suspension design of a off road vehicle...by rajeev mokashi.

1. Suspension Design notes
BAJA SAEINDIA 2011 Workshop
23rd July 2010
By Rajeev Mokashi

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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

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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
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
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)
• 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

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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 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
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 adjustment
Affect on vehicle handling,
limit adjustment
Suspension adjustment

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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 adjustment
Affect on vehicle handling,
limit adjustment
Suspension
adjustment

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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