Slide 1

STEADY-STATE HANDLING CHARACTERISTICS OF A TWO AXLE VEHICLE

Theory of ground vehicles 369

1

The slip angles f and r, are dependent on the side forces acting on the tires and their cornering stiffness

Within a certain range, the slip angle and cornering force may be considered to be linearly related with a constant cornering stiffness

Dependent on the values of the understeer coefficient Kus the steady-state handling characteristics may be classified into three categories

neutral steer,

understeer,

oversteer

Kus = 0

Kus > 0

Kus < 0

Neutral SteerWhen the understeer coefficient Kus = 0 which is equivalent to the slip angles of the front and rear tires being equal.

When a neutral steer vehicle originally moving along a straight line is subjected to a side force acting at the center of gravity, equal slip angles will be developed at the front and rear tires. As a result, the vehicle follows a straight path at an angle to the original.

UndersteerWhen the understeer coefficient Kus > 0, which is equivalent to the slip angle of the front tire being greater than that of the rear tire.

Oversteer

When the understeer coefficient Kus < 0, which is equivalent to the slip angle of the front tire being less than that of the rear tire

Relationships between steer angle and speed of neutral steer, understeer, and oversteer vehicles.

Curvature responses of neutral steer, understeer, and oversteer vehicles at a fixed steer angle.

Directional responses of neutral steer, understeer, and oversteer vehicles to a side force at the center of gravity.

Variation of turning radius with lateral acceleration of a four-wheel-drive car

Variation of understeer coefficient with lateral acceleration of various types of car

I -a conventional front engine/rear-wheel-drive car;

2-a European front engine/front-wheel-drive car;

3-a European rear engine/rear-wheel-drive car;

4-an American rear engine/rear-wheel-drive car.

A passenger car has a weight of 20.105 kN (4520 Ib) and a wheelbase of 2.8 m (9 ft, 2 in.). The weight distribution on the front axle is 53.5%, and that on the rear axle is 46.5% under static conditions.

a) If the cornering stiffness of each of the front tires is 38.92 kN1rad (8750 lb/rad) and that of the rear tires is 38.25 kN/rad (8600 Ib/rad), determine the steady-state handling behavior of the vehicle.

b) If the front tires are replaced by a pair of radial-ply tires, each of which has a cornering stiffness of 47.82 kN/rad (10,750 lb/rad), and the rear tires remain unchanged, determine the steady-state handling behaviour of the vehicle under these circumstances.

STEADY-STATE RESPONSE TO STEERING INPUT

Yaw Velocity Response

It is defined as the ratio of the steady-state yaw velocity to the steer angle. Yaw velocity R, of the vehicle under steady-state conditions is the ratio of the forward speed V to the turning radius R.

From the point of view of handling response to steering input, an oversteer vehicle is more sensitive than a neutral steer one.

Yaw velocity gain characteristics of neutral steer, understeer, and oversteer vehicles.

Lateral Acceleration Response

It is defined as the ratio of the steady-state lateral acceleration to the steer angle, is another commonly used parameter for evaluating the steering response of a vehicle.

For an oversteer vehicle, the value of the understeer coefficient is negative. The lateral acceleration gain increases with an increase of forward speed at an increasing rate, as the denominator decreases with an increase of speed.

Lateral acceleration gain characteristics of neutral steer, understeer, and oversteer vehicles.

The ratio of the steady-state curvature 1/R to the steer angle is another parameter commonly used for evaluating the response characteristics of a vehicle.

Curvature Response

Curvature responses of neutral steer,understeer, and oversteer vehicles.

TESTING OF HANDLING CHARACTERISTICS

Constant Radius Test

The steer angle or the angle of the steering wheel required to maintain the vehicle on course at various forward speeds together with the corresponding lateral acceleration are measured

The handling behaviour of the vehicle can then be determined from the slope of the steer angle-lateral acceleration curve.

If the steer angle required to maintain the vehicle on a constant radius turn is the same for all forward speeds the vehicle is neutral steer. The vehicle is considered to be understeer when the slope of the steer angle lateral acceleration curve is positive.

The vehicle is considered to be oversteer when the slope of the curve is negative.

Constant Speed Test

In this test, the vehicle is driven at a constant forward speed at various turning radii. The steer angle and the lateral acceleration are measured.

If the vehicle is neutral steer, the slope of the steer angle-lateral acceleration line will be a constant of gLIV2 .

The vehicle is considered to be understeer when the slope of the steer angle-lateral acceleration curve is greater than that for the neutral steer response at a given forward speed. The vehicle is considered to be oversteer when the slope of the curve is less than that for the neutral steer response at a given forward speed (i.e., gLIV2).

Assessment of handling characteristics by constant speed test.

Constant Steer Angle Test

In this test, the vehicle is driven with a fixed steering wheel angle at various forward speeds. The lateral accelerations at various speeds are measured.

If the vehicle is neutral steer, the value of the understeer coefficient will be zero, and the slope of the curvature-lateral acceleration curve is zero.

The vehicle is considered to be understeer when the slope of the curvature lateral acceleration curve is negative, which indicates that the value of the understeer coefficient is positive.

In general, the constant radius test is the simplest and requires little instrumentation. The steer angle of the front tire and forward speed are the only essential parameters to be measured during the test.

The constant speed test is more representative of the actual road behaviour of a vehicle than the constant radius Test.

The constant steer angle test, on the other hand, is easy to execute.