Co-ordinate axes
Forces are exerted on the chassis and therefore on the bodyshell when the vehicle is in motion. This gives rise to self-movements in the bodyshell which can be subdivided into and represented as three categories.
In design terms, a system of coordinates is used which possesses three spatial coordinate axes. Three main movement directions are derived from this system.
- Longitudinal dynamics
- Lateral dynamics
- Vertical dynamics
Longitudinal dynamics
The main movement direction or direction of travel is defined by the x or longitudinal axis of the coordinate system. Driving situations involving longitudinal dynamics, such as accelerating or braking, cause the vehicle to pitch and result in a movement about the y-axis.
Lateral dynamics
Lateral dynamics are said to occur when the direction of movement is along the y or lateral axis, as is the case with steering or swerving.
This causes the vehicle to move about the vertical axis (z-axis). This rotating motion about the vertical axis is called "yawing.
As a side effect of the lateral movement, a rotating motion is also introduced about the x-axis. This so-called "rolling", however, is described later in the section on vertical dynamics since, under these circumstances, the bodyshell moves in a vertical direction.
Vertical dynamics
In the body moves along the z or vertical axis, we speak of vertical dynamics and describe oscillating up and down movements of the body as kangarooing.
A rotating motion about the x-axis, or rolling, is also included in vertical dynamics. It is caused when the suspension of the left and right wheels is compressed to differing degrees.
A triggering factor for this may area of unevenness along one side of the road only. But it may also occur on bends where the centrifugal force created when cornering leads to a rolling movement of the bodyshell. The centrifugal force affects the vehicle's center of gravity thus creating a torque which rotates the bodyshell about x-axis.
Vehicle handling when cornering
A vehicle's handling when cornering is also referred to as its self-steering characteristics. This handling performance is considerably influenced by the changing ratio of lateral force to wheel load on the front and rear axles. Lateral force increases as centrifugal force increases.
Neutral handling
In order for the vehicle to remain stable when driven through bends, the sum of the lateral forces on the wheels must counteract the total centrifugal force exerted on the vehicle.
The slip angles arising as a result of lateral force are the same on the front and rear axles. Neutral cornering facilitates the best use of lateral forces and thereby the highest limit cornering speeds.
It also however, reduces the subjective perception of how close the vehicle already is to the physical limit. The limit referred to here is that of the maximum transferable force (or the sum of the forces). If this limit is exceeded, it is not possible for the driver to calculate in advance whether the vehicle :
- will exit the curve at a tangent
- will experience a front wheel slide out of the corner (understeer)
- lose traction at the rear axle (oversteer)
Understeer
The ratio of lateral force to wheel load is greater at the front axle than at the rear axle. The vehicle follows a larger cornering radius than that corresponding to the steering angle. The driver will have the impression that the vehicle is experiencing a front axle slide towards the outside of the bend. When designing the chassis, this road behavior is often the preferred option, because when the vehicle loses traction, this behavior will produce a straight ahead course which it is possible to calculate.
Take for example a vehicle which begins to break away via the front axle whilst being driven to the limits, if the steering angle is then reduced, the vehicle will recover to assume a straight line course.
Oversteer
The ratio of lateral force to wheel load is greater at the front axle than at the rear axle. The vehicle follows a smaller radius than that corresponding to the steering angle. The vehicle experiences a rear axle "slide" towards the outside of the bend.
Reference parameters