Roll center, roll axis, and their relationship with a car’s center of gravity (CG) are fundamental concepts in vehicle dynamics, particularly in the context of radio-controlled (RC) cars. Understanding these concepts is crucial for optimizing handling characteristics and overall performance.

Roll Center and Roll Axis: Basics

Roll Center:The roll center is an imaginary point around which the car's chassis rolls in a corner. It's not a fixed point but changes based on the suspension geometry and the dynamics of the car.

Roll Axis:The roll axis is an imaginary line connecting the front and rear roll centers. This axis represents the pivot point around which the car rolls during cornering.

Relationship with Center-of-Gravity (CG)

The CG is the point where the weight of the car is considered to be concentrated. The vertical distance between the CG and the roll axis significantly influences how a car behaves in corners.

• Lower CG: Enhances stability and reduces the tendency to roll.
• Higher CG: Increases sensitivity to roll, potentially leading to a loss of grip in extreme cases.

Impact on RC Car Handling

The positioning of the roll center affects the car’s handling:

• Lower Roll Center: Increases body roll, which can improve grip on loose surfaces by increasing weight transfer. However, it can also make the car feel more sluggish in response to steering inputs.
• Higher Roll Center: Reduces body roll, leading to a stiffer feel and quicker response. This is beneficial on high-grip surfaces but can reduce overall grip on loose surfaces.

Determining Roll Center

Calculating the roll center involves analyzing the suspension geometry. This includes looking at the angles and lengths of control arms, the positioning of the suspension joints, and the geometry of the steering mechanism.

Adjusting Roll Center in RC Cars

Adjustment methods vary based on the suspension design:

1. C-Hub Design:Adjustment is typically achieved by changing the height of the suspension arms or adjusting the angle and position of the C-hubs.
   Shims or spacers are often used to raise or lower the arms, thereby affecting the roll center.
   
2. Pivot Ball Design:Involves adjusting the pivot balls that connect the suspension arms to the chassis.
   Altering the height or angle of these pivot points changes the suspension geometry and hence the roll center.
   

Cause and Effect

Adjusting the roll center has a domino effect on other settings:

• Suspension Stiffness: A higher roll center may necessitate softer springs to compensate for reduced body roll.
• Dampers: Adjustments might be needed to control the rate of weight transfer.
• Camber and Toe Settings: As the roll center changes, so does the dynamic alignment of the wheels.

Conclusion

In conclusion, understanding and adjusting the roll center and roll axis in relation to the CG is vital in RC car tuning. It's a balancing act that requires considering various factors, including the type of surface and the desired handling characteristics. Effective adjustments can lead to significant improvements in performance, making the RC car more responsive and better suited to specific racing conditions.