Roll

Roll Motion is when the vehicle’s chassis rotates around the Roll Axis. The Roll Axis is defined by the suspension geometry and can move as the suspension articulates. Positive Roll is when the chassis rolls to the right (when viewed from the behind of the car) while negative roll is when the chassis rolls to the left.

Instant Center

Instant Center (IC) is defined as the imaginary 2D pivot point of the suspension linkage, when viewed from the front of the vehicle. In a double A-arm suspension setup for example, the IC is the point of intersection of the projected upper and lower suspension arm. The term “instantaneous” means that the point is valid for a particular position of the suspension linkage, but is subject to displacement along with the movement of the suspension.

Roll Center

The 2D kinematic Roll Center (RC) is defined as the instantaneous center of rotation of the chassis about the ground. It is formed by the intersection of the lines connecting the contact patch of the tire to the IC of the opposite side. Similar to ICs, the RC is also dynamic and varies its position based on the movement of the suspension linkages.

Roll Axis

The Roll Axis is the axis or line which connects the Roll Centers of the front and rear suspension. The Roll Axis is the more appropriate method of determining the chassis roll characteristics of a 3D suspension model.

If both front and rear suspension geometries are selected for analysis, OptimumK performs all of the roll calculations by pivoting the chassis on the roll axis. OptimumK calculates both front and rear roll centers simultaneously when performing the chassis motion calculations. This results in a more accurate representation of vehicle dynamics characteristics.

The example below highlights the importance of using the roll axis to define the roll characteristics of the chassis:

The image on the left shows a vehicle which is at static ride height. The roll axis and pitch axis are defined by the blue and red line respectively. The front and rear roll center heights are equal and located on the centerline of the car. Hence, the roll axis is parallel to both the vehicle longitudinal axis and the ground plane. For small values of pure chassis roll, the chassis will follow a cylindrical rotational motion about the roll axis.

However, when subject to multiple chassis motions comprising of bump, heave and roll, it is observed that the roll axis is skewed relatively large, as shown in the image on the left. In such a situation, the chassis will assume a conical rotational motion about the roll axis. This kind of motion can only be deduced when the complete 3D suspension geometry is analyzed simultaneously.

The image below shows the skewed roll axis in plan view: