Combining the two vertical dotted arrows (red and blue) represents the sum of the vertical force. It is equal on both sides and equal to the original vertical force. However, the vertical force’s proportional split between the spring/damper and the suspension links varies. On the left-hand side of figures 6, 7 and 8, which represent the higher roll centre, more vertical load is applied through the suspension links, resulting in less body roll.
Another way to interpret this is to consider the roll centre as the force coupling point between the suspension and the chassis. Tyre forces transfer to the chassis into this point, while inertial forces apply at the centre of gravity. The distance between these points creates a moment arm, and the difference in force makes a roll moment of this arm. The stiffness of the spring and dampers must resist this roll moment, and the lower the total moment generated, the less the body will roll.
A higher roll centre will reduce the moment arm, and therefore roll moment. Once the roll centre and centre of gravity are at the same height, no kinematic roll occurs. The roll centre helps us understand the application of forces across the vehicle.
Roll centre as a tuning tool
For racing series where suspension adjustment is allowed, the roll centre is one of many tools in the race engineer’s proverbial toolbox when trying to fine-tune vehicle behaviour. However, as the roll centre plays a very complex role in the suspension kinematics and performance, there are many influences one needs to understand before adjusting.
Raising the roll centre can decrease the roll couple generated in a corner and reduce overall body roll. In addition, lowering the moment arm between the roll centre and the centre of gravity will reduce the roll inertia, improving the roll response.
Diverting forces away from the spring and damper and into the suspension links means the suspension arms will experience increased loading. They may need to be sized larger to maintain reliability as a result. In addition, any vertical force on the suspension links pushes the chassis up during a corner which can be an advantage, or a disadvantage, depending on the vehicle’s characteristics.
In a cornering scenario, lateral load transfer is distributed proportionally between the suspension links and the spring/damper units by the height of the roll centre. Load transfer through the spring and damper units is known as elastic load transfer because it is only complete at peak roll. By comparison, the suspension links exhibit geometric load transfer, which happens much faster than elastic load transfer. Adjusting the load on the suspension links is a method of tuning the speed at which load is transferred across the car, altering the dynamic performance and balance.
The inclination of the suspension swing arm influences tyre wear. The higher the roll centre and the more inclination of the swing arm, the more potential side to side movement of the tyre is possible. This will drag the tyre across the asphalt and induce higher temperatures and more wear over a stint.
In summary, though derived from kinematics, the concept of a roll centre goes far beyond the location of suspension linkages. It represents a powerful tool to control the forces and load transfer across the car. This is only a narrow slice of the full picture, as in this article, we explored the roll centre in a 2-dimensional context. In reality, race car suspension is a 3-D system, and one can’t merely consider forces in isolation. The roll centre must be regarded alongside kinematics and compliance, migration of the roll centre, tire flex, and other factors. Hopefully, this information offers insight into the nature of load transfer and can help you strengthen the intuition required to tackle more complex vehicle dynamics problems in the future.