In the coilover, the spring...
In the coilover, the spring angle is nearly zero and insignificant and the motion ratio is found by dividing B by C. This number is squared and multiplied times the installed spring rate to find the wheel rate.
No matter what the mounting design is, the end result is that the sprung portion of the car over the straight axle suspension is riding on two springs. The roll angle amount for this suspension is determined by 1) the magnitude of the g-force, 2) the height of the CG, 3) the combined spring rates, 4) the height of the rear moment center, 5) the width of the spring base, 6) the difference in spring rates, if different.
1. Same as in AA suspension.
2. Same as in AA suspension.
3. We use the installed spring rate corrected for spring angle and in the case of a spring mounted on a trailing arm, factoring in the motion ratio.
4. The height of the rear MC is usually the average height of the ends of the Panhard/J-bar, in some cases the height of the metric four-link MC, or the height of the Watt's link MC. Width of the MC has no effect on roll angle, but does have some effect on weight jacking as does bar angle, but we won't get into that here.
5. The spring base is the width of the top mounting points of the springs. In a leaf-spring car, it's the width measured to the center of the two leaves.
In a straight axle car with...
In a straight axle car with the rear springs mounted on the trailing arms, there is a motion ratio involved. To find the spring rate that the car will feel, you multiply the motion ratio squared times the installed spring rate. The MR is B divided by C. As in the front coilover sketch explanation, if there is a spring angle relative to the swing arm, then you would square the Cos of that angle (difference from 90 degrees off the arm) times the MR times the installed spring rate to find the true spring rate that the car feels.
6. Spring split has a significant effect. If we install different rate springs in a solid axle suspension, our roll angle will be influenced quite a bit. If the outside spring is softer, the roll angle will increase. If the inside spring were softer, the roll angle will decrease over that of a system with equal spring rates.
Roll Angle Comparison
Once we grasp the influences of the different suspension systems and how each part plays a role in creating and determining the amount of chassis roll, we can now proceed on to looking at the entire car and how all of that affects our setup.
Each axle, or to better explain it, each pair of tires, front and rear, are the points ultimately that resists the centrifugal force that tries to take the car to the outside of the turns. As such, for proper analysis, the suspensions at each end and the sprung weights at each end, should be combined into two separate systems or vehicles unto themselves.
Imagine that the race car were cut in half, sideways down the middle in a line through the CG perpendicular to the centerline. And imagine that we could create a swivel where the ends of the car would rotate so that the front and rear could roll free of influence from the other end.
The truth of the matter is that when we install springs and control arms and J-bars, we're creating a system that will ultimately want to roll to a certain angle. What we create, or have for a front suspension and spring rates will result in its very own roll angle while going through the turns at our local track.
What we have for a rear suspension and corresponding spring rates will determine our rear roll angle in this imaginary cut up car. If we could visualize this car going around the racetrack, we might see where the car looks normal through the turns with no apparent distortion and this we can now call a balanced setup. Both suspension ends are rolling to the same angle, the two halves are inline just like when the car is parked and everything is working well.