This IMCA dirt Modified utilized...
This IMCA dirt Modified utilized a Z-link rear suspension. Note there are two holes for the coilover mount on the lower arm. Moving the mount changes the motion ratio of the arm and the spring rate that is felt by the chassis. Using a 200-pounds per inch (ppi) rated spring, the front hole would yield a net spring rate of about 110 pounds. Moving the spring to the back hole would increase the net spring rate by 20 pounds to about 130 ppi.
There are probably several hundred different configurations we can use in typical four-link or Z-link rear suspensions. If we combine the different bar angle possibilities with the various spring placements, indexing properties of the birdcages, pull bar angles and spring rates, lift arm rates that regulate lift on acceleration and compression during braking, shock rates for the four corners, plus the fifth and sixth coil, we begin to get the picture. The adjustment combinations are even much more than those for a Formula One or Indy car.
We need to decide what we want to achieve and then set the bars, etc., so that the car will do what we want. The various linkage designs from the four-bar to the Z-link offer possibilities for changing the rear steer characteristics as well as the distribution of weight while decelerating and while under acceleration loads.
Each side can be adjusted with respect to "front to rear movement" as the chassis moves vertically, or to steer either direction depending upon what the chassis is doing on a particular side of the car. The four-bar can be set up to move the wheel considerable distances fore and aft, and that movement can either help performance or put you out of contention.
Most Late Model cars utilize...
Most Late Model cars utilize birdcage-type attachment points where the linkages connect to the rear axle tubes. This allows the rear end to be free to rotate.
Rear geometry deals not only with wheel movement, but also the forces associated with acceleration and braking. As we accelerate, there is a great amount of torque applied to the rear axle housing, and dirt car manufacturers have designed several systems to take advantage of that force. Among the most popular systems are the pull bar and the lift arm. Both help cushion the torque related to instant application of power to help the rear tires maintain grip with the racing surface, and to control the forces of engine and wheel braking when decelerating.
Both systems can utilize a shock that will control the speed of movement both ways. This is essential, because a spring will react relatively slowly when being compressed, and violently when released from compression. The shock controls both, but obviously must do much more work in the rebound mode.
The changing of the static distribution of weight in a dirt car can be accomplished in two ways: by moving weight around in the car or by adjusting the distribution of weight on the four wheels without moving actual weight. The latter involves changing the height of the spring by turning the adjusting ring at the top or bottom of each spring to regulate the amount of crossweight, or, in more familiar terms to dirt racers, the amount of left-rear (LR) weight.
There are two devices that...
There are two devices that can be used to control and regulate the rotation of the rear end as the car is accelerating and braking. They are the lift arm (shown on the left) and the pull bar.
LR weight refers to the number of pounds of weight supported by the left-rear wheel versus the weight supported by the right-rear (RR) wheel. One hundred pounds of LR would indicate that if the car were weighed, the LR scale would read 100 pounds more than the RR scale. Setups can be developed around a particular LR weight number or the LR weight around a particular setup to tighten or loosen a car.
In some cases, high amounts of LR weight can produce less traction than smaller amounts. We generally relate high crossweight, or LR weight, with improved traction, but that's not always the case. If we have a car that utilizes all four tires (all on the ground with sufficient weight on them to make them work), we can definitely use a higher amount of LR weight to try to balance the weight across the rear tires. The more equally loaded a pair of tires is, the more available traction.
This does not work in the case of a setup that lifts the LF tire off the ground. Once the LF is elevated, all of the front weight of the car is supported by the RF tire. Since diagonal tires share the loads in combination, if the RF weight goes up, so does the LR weight. If there is already a high amount of LR weight supported by that tire, imagine what it is after the LF comes off the ground. For this reason, we need to run a much lower amount of LR weight with a three-wheel setup.