Shocks mostly determine the entry and exit characteristics of the racecar. To get the low and fl at attitude shown here, you need soft front springs, a stiff right rear spring and shocks that will hold the front end down for a period of time. But be careful what you ask for, the desired results can be elusive.
In the modern short track racing world of both dirt and asphalt competition, shocks have become one of the most important tuning tools we have. They compliment the current setups, especially the radical big bar and soft spring setups. And so, we complete our trilogy of shock technology articles with the final installment on how to tune the transitions.
This information is useful whether you are running more conventional setups or the more radical ones. Think along as we discuss how shocks affect the speed of movement and the load distribution at the four corners of the car as we transition from high speed to minimum and then to high speed again.
We have learned that shocks regulate the timing of movement of the four corners of our racecar. Matching the shocks to the springs on a conventional setup is fairly simple and can be done with off-the- shelf shocks. Matching the shocks to non-conventional setups can be a bit tricky. Let's take a look at how we can improve our overall setup package by helping our car negotiate the transitional phases of the racetrack.
This sketch shows how increasing or decreasing the compression or rebound at each corner can tighten the car during transitions involving deceleration and acceleration. The changes shown will have the effect of making the car tighter by increasing the crossweight percent.
Entry Tuning With Split Valve Shocks
If we split the front shock compression rates with a LF 5/4 (5 rebound and 4 compression) and a RF 5/5, then, while the suspension is in motion due to weight being transferred on to 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 load transfers almost immediately when a force is presented to enact that transfer. As we brake into the corner, the load transfer happens quickly. If on entry we transfer 300 pounds 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 influenced by the shocks and springs.
Here, we see how increasing or decreasing the compression or rebound at each corner can loosen the car during transitions involving deceleration and acceleration. The changes shown will have the effect of making the car looser by decreasing the crossweight percent.
Reasoning out the effect of load transfer onto the front suspensions that are dissimilar in stiffness, the slower moving (or stiffer) corner will momentarily retain more of the transferred load 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 load.
Crossweight is defined in short track racing as the percent of the combined RF and LR weight vs. the total vehicle weight. If the crossweight percent increases, then the car will be tighter on entry and the car might be faster if that is the desired affect. 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 load 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, then 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), then 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.
This is a graph generated by a shock dyno that shows a relatively normal shock. The rebound (bottom of the graph) is a higher rate than the compression. Relate the pounds of resistance reading the left side with the speed which is along the bottom of the graph in inches per second. | 
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. | 
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. |