In this presentation the stiff spring and soft spring effects on vehicle dynamics is presented

1. Influence of suspension stiffness
By
Intakhab Khan (VTP2984)
Under the guidance of
Dr. Padmanabhan S
Vehicle mechanics
2161AU117

2. Table of contents
1. Introduction
2. Spring and Spring stiffness
3. Effect of stiffness on comfort
4. Effect of stiffness on Road holding capability
5. Effect of stiffness on Suspension travel
6. Dynamic stability and stiffness
7. Soft Spring and Stiff Spring
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Introduction
• Suspension system is the system consisting springs, shock absorbers and having linkages
which is used to isolate sprung mass from the road shocks, vibrations and provide stability.
They consist of spring and damper. Spring takes the vibrations and dampers reduces the
amplitude of vibration and damp the vibration. The stability, ride comfort and suspension
travel depends on the suspension stiffness, masses and damping factor.
• Main functions of suspension system are
• To isolate body from road shocks
• To support the weight of the vehicle
• To provide smooth ride
• Allows rapid cornering without extreme body roll
• It helps to maintain the contact between road and tyre.
• Prevent squat during acceleration and dive during braking
• Gives particular height to vehicle body
• Maintains proper steering geometry

4. Springs in suspension system
• Amount of force required to compress or extend the spring
by unit length is known as spring rate.
• It’s SI unit is N/m and is denoted by S or K
• It is also defined by ability of spring to resist a force
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Spring rate(Spring stiffness)
Coil springs Leaf springs Torsion springs Air springs
Rubber springs

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How Spring Stiffness is important parameter in Vehicle Dynamics
and ride comfort
Parameters varied
due to K
Comfort Stability of vehicle

6. Equivalent suspension system(Quarter car model)
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7. Ride comfort and Spring stiffness
• We call the ride as comfortable when the transmissibility of vibrations of road is
minimum and have the cushioning ride where the suspension travel takes the
irregularity of surface.
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• From this curve it is clear that if the spring is softer
the transmissibility ratio is observed less hence soft
spring provide better isolation from vibration and
comfortable ride.

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Stiff spring

10. Road Holding capability and Spring stiffness
• The suspension takes up the irregularity and damper dampens the vibration but
the spring must have stiffness in such a manner that it won’t leave the road in
bumps or else the vehicle will loose stability.
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• From this curve it is clear that softer spring have
poor road holding capability at high frequency or
speed hence race cars have stiffer spring

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12. Suspension travel and Spring stiffness
• While hitting a bump or irregularity the spring deforms up to certain extent
depending on spring stiffness and frequency.
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• From this curve it is clear that softer spring have
larger suspension travel at frequency near to natural
frequency due to which the vehicle with soft spring
must have greater clearance for travel of wheel
which leads to rise in centre of gravity height and
hence harder less stable while cornering.

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14. Dynamics dependence on spring stiffness
Stiffer the tire spring is more amount of weight will be transferred to that tyre in
turns
So softer spring at front right and left rear makes vehicle understeer
Softer spring in left front and right rear makes the vehicle over steer
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Understeer Understeer
Oversteer Oversteer

15. Soft spring
• Max grip and more contact on road in rough road implies it will be safe and
better handling
• More suspension travel while force from road irregularity
• More compression under vehicle weight
• Ride height changes
• Used for commercial vehicle
• Comfortable ride and cushioning ride due to less transmissibility
• Cornering, weight transfer and steering ability performance is bad for softer
spring compared to stiff springs
• Less control at higher speed due to higher COG
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16. Stiff Spring
• Low COG can be achieved, no stability issue in plain road or cornering
• Constant ride height
• Less rolling moment, less pitching while braking
• Gives lower clearance(due to less Suspension travel) and also better aerodynamic
results
• Contact is less while in potholes or sudden depression
• Losses contact while in sudden bumps since more force is applied to the vehicle
without compression
• Usually used for race cars
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