The roll center of a

vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), ...

is the notional point at which the cornering forces in the suspension are reacted to the vehicle body. There are two definitions of roll center. The most commonly used is the geometric (or kinematic) roll center, whereas the Society of Automotive Engineers uses a

force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...

-based definition.An Introduction to Modern Vehicle Design edited by Julian Happian-Smith P293

Definition

* Geometric roll center is solely dictated by the suspension geometry, and can be found using principles of the instant center of rotation. * Force based roll center, according to the US Society of Automotive Engineers, 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 location of the roll center is typically at the center-line of the vehicle when the suspension on the left and right sides of the car are mirror images of each other. The significance of the roll center can only be appreciated when the vehicle's center of mass is also considered. If there is a difference between the position of the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

and the roll center a moment arm is created. When the vehicle experiences

angular velocity In physics, angular velocity or rotational velocity ( or ), also known as angular frequency vector,(UP1) is a pseudovector representation of how fast the angular position or orientation of an object changes with time (i.e. how quickly an object ...

due to cornering, the length of the moment arm, combined with the stiffness of the springs and possibly anti-roll bars (also called 'anti-sway bar'), defines how much the vehicle will roll. This has other effects too, such as dynamic load transfer.

Application

When the vehicle rolls the roll centers migrate. The roll center height has been shown to affect behavior at the initiation of turns such as nimbleness and initial roll control.

Testing methods

Current methods of analyzing individual wheel instant centers have yielded more intuitive results of the effects of non-rolling weight transfer effects. This type of analysis is better known as the lateral-anti method. This is where one takes the individual instant center locations of each corner of the car and then calculates the resultant vertical reaction

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due to lateral force. This value then is taken into account in the calculation of a jacking force and lateral weight transfer. This method works particularly well in circumstances where there are asymmetries in left to right suspension geometry. The practical equivalent of the above is to push laterally at the tire contact patch and measure the ratio of the change in vertical load to the horizontal force.

References

{{carDesign nav Classical mechanics Geometric centers Vehicle technology

Definition

Application

Testing methods

See also

References