The vehicle’s center of roll is the assumed point at which the cornering forces in the suspension are reacted to the body of the vehicle.
WHAT IS ROLL CENTER?
The roll center is an imaginary, but precisely defined, point on the centerline of a car on which the car rolls on its suspension. The roll center can be high above the ground, low or even below the ground (remember that’s just fantasy). A line connecting the roll center of the rear suspension to the roll center of the front is called the roll axle. If the axle is facing down, the car tends to Oversteer. If the axle runs upwards, the vehicle tends to Understeer. The center of the vehicle’s rollover is where the vehicle will lean (when cornering) when viewed from the front (or rear).
“Roll Center” (RC) is a theoretical point where the frame/body leans/overturns and is determined by the design of the suspension. The front and rear suspension have different roll centers.
There are two definitions of roll center. The most commonly used is the geometric (or kinematic) roll center, while the Society of Automotive Engineers uses a force-based definition.
- The geometric roll center is determined by the suspension geometry and can be found using the principle of instantaneous center of rotation.
- The force-based roll center, according to the US SAE, is “The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll”.
The lateral position of the roll center is usually at the centerline of the vehicle when the suspension on the left and right sides of the vehicle are mirror images of each other.
The importance of roll center can only be appreciated when the mass of the vehicle is also considered. If there is a difference between the position of the center of mass and the center of the roll, a swing arm is created. When the vehicle is subjected to angular velocity due to cornering, the length of the swingarm, combined with the stiffness of the spring and possibly the anti-roll bar, determines how much the vehicle will roll. This also has other effects, such as dynamic transmission.
When the vehicle is tilted the roll centers move. Roll center height has been shown to affect behavior when starting through turns, such as agility and initial roll control.
The mass with which the chassis leans at an angle depends on the position of the roll axis relative to the vehicle’s center of gravity (CG). The closer the roll shaft is to the center of gravity, the less roll the body has at an angle. The body of the wheelchair at one end or the other of the vehicle provides a stronger grip to that end of the vehicle.
The roll center is one of the least used adjustments in cars, but one of the most powerful. This is because the roll center has an immediate effect on the car’s handling, while the anti-roll bars, shock absorbers and springs require the car to tilt before they have an effect.