Importance of A Vehicle’s Roll Centre Explained

To understand the roll centre, let's start with an example. Consider a car is travelling at 120 km/hr on a highway and wants to take a u-turn, so the driver will gradually reduce the speed and then take a turn.

If the driver turns the vehicle without reducing the speed, imagine what will happen  OMG! ><, definitely there will be a rollover, it is an exaggerated case.

But think of normal turning, in this case, as well, there will be a roll due to the lateral force and Cg shift, but it will be within the limit. To optimise this cornering behaviour of the vehicle, we can play with the roll axis and Center of gravity. Anti-roll bar, spring and shock will also play a major role in reducing roll, but changing the roll centre position with respect to Cg has an effective result. Since it determines the magnitude of the rolling moment.

 

Roll Axis:

The roll axis is the axis about which the body rolls, it is obtained by joining the front roll centre and rear roll centre.

 

Roll Centre:

The roll centre is the virtual point where the entire body rolls while cornering. The roll centre position can be adjusted by changing the front and rear suspension linkages.

 

Roll Angle:

                 Roll angle measures the angle at which the body rolls while cornering. It is denoted as ‘φ’.

 

Roll gradient:

                     Roll gradient measures the vehicle body roll angle concerning the lateral acceleration at the centre of gravity. The unit of roll gradient is deg/g.

 

The following are the ways to find the roll centre,

 

During Positive Swing arm Geometry:

 

During Negative Swing arm Geometry:

 

Rolling Moment:

              The rolling moment will roll the vehicle about the x-axis due to the load transfer at Cg due to cornering.

       

harm is the vertical distance between the Cg point and the roll axis.

 

Non-Rolling moment

                 The non-rolling moment is due to the reaction force at the tire contact patch.

 

Non-rolling moment = Fy * RCH

 

• Fy    ⇒ Lateral force
• RCH ⇒ Roll centre height from the ground

 

Optimal Condition:

      To get better performance and avoid roll-over, the Cg height should be as minimum as possible, and the roll centre should be between the ground and Cg. 

 

Effects of Roll Centre Positions

Case-1: RC at Cg

                 Since there is no h_arm value, there will be no rolling moment. There will be only a non-rolling moment, hence there will be no rolling effect. At this condition, all the lateral force at the Cg point will be taken by the suspension links. 

Case-2: RC below Ground

                  Normally during cornering, the outer wheel will be loaded and the inner wheel will be unloaded, this is called the Jacking effect. Higher the roll centre there will be more jacking.

If the roll centre is positioned below ground level, it will result in an inverse jacking effect. In this case, the inner wheel will be loaded and the outer wheel will be unloaded.          

Case-3: RC at  Ground

                If the roll centre is at the ground level, there will be zero non-rolling moments; there will be only a rolling moment. At this condition, suspension links will not take up the lateral load transfer, hence the rolling effect will be aggressive.

 

                 To get a good performance car, it is essential to consider all the aspects and to get an optimal solution. The roll centre is based on the suspension geometry, also the anti- characteristics depend on the suspension geometry. Usually, suspension geometry will be fixed first at the design stage. To tune the rolling effect we can also play with the stiffness of springs, dampers and Antiroll bars.