Suspension Design Rajeev_Mokashi

Suspension Design notes By Rajeev Mokashi

BAJA SAEINDIA 2011 Workshop 23rd July 2010

Acknowledgements „

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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

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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

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Process of Suspension Design „ „ „ „ „ „ „ „

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

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Vehicle level targets (main) „ „ „ „ „ „ „ „ „

Ride heights at various states of load Ride frequencies Roll stiffness (Deg / g of lateral acceleration) Distribution of load – front to rear Jounce travel (Bump / Compression) Rebound travel (Droop / Extension) Camber Caster Toe In / Toe Out

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Basic Suspension Terminology

Ride Height

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Desirables for Vehicle level targets

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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.

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Suspension Architecture

Double Wishbone (Equal or Unequal arm)

McPherson Strut / semi –strut

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

• Compact suspension • Less adjustability

Recommendation: Double wishbone – unequal arm Suspension Design by Rajeev Mokashi

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Double wishbone suspension

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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 trail „ Kingpin inclination (normally 7° ~ 8°) „ Scrub radius „ Spring / Shock absorber motion ratios Hard points also affect Handling of the vehicle in dynamic state.

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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.

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Design of suspension components Wishbone – Control arms etc. „ Keep calculations simple. „ Draw Free Body Diagrams for loads on wishbones. „ Design for Stiffness Æ 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 / bushes „ Use 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

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Design of suspension components Coil springs / shock absorbers „ Coil springs over shock absorber designs are easy for tuning, by providing 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.

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Effect of Suspension geometry on handling of vehicle Suspension adjustment

Affect on vehicle handling, limit adjustment

Front Spring rate increase

More understeer

Terminal understeer, front of car hops in corners.

Front spring rate decrease

Less understeer

Oversteers. then understeers as car bottoms excessively with jolting ride

Rear spring rate increase

More oversteer

Too much oversteer, hop in corners, twitchy.

Rear spring rate decrease

Less oversteer

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

More negative camber on front wheels

Less understeer /limit -3 degrees

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

More positive camber on front wheels

More understeer, can make the tyres last longer

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

More negative camber on rear wheels

Less oversteer, more rear grip / limit -3 degrees

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

More positive camber in rear wheels

More oversteer, more forgiving limit

Twitchy in back, tires wear on outer edge.

Symptom of too much adjustment

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Effect of Suspension geometry on handling of vehicle

Suspension adjustment

Affect on vehicle handling, limit adjustment

Symptom of too much adjustment

Toe In front

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

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

Toe In rear

Less likely to oversteer when throttle is lifted

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

Toe Out front

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

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

Toe Out rear

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

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

Positive front caster

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

Can increase understeer, increases steering efforts.

Negative front caster

Not usable

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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 Deshmukh R & D, Mahindra & Mahindra

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