VEHICLE BODY ROLL AND
6.2.3 CAMBER CHANGE AND ROLL STEER
If the ground contact points of the wheels are fixed, as the vehicle body rolls, the unsprung mass, including the wheels, tilts relative to the ground. This gives the camber change of the wheel, which is measured relative to the ground and is due to body roll. The vehicle roll also gives the wheels an up-and-down displacement relative to the vehicle body. At such time, depending on the structure of the suspension system, the wheels may produce some angular displacement in the horizontal plane along with the up-and-down movement relative to the vehicle body. This is called roll steer.
The camber change and roll steer are dependent on the structure of the suspension system.
The suspension system is designed with keen consideration of these characteristics, often using them to affect the vehicle dynamics or sometimes trying to avoid them completely. This chapter will skip the detailed explanation of camber change and roll steer mechanism for various suspension systems, and it will only look at the basic characteristics of camber change and roll steer. The collective term for camber change and roll steer is sometimes called alignment change due to roll.
In axle-type suspensions, the wheel does not produce any camber change due to vehicle roll.
The camber change due to roll only occurs for independent suspension systems, where depending on the suspension structure, there could be one of two cases: camber change in the same direction as roll, which is called positive camber, or in the opposite direction, negative camber. This is shown inFigure 6.8.
Independent suspension systems are constructed by a linkage mechanism, and the vehicle roll angle and camber change can be determined from geometric analysis of the linkage.
Figure 6.9shows the actual measured value and calculated value of the camber change for a wishbone-type suspension system. This relationship varies substantially with the arrangement of the links, even for suspension systems of the same type. From the figure, if the roll angle is not large, the camber change can be considered as nearly proportional to the roll angle. As the roll angle becomes large, this linear relation is lost, and nonlinearity appears. This is generally true for other types of suspension systems. The nonlinear characteristic of the camber change is one of the main factors that influences the vehicle motion at large lateral accelerations. For
lower lateral accelerations, the camber change could be considered to be proportional to the roll angle, in either direction shown inFigure 6.8.
Example 6.1
Investigate the geometric condition of the suspension mechanism, such as inFigures 6.2 or 6.3, which basically determines the positive or negative camber to the body roll.
Solution
Any part of the left half of the vehicle body is going downward and the right half upward due to the positive body roll. The unsprung mass rolls around the tire contact point, for example point A in Figure 6.2, which is a camber change. So any point of the left side of the unsprung mass relative to the centerline of the left wheel is going downward and the right side goes upward due to the positive camber of the unsprung mass. Due to the negative camber, the left side is going upward and the right side is going downward.
measured calculated bump (mm) rebound (mm)
camber change (deg)
FIGURE 6.9
Chamber changes to suspension stroke.
positive camber negative camber FIGURE 6.8
Camber change due to body roll.
As the virtual point, O1, is the common point fixed to the vehicle body and the left-side unsprung mass, it must move in the same direction as the body roll and the camber change during roll motion. If point O1is to the left of the body center and to the right of the wheel centerline, then the suspension has negative camber. If point O1is to the left of the body center and to the left of the wheel centerline, or to the right of the body center and the wheel centerline, then the suspension has to show the positive camber.
The roll steer due to the roll of the vehicle body is also dependent on the suspension system structure. For an axle-type suspension, the sprung mass and unsprung mass are often connected using leaf springs. The mounting point of the spring at the vehicle axle moves in the rear-and- forward direction and causes the axle to produce an angular displacement relative to the vehicle body in the horizontal plane. This is roll steer for an axle-type suspension, which is sometimes called axle steer due to roll.
For independent suspension systems, the amount of roll steer can be determined from geometric analysis of the linkage. Similar to camber change, the roll steer direction and magnitude, relative to the roll angle, can vary substantially with the arrangement of the links.
The suspension systems are usually designed to control the amount of steer by careful arrangement of the links. For independent suspensions, if the roll angle is small, the roll steer can be considered to be proportional to the roll angle. The direction of the roll steer can sometimes be in either the positive or negative direction, depending on the suspension system structure.
The roll steer for an independent-type suspension is sometimes called the toe change due to the vertical stroke of the suspension.Figure 6.10shows an example of toe change to suspension stroke. Roll steer in the direction toward the inner side of the vehicle is called toe-in, and that in the opposite direction is called toe-out.
toe in (deg) toe out (deg)
bump (mm)
rebound (mm)
FIGURE 6.10
Roll steer (toe change to suspension stroke).