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 and at a height that is the average height of the two ends.
Both suspension systems of the car are connected by a rigid chassis, however stiff this connection may be, and what each desires to do is influenced by what the other wants to do. I have used an analogy in the past that helps to explain this concept. Have you ever seen a circus act in which two people are in a horse suit? The horse moves around well, 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.
There are six primary factors influencing the amount of roll angle in the front suspension. They are as follows: 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, and so on; 2) The magnitude of the lateral force measured in g's. A 1g lateral force would equal the sprung weight of the front end; 3) The moment center location, both in height and width after the car dives and rolls in the turns; 4) 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 versus left-front spring, and so on); 5) The front sway bar has an effect of antiroll and must be taken into account. The larger the bar is, the more resistance there is to roll; 6) The track banking angle.
A typical rear sway bar installation...
A typical rear sway bar installation uses a high mount and arms with several holes used to adjust the length of the arm, which changes the rate of the assembly. Note that the link between the sway bar arm and the rear end is 90 degrees off the arm and directly above and in line with the motion of the axle tube.
The rear suspension system is much different from the front, and we look at it differently. At the front, the spring base is felt at the wheels. With a rear solid axle system (and this relates also to a front straight-axle car), we have a spring base that is felt on the top of the actual springs. This dynamic model is not a new concept-it was developed and published some 60 years ago.
Like the front suspension, the rear suspension has a center of gravity of the sprung weight of the car that represents the top of the moment arm. 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 four-link, leaf springs, or Watt's 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 height 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.
Note: The car "feels" the spring base at the top of the springs. In a car with coilover springs, the distance between the tops of the mounting bolts is the rear spring base.
The leaf spring rear locating...
The leaf spring rear locating device is unique in that the mounting points of the springs are also the lateral locating devices. Therefore, the average heights of the ends equal the height of the rear moment center. The leaf-spring system provides a wide spring base and is superior to other suspension systems in some respects.
There are additional effects created by the Panhard/J-bar angle that tend to leverage and exert influential forces on the suspension, and those effects will be covered in future articles on chassis dynamics.
There are seven primary factors that affect the magnitude of the rear-roll angle. These are: 1) The sprung mass of the portion of the car supported by the rear suspension (scale weights at the LR and RR tires minus unsprung components); 2) The height of the rear moment center; 3) The magnitude of the lateral force; 4) The overall spring stiffness as well as side-to-side spring split; 5) The width of the spring base, which is the distance between the centers of the top of the springs; 6) The rear sway bar (if used) has a large effect of antiroll and must be taken into account; 7) The track banking angle.
Now that we know the importance of matching the desires of each suspension system, the key to creating a balanced setup is to change springs and moment center locations so that each end of the car will want to have the same desires. The result of doing that is a faster race car that is more consistent in handling balance.
The Watt's link is mostly...
The Watt's link is mostly used in road racing cars. The advantage of this system is that the rear solid axle does not move laterally as the chassis moves vertically. With some three-link and metric four-link support systems, lateral movement of the rear end would cause adverse rear steer. The Watt's system eliminates the lateral movement that creates the rear steer. The rear moment center location also stays very consistent with right and left turns encountered in road racing.
In next month's issue, we will examine how we can apply this technology to set up some typical types of stock cars. Understanding these basic principles of stock car dynamics will help you make correct setup decisions.
A sway bar is a device that produces a resistance to roll when used in either the front double A-arm or straight-axle suspension system. Its influence in a double A-arm suspension is much less than when used in the solid-axle suspension.
The sway bar has a stabilizing effect on the double A-arm system and helps control oscillations that are common with that type of suspension. The sway bar is more effective in a straight-axle suspension when, for reasons of design limitations, the rear-roll angle is hard to control. Such is the case with big-spring/truck-arm suspensions like those used in Nextel Cup cars.
The tendency in recent years for asphalt circle track racing has been to use softer spring setups and stiffer sway bars. Racers have found both success and limitations to the usefulness of this trend. In some cases, they have found that softer springs and larger diameter sway bars help to lower the car's center of gravity and to also promote an efficient aerodynamic configuration when the car is at racing speeds.