Roll Centre

A line is projected from the ground contact patch, at the contact point of the Steering Axis Inclination, through the wanted Roll Centre and to the Instant Roll Centre located at the desired Swing Arm Radius (fvsa) length distance from the ground contact patch. From the instant centre one line is projected back to the location of the lower ball joint and one line to the location of the upper ball joint.

image: Roll Centre

fvsa =        tw / 2            =                      tw / 2                                  
1 – roll centre          1-wheel camber angle / roll centre

The length of the lower control arm shall be made as long as possible but is limited by packaging. The length of the upper control arm will determine the curvature of the camber curve. If upper and lower control arms are of the same length the camber curve will be a straight vertical line and if the upper arm is shorter than the upper the curve will be concave toward negative camber which is preferable. The shorter the upper arm is the more concave the camber curve will be. It is possible to design geometry that will have progressive camber in bump with much less in drop.

Having a slightly higher roll centre at the rear has at least two advantages:

  • Softer springs can be used at the rear axle since less rolling moment will appear here.
  • The roll axle is closer to parallel with the cars main inertia axle.

The roll centre height and movement affects the handling qualities of the car. It is generally kept as low as possible

The parameter having the biggest positive influence on the roll centre height and that which raises the roll centre most up from the ground, is the angle of the the lower A-arms in the front view.

Increasing the angle of the lower A-arms in the front view, will increase the lateral movement of the roll centre for the front suspension while decreasing the angle of the upper A−arms in the front view will make the lateral movement of the roll centre smaller.

image: Lateral movement of the Roll Centre

Changes in roll centre height due to roll are often small but lateral movement can be greater.

Making the angle of the upper A-arms in front view smaller will also lower the roll centre.

Adding anti dive will lower the roll centre and decrease the lateral movement of the roll centre.

Adding anti effects will lower the roll centre, but making the upper A-arm angle smaller will instead raise the roll centre.

Instant centre or Instant Roll Centre is the momentary centre which the suspension linkage pivot around. As the suspension moves the instant centre moves due to the changes in the suspension geometry.

Instant centres‘ can be constructed in both the front view and the side view. If the instant centre is viewed in front view a line can be drawn from the instant centre to the centre of the tire’s contact patch. If done for both sides of the car the point of intersection between the lines is the Roll centre of the sprung mass of the car. The position of the roll centre is determined by the location of the instant centres.

High ‘Instant Centres’ will lead to a high roll centre and vice versa. The roll centre establishes the force coupling point between the sprung and the unsprung masses of the car.

When the car corners the centrifugal force acting on the centre of gravity can be translated to the roll centre and down to the tyres where the reactive lateral forces are built up. The higher the roll centre is the smaller the rolling moment around the roll centre is. This rolling moment must be restricted by the springs.

Another factor is the horizontal-vertical coupling effect. If the roll centre is located above the ground the lateral force generated by the tyre generates a moment about the Instant Centre, which pushes the wheel down and lifts the sprung mass. This effect is called ‘Jacking’.

If the roll centre is below the ground level the force will push the sprung mass down. The lateral force will, regarding the position of the roll centre, imply a vertical deflection. If the roll centre passes through the ground level when the car is rolling there will be a change in the movement direction of the sprung mass.

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