Mick: Roll centers are independent of cg. There's a roll center at the front and one at the rear.
On an independent SLA type front suspension such as used on all Cobras, it's found graphically by extending a line inboard from the ball joint of the upper control arm through its pivot and beyond, then doing the same for the lower arm. Where these two lines intersect is called the Instant Center. A line is then drawn back from the IC to the center of the tire contact patch with the ground. Where this line crosses the center of the vehicle is the suspension's roll center.
The same is done with an SLA type IRS such as the Jag setup. Typically the front rc is lower than the rear.
By the way, the cg isn't theoretical, it's real. You just can't see it. Same with roll centers. They are points in space. What's theoretical with the cg is it's a point where all the weight of the car could be located to provide the same front/rear weight distribution and weight transfer during cornering, braking and accelerating. But, then there's polar moment . . . another can of worms.
