ADJUSTMENT IS POSSIBLE.

•SUSPENSION •RIDE HT / SUSPENSION TRAVEL •BODY ATTITUDE •WHEEL LOADS/ SPRINGS •DAMPERS •STATIC SETTINGS OF WHEEL ALIGNMENT GEOMETRY •ROLL CENTRE HT FRONT AND REAR •SWING ARM LENGTH •HEIGHT OF STEERING RACK.

ROLL CENTRE HEIGHT ESTIMATE IT ON DRAWING • IT SIGNIFIES THE COUPLING BETWEEN UNSPRUNG AND SPRUNG MASS. •IT CAN CHANGE THE ROLL STIFFNESS OF THE VEHICLE

BUMP STEER •AFFECTS STEERING FEEL AND UNDERSTEER/OVERSTEER

MEASURE BY TEST. CAN BE MODIFIED BY CHANGING THE HT OF THE STEERING RACK. DESIGN FOR TOE OUT IN FRONT AND TOE IN IN REAR. SET AND TEST .THERE IS NO THUMB RULE.

DESIGN OF SUSPENSION COMPONENTS WISHBONES KEEP CALCULATIONS SIMPLE. DRAW FREE BODY DIAGRAMS FOR LOADS ON WISHBONES. DESIGN FOR STIFFNESS. STRENGTH FOLLOWS. CREATE CLEAN DRAWINGS /SKETCHES FOR FABRICATION. KEEP SHAPES SIMPLE. CALCULATE STRESSES FOR SINGLE EVENTS LIKE 5G IMPACTS AND KEEP BELOW Y S

DESIGN OF SUSPENSION COMPONENTS WISHBONES PIVOTS

•USE RUBBER BUSHINGS / OR SOLID BUSHINGS •ENSURE WISHBONE MOVE FREELY AND DO NOT RUB AGAINST ATTACHMENT BRACKETRY. •RUBBER BUSHINGS IF READILY AVAILABLE ARE PREFERRED . CAN WORK AROUND THE COMPLIANCES DURING TUNING.

DESIGN OF SUSPENSION COMPONENTS COIL SPRINGS/ SHOCKS •COIL SPRING OVER SHOCK DESIGNS ARE EASY FOR TUNING. USE SCREWED TYPE SPRING SEATS ON SHOCKS •CHECK THAT COIL SPRING DOES NOT BECOME SOLID AT COMPLETE BUMP TRAVEL. •CHOOSE SHOCK ABSORBER LENGTHS SO THAT THEY DO NOT BOTTOM OUT AT FULL JOUNCE TRAVEL. •IF AVAILABLE TRY USING DIFFERENT DAMPING VALUES DURING TUNING. •REMEMBER YOU ARE MAKING A RECREATIONAL VEHICLE.

USE BALL JOINTS AT UPRIGHTS . ALLOW MORE ARTICULATION

ONE LAST WORD •FIGURING A SUSPENION OF A CAR IS ALMOST ENTIRELY A MATTER OF MAKING USEFUL APPROXIMATIONS. •IT IS NOT A EXACT SCIENCE •BUT NEITHER IS IT A BLIND APPLICATION OF RULE OF THUMBS

My e-mail address deshmukh.ravi @ mahindra.com