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Introduction

In a racing event, one parameter that is left to be decided last, just before the start of the race is – tyre to be used. This decision is not the only difference between a win or lose but even whether a driver will be able to finish the race or not. When we move out of test track to normal road, though most of us do not decide the tyres, here too they are one of the biggest contributors to our capability on road with a vehicle. During manoeuvring events like acceleration, braking and cornering, forces get transferred to the road through tyres only and successful completion of that event largely depends on tyres capability. In this blog, we will be discussing how tyre parameters affect car handling performance and how this behavior can be objectively measured.

Effect of tyre on car handling

Tyre parameters are inherently linked to each other and thus single parameter impact multiple outcomes on road like ride and handling, noise, braking, etc. Some of the outcomes can be broadly categorized as:

  • Ride and Handling
  • Tire impact on noise- (read blog here)
  • Rolling resistance and its effects- (read blog here)
  • Ride quality we will be discussing in an upcoming blog.
  • Tyre wear

Effect Of Tyre Parameters On Car Handling

We will be focusing on the effect of tyre parameters on car lateral grip i.e. car handling performance. These tyre properties not only affect the handling behavior but also other car parameters.
Tyre Parameters That Affect Vehicle Handling
1. Hysteresis
Tyre-road friction is caused by adhesion and hysteresis. Adhesion is the result of an intermolecular bond between tyre and road. Hysteresis is the property of rubber by virtue of which it absorbs more energy during deformation than the energy released when it returns to its normal state. Hysteresis is the main cause of Rolling friction, also the main factor in lateral grip, especially in wet conditions where the effects of adhesion are heavily reduced.

2. Vertical stiffness
Tyre stiffness plays an important role in ride quality and takes the first impact from the road. Low tyre stiffness is good for ride quality as it absorbs most of the shocks but reduces tire grip which affects car handling behaviour. Finding the right balance in this trade-off is key.

3. Tyre compound
Tyre’s compound determines the friction level between the tyre and road. A softer compound results in the higher grip but is susceptible to higher tyre wear, which decreases the life of the tyre. Choosing the right compound for the right application is important.

4. Contact area
Contact area is defined by the physical dimensions of the tyre and tyre inflation pressure.  Higher contact area is good for lateral grip, tyre wear and durability but increases rolling resistance.

5. Cornering stiffness
When the tyre is under lateral force, it starts slipping laterally giving rise to slip angle. At low slip angles, the relation between the lateral force and slip angle is linear and cornering stiffness is the slope of the plot between the lateral force and slip angle. Higher cornering stiffness is good for car handling.

Evaluating the effect of tyre on car handling

Some tests which help us in evaluating objectively the effect of tyre on car handling performance are listed below-
Tyre Testing Methods And Impact Of Tyres On Vehicle Handling Performance

Understanding the effect of tyre parameters on test outputs

The below table summarises how change in tyre parameters changes a car’s handling behaviour and how they are reflected in the test results.
How Variation In Tyre Parameters Affects Test Outputs
Let’s discuss some test outputs in details to understand how vehicle level results are impacted with tyre parameter-
1. Understeer Gradient
Understeer gradient represents how much the steering wheel should be turned to keep the car in the same path for every unit change in lateral acceleration. Positive slope represents understeer and negative slope represents oversteer. Increase in lateral grip make the vehicle understeer/oversteer less, this is reflected as the decrease in slope.

Understeer Gradient- Vehicle Dynamics
Figure 1 - Understeer Gradient for an understeering and an oversteering vehicle from steady-state circular pad test

2.  G-G Diagram
G-G diagram represents a vehicle’s acceleration limits, both in the lateral and longitudinal direction. Increase in lateral grip translates as the increase in limits of the diagram, which increases the overall area of the graph.

The GG Diagram
Figure 2 - G-G diagram

3. Torque Steer
Torque Steer is the steering effort a driver has to use to turn the vehicle. Torque Steer is increased due to the increase in self-aligning torque. Self-aligning torque increases when there is an increase in lateral grip or due to increase in pneumatic trial resulting from an increase in the contact area. The graph below represents the change in Torque Steer with Lateral Acceleration during a steady state circular pad test.

Steering Torque Against Lateral Acceleration
Figure 3 - Variation of torque steer with lateral acceleration in a circular pad test

4. Steering Response
Steering response is defined as a time required for the car to respond to any steering input. The graph below shows steering wheel angle, yaw rate and lateral acceleration during a double lane change manoeuvre. The phase lag of lateral acceleration and yaw rate with respect to steering angle represents the steering response.

Wheel Steering Response
Figure 4 - Steering response from a double lane change test.

 

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Prajwal P

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