In the modern world of short track racing for both dirt and asphalt competition, shocks have become one of the most important tuning tools we have. They complement the current setups, especially the radical 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 and critical for 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.
To get the low and flat attitude...
To get the low and flat attitude at the front shown here, you need soft front springs, a stiffer right rear spring and shocks that will hold the front end down. This is our test car running on bump springs combined with the right shocks.
This sketch shows how increasing...
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. The size of the circle relates to the amount of loading, the larger circle means more loading.
This is a graph generated...
This is a graph generated by a shock dyno that shows a relatively normal shock. The rebound (the bottom half of the graph) is a higher rate than the compression. Relate the pounds of resistance reading on the left side with the speed which is along the bottom of the graph in inches per second. This shock has around 75-100 ppi of resistance in compression and 200 ppi of resistance in rebound at 3.0 inches per second of shaft speed.
We have learned that shocks regulate the timing of movement of the four corners of our race car. Matching the shock rates to the spring rates 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.
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, 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's 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.
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 transferred load.
Crossweight is defined in short track racing as the percent of the combined RF and LR weight divided by 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 effect. This is exactly why it has been said that a stiffer RF shock will speed up load 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 will return to the LF tire as the car reaches steady state or a steady ride height at mid-turn.
Here we see how increasing...
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.
By using split rate shocks...
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 load due to deceleration.
If the car is already tight on entry, after having eliminated common causes of tight entry such as rear misalignment, rear steer 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.
We can also work with the rear shock rebound rates to help effect changes in load 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 load 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 which loosens the handling momentarily while the suspensions are in motion.
Modern soft spring setups use a much lower compression setting due to the high spring rate of the bump. We can still utilize this method by creating a compression split. The numbers will be lower than with a conventional setup, but still effective.
Once the shocks are firmly on the bumps, the load distribution will equalize and the advantage of shock compression split will be nullified. If the shock rebound settings are high enough, the shock will never leave the bump and no amount of compression split will work.
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 load is transferred to the rear while the rear suspension is in motion to tighten the car with more crossweight percent. 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 load off of that corner which in turn increases the percentage of load supported by the RF and LR tires.
With the soft spring setups, the rear rebound settings can really help a car that is otherwise limited in adjustments. If the RR spring is very stiff, and this is both common and mostly unnecessary, there will be a loosening effect on acceleration when load transfers to the rear.
The stiffer RR spring will take much of the load transfer from acceleration until the car is settled and even beyond. What helps is to increase the stiffness of the LR shock compression and reduce the compression of the RR shock. This serves to equalize the resistance to compression due to load transfer.