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The vertical force component that acts on the sprung mass through the suspension geometry during cornering. When the roll center is above ground, lateral tire forces create an upward (jacking) force on the chassis.
Carroll Smith's Racing Chassis and Suspension Design defines it precisely: "Jacking force is the vertical component of the force vector formed between the tire contact patch and the roll center." Gillespie's Fundamentals of Vehicle Dynamics explains the mechanism: "a lateral force from the left-side wheel reacting along that line must have an upward (vertical) force component, thus explaining the source of jacking forces inherent to independent suspensions. This suspension geometry is referred to as the positive swing arm because the roll center is located above the ground." The practical effect is that a car with high roll centers will rise vertically during cornering as the lateral tire forces push the chassis upward. This raises the CG dynamically, partially negating the handling benefit of a lower static CG. Lowering the roll center reduces jacking but increases body roll — another fundamental suspension trade-off.