Under the influence of the exterior disturbances with regard to the given choice of course, the driver-Vehicle system should remain stable. The driver as a control mechanism has limited possibilities to intervene. Modern electronic system such as EBS and ESP are able to stabilise many critical and unstable driving situations by means of specific intervention. E.g. on individual wheels.
Driving characteristics refer to properties of the vehicle. Good driving characteristics are
– When there is a sensible relationship between steer angle and change in course.
– When the driver is provided with useful information (e.g.Tyre noise) on the state of movement of the vehicle.
– When outside disturbances (e.g. Cross wind) only lead to slight course deviations.
– When the achievable cornering speed and transversal acceleration are high with regard to driving safety and performance.
During cornering if the external forces like cross wind and centrifugal forces affect the vehicle, static friction in the tyre contact area leads to oppose reaction forces. These cornering forces have the objective of keep the tyre on the rolling direction and thus keeping the vehicle moving. However the tyre deforms under the exterior forces and the Static friction is limited tyres then always rolls at a tyre slip angle.
The level of cornering forces Fs depend on following conditions.
– Cornering forces rises with tyre slip angle.
– Cornering forces rises with the wheel load.
– Cornering forces rises with air pressure up to a maximum and then falls again.
– Cornering forces rise with the co efficient of static friction of the road surface.
– The tyre and the tread of the tyre also influence the cornering forces Fs.
LEGEND: Fs – Cornering force , GR- Wheel load , α – Tyre slip angle.
For driving stability, the exterior forces and cornering forces built by the tyres should be in equilibrium at all times while the vehicle in motion. Other wise unstable and uncontrolled vehicle movement will occur. In order to prevent this ESP (Electronic Stability Programme) has been incorporated in the design.
The transverse movements and Yawning are of particularly significance in relation to transfer dynamics. As rolling in curves leads to drastic changes in wheel load and thus has a great effect on the cornering forces, this vehicle movement is also viewed from the perspective of transfers dynamics.
Next session : Transfers dynamics and Cornering ability.