Entry Tuning with Split-Valve Shocks
If we split the front shock compression rates with a left front (LF) 5/4 and a right front (RF) 5/5, then while the suspension is in motion due to weight being transferred onto it, then the RF suspension will move slower than the LF suspension. Additional weight will be transferred onto the RF and LR tires, causing a momentary increase in the crossweight percent in the car. This obviously tightens the car.
It is important to note that, contrary to some opinions, the weight transfers almost immediately when a force is presented to enact that transfer. As we brake into the corner, the weight transfer happens quickly. If we transfer 300 pounds on entry from the rear to the front, the 300 pounds goes to the front in an instant. The distribution of that 300 pounds between the two front wheels, while the suspension is assuming a new attitude that will support the additional weight, will depend entirely on differences in stiffness of the suspension systems at all four corners. Stiffness is defined as the resistance to movement of the shocks and springs.
By using split-rate shocks between pairs on each end of the car, we can tighten the car on entry to the corner. If the RF shock is stiffer in compression than the LF, and/or the RR shock is stiffer in rebound, then the crossweight percent (RF + LR combined weights) will increase momentarily while the suspension is moving and adjusting to the transfer of weight due to deceleration.
Reasoning out the effect of weight transfer onto the front suspensions that are dissimilar in stiffness, the slower moving (or stiffer) corner will momentarily retain more of the transferred weight while the suspension is moving to a new attitude to support the added weight. If the RF suspension is stiffer than the LF suspension, then both the RF and LR tires will support more of the total sprung weight of the race car.
Crossweight is defined in short track racing as the percent of the combined RF and LR weight versus the total vehicle weight. If the crossweight percent increases, the car will be tighter on entry and the car might be faster if that is the desired effect. This is exactly why it has been said that a stiffer RF shock will speed up weight transfer to that corner. In truth, some of the momentary weight that has been transferred onto the RF due to that corner being stiffer than the LF corner may well return to the LF tire as the car reaches a steady state or a steady ride height at mid-turn.
If the car is already tight on entry, after having eliminated common causes of tight entry such as rear misalignment or brake bias issues, an opposite effect can be utilized. If we increase the compression of the LF shock and/or increase the spring rate on that corner (which is usually a good idea for flat tracks), we can effectively reduce the crossweight in the car on entry while the suspension is in transition by loading the opposite diagonal, the LF and RR. As one diagonal goes up in percentage of supported weight, the other goes down. We can also work with the rear shock rebound rates to help effect changes in weight distribution and corner entry characteristics. To neutralize a car that is tight on entry, a stiffer rebound setting in the LR shock and/or a softer rebound setting in the RR shock will cause more of the transferred weight to be taken from the LR than the RR tire during the transitional period. This too causes a decrease in the RF and LR weight distribution percentage that loosens the handling momentarily while the suspensions are in motion.
Exit Tuning Using Split-Valve Shocks
Corner exit performance that utilizes the shocks is primarily tuned by splitting the compression settings in the rear shocks and/or the rebound settings in the front shocks. A stiffer compression setting in the LR shock will load the LR and RF corners as weight is transferred to the rear while the rear suspension is in motion. A stiffer shock in rebound at the LF corner can help accomplish the same effect by causing a slower movement of that suspension and a more rapid transfer of weight off of that corner which in turn increases the percentage of weight supported by the RF and LR tires.
The term "tie down" is often used to refer to a shock that has a high resistance to rebound. If the rebound rates are higher for both left side shocks than those of the right side shocks, then as the car turns left (especially with quicker turning rates associated with smaller radius turns), the tendency for the left side suspensions to quickly rebound as weight is transferred from the left side to the right side is reduced.
By using split-rate shocks between pairs on each end of the car, we can tighten the car on exit off the corners. If the LF shock is stiffer in rebound than the RF, and/or the LR shock is stiffer in compression than the RR shock, then the crossweight percent (RF + LR combined weights) will increase momentarily while the suspension is moving and adjusting to the transfer of weight due to acceleration.
If we can stop the sudden motion, we can keep the left side down on initial turn in and the chances are good that the whole attitude of the car through the middle of the turns will be lower. A lower center of gravity (CG) means less weight transfer off the left side of the car and more retained left side weight. For asphalt stock cars and dirt cars on higher-banked tracks with grip, a higher left- side weight means more equally loaded tires, left to right, and more traction. The opposite is true of dirt cars on slick tracks.
The reverse term, or "Easy Up" shocks, are used to help raise the suspension quickly which does also raise the center of gravity of the sprung mass and a higher CG promotes more weight transfer. Drag cars use this effect on the front of their cars to promote more weight transfer to the rear tires for added traction. On dry, slick dirt tracks, teams can utilize less rebound in the left side shocks and in the front shocks to promote weight transfer to the right side for better side bite and to the rear for better traction off the corners.