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Originally Posted by Dean
But what is roll? I beleive roll is the CG applying it's force/momentum through the leverage of the susension all the way to the tire contact causing the body mass to rotate. Now you have rotating mass, in addition to the momentum of the CG when you run out of suspension travel.
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Your definition was pretty close. The sprung mass (body/chassis) rolls simply because the instantaneous axis that it rotates about (the vehicle roll axis, defined by drawing a line between the front and rear roll centers - all of which is defined by the suspension geometry) is typically below the C.G. of the sprung mass. Imagine an upright pendulum with a pivot at the bottom; if you grab that bottom pivot and accelerate it to the side, the pendulum is going to lag behind and lean over to the opposite side. When the vehicle is in a turn, the "centrifugal" force (due to the lateral acceleration) acting on the C.G. of the sprung mass causes a torque about the roll axis, which is typically called the vehicle's Roll Couple.
The amount of body roll present for any amount of lateral acceleration is dictated by the value of the roll couple, and the roll stiffness of the springs and swaybars which resists the couple.
This might lead someone to wonder why we don't just design the suspension such that the roll axis passes right through the C.G. of the sprung mass? That way there would be zero roll. Well, since the C.G. is typically fairly high, probably somewhere between the engine block midpoint and the midpoint of the chassis, there are some unpleasant side effects that result from having high suspension roll centers (and therefore a high roll axis). Jacking is the most noticeable; the higher the roll center of an independant suspension is, the more lifting force is created on the chassis during cornering; that's why many racecars have suspension roll centers that are pretty close to the ground.