The term "tie-down" is often used to refer to a shock that has an unusually 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 load is transferred from the left side to the right side is reduced.
If we can stop the sudden motion, then we can keep the left side down on initial turn in and the chances are that the whole attitude of the car through the middle of the turns will be lower. A lower center of gravity (CG) means less load 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 CG of the sprung mass and a higher CG promotes more load transfer. Drag cars use this effect on the front of their cars to promote more load 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 load transfer to the right side for better side bite and to the rear for better traction off the corners.
Adjustable shocks can be run...
Adjustable shocks can be run on a dyno and graphed to show the rates at various settings. Here we see quite a wide range of resistance for rebound and compression. This shock can be used for a wide range of setups from fairly normal, conventional rates shown in red to the more current asphalt trend for high rebound and less compression shown in gray.
The trend in asphalt Late...
The trend in asphalt Late Model racing is towards softer settings on compression and higher rebound rates. Here, might be a typical layout using common numbers representing rates. For entry, this might create a slightly tight entry which is desirable for deeper entry and control.
If the shock rates are arranged...
If the shock rates are arranged in a certain way, the load distribution will not be adversely affected with the high rebound rates at the front countered by reverse emphasis at the rear. Note the higher rebound on the LF countered by the higher compression at the RR and lower rebound at the RF countered by the lower compression at the LR.
Putting All of This to Use
In order to utilize the configurations we have discussed here, we must be able to use a range of different rates of shocks in order to find the right combination for our car at a particular racetrack for a particular setup. For a team that races at only one track, the process is fairly simple.
You would experiment to find the fastest set of shocks and ones that suit the driver's style and stick to those. For teams that travel to different tracks, some changes will be necessary if the setup needs to change and/or the track layout is different from track to track.
Most shock experts agree with certain basics, such as:
1 The shock package should be softer overall when racing on dirt and when the track is flatter when on asphalt for the conventional setups.
2 Get your basic setup close to being balanced before trying to tune with shocks. Shocks can't solve basic handling balance problems.
3 Higher banked tracks require a higher overall rate of shock as opposed to flat tracks. This is because of the higher speeds and the extreme amount of downforce.
4 Shocks that are mounted farther from the ball joint should be stiffer than if they were mounted close to the ball joint. That is because with each inch of travel of the wheel, the shock mounted farther away will move at a lesser speed which means less resistance in both rebound and compression.
5 Tune entry performance first. If there are no entry problems, make small changes if you want to experiment to see if entry can be improved. Entry problems include a tight car or a loose car. By far the worst problem would be the loose-in condition. Nine times out of 10 this is an alignment problem and not shock related.
6 Tune exit performance last. If there are no exit problems, don't make any significant changes. Exit problems can include a car that pushes under acceleration or one that goes loose under power. Be sure that you do not have a tight/loose condition where the car is basically tight in the middle and goes loose just past mid-turn. This is fixed with spring rate changes, Panhard bar adjustment, and more.
7 On dirt racetracks, reduce rebound settings on the left side and decrease the compression rates on the right side for dry slick surfaces to promote more chassis movement. This helps to maintain grip as the car goes through the transitional phases of entry and exit.
8 On asphalt tracks, once the car has been tuned with shocks for optimum entry and exit performance, increase the overall rebound rates a small step at a time, especially on the left side, to possibly increase overall performance. This is especially good for low to medium banked racetracks. For highly banked tracks, the added rebound is not necessary or advisable.
9 For the soft spring setups on asphalt, the whole shock package is much different than when running conventional or soft conventional setups. The bump rates (either bump rubbers, spring rubbers, coil bind, or bumpsprings) will be very high and the shock rebound rates must match those high rates in order to control the device.
A bump that is used in the 1,000-ppi spring rate range will need a shock that is rated at 1,000 ppi at around 3 inches of movement per second. Usually, the low speed rate of the shock will be comparatively high too and often we see a "nose" rate of between 500 and 800 ppi or more at close to zero movement.
This was considered extreme...
This was considered extreme tie down a few years ago and we can see where there is very little in the way of compression resistance and a lot of rebound resistance. In order to control the high spring rates associated with bumps, the shock rebound rates must be equal to those spring rates.
This is a graph of the shocks...
This is a graph of the shocks we used in our bump spring testing. We can see the “nose” where the rebound starts at 900 ppi for the LF shock and 500 ppi for the RF shock. The rebound rate for the LF shock is 1,350 ppi and 800 ppi for the RF shock. Those rates controlled the bump springs when used in conjunction with the rates of the ride springs.
A Closing Caution
The suggestions provided here are representative of trends that can enhance your handling package. Before any of this can work, the setup must be balanced, the steering characteristics must be ideal and the car must be aligned properly. If not, you will probably chase the setup and experience a lot of frustration and expense.
Shock tuning is the last thing to experiment with in order to try to increase your race car's performance, but it is nonetheless a necessary step in finding the ideal total handling package. That said, before you set up your car and chose your shocks, evaluate what you will need to match the spring rates you will run.
If you're experimenting with the soft spring setups, consult your shock expert so that you can match your shocks to the setups. Most racing shock companies have technicians who are very familiar with those setups and can advise you in the best rates to use to match your bumps.