baja sae india suspension design

Suspension Design notes

BAJA SAEINDIA 2011 Workshop23rd July 2010

By Rajeev Mokashi

Suspension Design by Rajeev Mokashi 2

Acknowledgements

Explanation of “Automotive Suspension” www.HowStuffWorks.com

SAEINDIA presentation “Suspension design” by Mr Ravindra Deshmukh, Dy GM (R & D), Mahindra & Mahindra, Nashik. 2007

“Suspension Design” presentation by Mr Rob Shanahan, 15-11-2005

“Automotive Suspension design”. Ref: http://en.wikipedia.org/wiki/Automotive_suspension_design

“Vehicle Dynamics – Theory & Application” by Mr Reza N Jazar


Introduction

What is an Automotive Suspension?An Automotive Suspension is the system of parts that give a vehicle the ability to maneuver.It is a 3 Dimensional Four Bar Linkage

What does a suspension do?“The job of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling”

Ref: www.HowStuffWorks.com


Process of Suspension Design

Selecting vehicle level targetsSelecting system architecture – type of suspension etc.Choosing location of ‘Hard points’.Selecting rates of the bushings.Analysing the loads in suspension.Designing Spring ratesDesigning Shock absorber characteristics.Designing structure of each component – Strong, stiff, Light, easy to manufacture and Cheap.Analysing Vehicle Dynamics of the resulting design


Vehicle level targets (main)

Ride heights at various states of loadRide frequenciesRoll stiffness (Deg / g of lateral acceleration)Distribution of load – front to rearJounce travel (Bump / Compression)Rebound travel (Droop / Extension)CamberCasterToe In / Toe Out


Basic Suspension Terminology

Ride Height


Desirables for Vehicle level targets

Stiffness – Design for maximum torsional stiffness and least weight. This is checked by holding rear shock absorber points & applying torque at front shock absorber points.Provide large suspension travels – typically 250 ~ 300 mm. For typical ATV, ratio of Jounce travel to Rebound travel is 2:1.Provide sufficient ground clearance – more than 200 mm.Use maximum track / overall width allowed. Place wheels at farthest corners.Design to provide tunable features – to adjust Camber, Caster, Toe In, damping forces in shock absorber, spring force on assembly etc.Keep aggregates like Fuel tank, Powertrain etc. as low as possible.


Suspension Architecture

Double Wishbone (Equal or Unequal arm)

• Lightest weight• Lowest unsprung mass• Greatest adjustability for roll center height, camber, caster etc.

McPherson Strut / semi –strut

• Compact suspension• Less adjustability

Recommendation: Double wishbone – unequal arm


Double wishbone suspension


Hard point location

Hard points determine Static settings:Toe (normally Toe In 3 ~ 5 mm)Camber (normally 0.5° ~ 2°)Caster (normally 2° ~ 4°)Roll center height at design load (vis-à-vis CG)caster trailKingpin inclination (normally 7° ~ 8°)Scrub radiusSpring / Shock absorber motion ratios

Hard points also affect Handling of the vehicle in dynamic state.


Roll center

Roll center moves as suspension travels.Goal of any suspension designer is to minimize Roll Center Migration.

Distance from roll center to CG is key to decide roll couple. Lower distance the better.


Design of suspension components

Wishbone – Control arms etc.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 – 5 g impact etc. Ensure maximum stress below Yield Stress with good factor of safety.

Wishbone pivots / bushesUse rubber bushings or solid bushings. Ensure wishbones move freely and do not rub against attachments / brackets etc.

Rubber bushes, if readily available, are preferred.Compliances may be worked out during tuning


Design of suspension components

Coil springs / shock absorbersCoil springs over shock absorber designs are easy for tuning, byproviding screwed type of spring seats on shock absorber body.Use wheel frequency of 100 ~ 125 cpm for designing spring stiffness. For passenger cars, this frequency is 60 ~ 80 cpm. Choose shock absorber length longer than required – by 10 mm or more, so that it does not bottom out with full bump (2.5 g).Check that coil spring does not become solid at full bump load.


Effect of Suspension geometry on handling of vehicle

Twitchy in back, tires wear on outer edge. More oversteer, more forgiving limit

More positive camber in rear wheels

More oversteer, car feels twitchy in back, tyres wear out on the inside edge

Less oversteer, more rear grip / limit -3 degrees

More negative camber on rear wheels

Poor braking, car is road crown sensitive, twitchy, tires wear out on outer edge.

More understeer, can make the tyres last longer

More positive camber on front wheels

Poor braking, car is road crown sensitive, twitchy, tyres wear out on the inside edge

Less understeer /limit-3 degrees

More negative camber on front wheels

Understeers, then oversteers as car bottoms out with a jolting ride.

Less oversteerRear spring rate decrease

Too much oversteer, hop in corners, twitchy.

More oversteerRear spring rate increase

Oversteers. then understeers as car bottoms excessively with jolting ride

Less understeerFront spring rate decrease

Terminal understeer, front of car hops in corners.

More understeerFront Spring rate increase

Symptom of too much adjustmentAffect on vehicle handling, limit adjustmentSuspension adjustment


Effect of Suspension geometry on handling of vehicle

Not usableNegative front caster

Can increase understeer, increases steering efforts.

Helps both stability, steady state cornering and turn in. Limit 6 degrees positive.

Positive front caster

Not good for street driving, causes lift throttle oversteer, car makes violent side to side rocking motions in rear.

Helps car rotate, useful on tight low speed courses and slalom events (limit 3 mm total toe out)

Toe Out rear

Twitchy under braking, car is road crown sensitive, car wanders on straight road.

Car turns in well, good in FWD cars. ( limit 6 mm toe out)

Toe Out front

Weird slow rocking movement in back, feels slow but unstable.

Less likely to oversteer when throttle is lifted

Toe In rear

Car has slow twitchiness under braking, feels odd, wears out outer edge of tyres

Car is stable while going straight. Turn in is average.

Toe In front

Symptom of too much adjustmentAffect on vehicle handling, limit adjustment

Suspension adjustment


One last word …….

Figuring a suspension of car is almost entirely a matter of making useful approximations.

It is not an exact science.

But neither it is a blind application of rule of thumbs.

- Quoted by Mr Ravindra DeshmukhR & D, Mahindra & Mahindra

baja sae india suspension design

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