The rear suspension is much...
The rear suspension is much different than the front suspension. The car "feels" the spring base at the top of the springs. The moment center resists the lateral centrifugal force that causes the suspension to roll. Changing the MC height is the primary method used to regulate rear roll.
The magnitude of lateral force and the length of the effective moment arm are contributing factors that help us predict exactly what roll angle the front of our car desires. If the front end of our car was not rigidly connected to the rear, it would roll to a predicted angle and a predictable amount of load would transfer from the left front tire onto the right front tire. But that is not the case. The two are connected and unless we know for sure what each one's desires are, we cannot accurately predict the load transfer at each end of the car, nor can we create a balanced setup.
Both suspension systems of the car are connected by a semi-rigid chassis, however stiff this connection may be. What each system desires to do is influenced by what the other wants to do as the car negotiates the turns. The two ends must work together in harmony in order for everything to work correctly. I have used an analogy in the past that helps to explain this concept.
Have you ever seen a circus act where two people are in a horse suit? The horse moves around fine as long as each end is in sync with the other. When the rear wants to go left and the front wants to go right, it gets comical. In our stock car, when the front moves/rolls differently than the rear, it is not so funny and performance suffers.
In past eras of stock car racing setup technology, the rear usually desired to roll more so than the front. In the more modern setups with soft front springs, large diameter sway bars, and high spring rates in the right rear corner, the front may actually want to roll more than the rear. We can now easily achieve a negative roll angle in the rear with a high enough right rear spring rate and a high rear moment center. This still represents an unbalanced state.
The rear moment center height...
The rear moment center height of a Panhard/J-bar system is the average height of the two ends of the bar. For the purpose of roll-angle prediction, the MC is located laterally halfway between the top of the springs, regardless of where the bar is mounted.
There are seven primary components that combine to influence the amount of roll angle in the front suspension. They are:
1.The weight of the sprung mass of the car supported by the front suspension. This is represented by the weight of the front end measured on the scales under the left front and right front tires minus the unsprung weight of the wheels, tires, brakes, and so on.
2. The height of the center of gravity of the sprung mass.
3. The magnitude of the lateral force measured in g's. One g lateral force would equal the sprung weight of the front end.
4. The dynamic moment center location, both in height and width measured with the attitude of the car as it dives and rolls in the turns.
5. The overall spring stiffness translated to wheel rate as well as the relationship of the two spring rates side to side (i.e., softer right front vs. left front spring).
6. The front sway bar has an effect of antiroll and must be taken into account. The larger the bar, obviously the more resistance to roll.
7. The track banking angle has an influence due to the dynamic downforce when the direction of the resultant force lies between the front tires.
The leaf-spring type of rear...
The leaf-spring type of rear locating device is unique in that the top of the springs are also the lateral locating devices. Therefore, the average heights of the ends are the height of the rear moment center. This system provides a wide spring base and is superior to other suspension systems in some cases.
The rear suspension is a much different system than the front and we look at it much differently. At the front, the spring base is felt by the chassis at the wheels, but a rear solid-axle system (and this relates also to a front straight-axle suspension) has a spring base that is felt on the top of the springs. We should never translate spring rate out to the wheel at the rear. This dynamic model is not a new concept, but was developed and published over 60 years ago. I first read about it while doing research in the Virginia Tech engineering library in 1995, and it has proven to be true beyond a doubt.
The rear suspension model has the center of gravity of the sprung mass of the car representing the top of the moment arm just like at the front. The bottom of the moment arm is the moment center created by the lateral locating device known to us by the terms Panhard/J-bar, metric 4-link, leaf springs, or Watts link. These four devices comprise the majority of lateral restraint systems used for straight-axle suspensions in stock cars. Each restraint system has its own moment center that represents the bottom of the rear moment arm.
The height of the rear moment center is fairly easy to determine, but there have been varying theories on both the height and lateral location. Again, going back to early writings on the subject, the car "feels" the rear moment center laterally halfway between the two springs. Subsequent experiments have proven that regardless of the lateral location of the locating devices, the rolling force remains the same, thus proving the early publications.