Performance gains are possible...
Performance gains are possible by knowing how shocks affect chassis movement and load transfer. It is important to know our individual shock rates and a little about how different rates produce desired affects. Through dynoing our shocks we can accurately predict the effect.
A lot has changed in the way of setups for both dirt and asphalt racing over the past five to ten years. Dirt setups have become more like asphalt setups and asphalt setups have become, well, crazy. In today's racing world, we have available pre-built shocks with any number of combinations of disk design and valving and with some types of designs, the ability to adjust shock rates quickly at the racetrack.
So, it would seem that we have what we need to choose the exact rate of compression and rebound necessary for a particular set of conditions if we know a little about the affect of rates on the setup. This wide range of choices can either be an advantage, or the proverbial "enough rope to hang..." syndrome. The more we can learn, the better we can make decisions regarding shock selection.
More and more, racers are being educated in all aspects of chassis tuning and they want to know more about shock technology. Today's racer is much more willing to learn than at any time in the past. The more we know about each of these subjects, the less fear we have. It is what we don't know that we fear the most. Shocks scare a lot of racers. Hopefully we can ease your pain and make understanding how shocks affect the car a simple process.
The information we present here is intended to be a guide to help you understand the basic principles of shock technology and the art of track tuning with shocks. The exact rates for the shocks that you need for your car are dependant on how your car is constructed, set up, and driven, as well as factors such as load distribution and racetrack characteristics.
We would really like to give you the exact shock values that will make your car as fast as it can be, but that would be impossible due to the many variables. That is exactly why you must work with your particular car and try not to follow what others are doing. Each car is a little different than another and each driver has his/her own style of driving.
An important aspect of shock technology is that there is no computer program or chart that will tell you exactly which shock rates to run on your car. This is, and always has been, a trial and error process. Where we have tell-tale signs and computer software that point us in the right direction for the chassis setup, with shocks we don't. It is all driver feel and stopwatch feedback that tells us when we are making progress or going backwards.
This QA1 shock has an adjustment...
This QA1 shock has an adjustment at the end of the shaft to control the amount of rebound force. This type of adjuster is easy to reach and change with the shock on the car.
Part one of this series dealt with the basic construction of the racing shock. We learned that the two strokes of the shock, rebound and compression, are looked at separately and perform functions related to different areas of track tuning. If we deal with rebound and compression separately, then we need to be able to tune each independently. There are also different designs of shock pistons including the Linear design and the Digressive design. Again, we are able to achieve varying results by utilizing all of the variables of shock design.
For most situations, we would use Split Valve shocks. Split valving means that we have different rates of resistance for rebound and compression because we need to tune each movement a little differently than the other. We can also rate the two movements differently for each corner of the car to further tune the setup.
If we want, we can buy (at a greater expense) shocks that have external adjustments for rebound or compression, or both (called double adjustable). That way, we can experiment with different shock rates without removing them from the car. Regardless of how we arrive at the different shock rates, we do need to know beforehand what we are looking for and how to get there.
Shock companies provide a system of numbers or letters to reference the rates of rebound and compression. Most of these companies try to provide a cross reference so their numbering system can be compared to the other systems used by competing shock brands. The ultimate match between shock brands is not exact because of differences in design of the valving and the fact that each company may rate its shocks at a different shaft speed.
Pro Shocks has had a double...
Pro Shocks has had a double adjustable shock on the market for some time now. Its range of adjustment is infinite rather than incremental. This means that you can run the shock "in-between the numbers" quite easily.
This Ohlins shock has a separate...
This Ohlins shock has a separate canister for the gas separator piston and the compression adjustment is located in this unit. The rebound adjustment is on the end of the shaft.
A double adjustable shock...
A double adjustable shock has external adjustments that can be easily changed to regulate the amount of resistance the shock will have in rebound and compression. By running the shock on a shock Dyno, we can measure and record the amount of resistance for each setting.
When a shock is run on a Dyno,...
When a shock is run on a Dyno, it is cycled at different speeds and the resistance is accurately measured by load cells. It is important to go through this process to not only know the true rating of each shock, but to also insure that every part is working properly. All shocks should be inspected, rebuilt and dynoed at least once a year.
If for example a "3" shock or an "A" shock were rated by each manufacturer at 100 pounds of resistance, then comparing them would depend on what speed of movement each company rated that 100 pounds. We know that the rate of resistance is directly related to shaft speed. Company X might rate the 100 pounds at 5 inches per second of shaft speed where company Y might rate the 100 pounds at 10 inches of shaft speed. We can see where the two would not feel the same to the driver. The X shock might well be 150 pounds of resistance at 10 inches per second of shaft movement whereas the Y shock might be only 75 pounds of resistance at 5 inches per second of shaft movement.
To simplify things, we will use the numbering system to relate to the amount of rebound and compression, the smaller number representing less resistance. Because we are not telling the exact rate in pounds of resistance for each number, the comparisons and trends will be good for either a dirt or asphalt stock car. The difference in the two types of racing as far as shock selection is concerned, is that generally speaking, the dirt cars require a softer overall package than the asphalt cars.
A basic starting shock setup for a medium banked racetrack might be a pair of 6 shocks on the front and a pair of 5 shocks on the rear. These should not be true 50/50 shocks where the resistance in either direction, rebound or compression, would be equal. Rather, we need more rebound control than compression control.
If the shocks were the split valve design, each shock would be numbered as say a 6/5, which for our purpose means that the first number will represent the rebound resistance and the second number will represent the compression resistance. Since the springs help resist compression, that number should be smaller than the rebound number.
Controlling wheel movement would be much easier if all we had to work with was the shocks. But in reality, our racecars are supported by a set of springs. If we wanted true equal resistance to wheel movement with the shocks installed along with the springs, we would want the rebound resistance to be greater than the compression resistance (reference Part One where we said that springs promote rebound and help provide resistance to compression). As we install stiffer springs, we would naturally increase the rebound resistance and decrease the compression resistance.
A shock that moves in direct...
A shock that moves in direct proportion to the spring moves at the exact same speed as the spring. On some designs, the shocks and springs are mounted at different distances from the wheel and move at different speeds in relation to the wheel speeds. This shock will work at 75 percent of the rate shown on the dyno due to the 30 degree installation angle.
The more balanced shock layout might well look like this: Front shocks = 6/5, rear shocks = 5/4. The greater rebound helps control the force of the preload on the spring as it is released and the softer compression works along with the resistance to compression provided by the spring.
Ideally, we would rate our shocks on a fixture that would simulate the installed spring and include the installation ratio. Some professional teams now have those fixtures. The rating is measured at the ball joint and takes into account the spring rate, installation ratio, spring angle, etc. When measured this way, it is an accepted theory that the rebound and compression numbers must be the same (i.e., it should take the same force to move the wheel vertically in both directions).
High Banked Setups
For a setup that uses higher rated springs in the front and rear for a higher banked racetrack, the compression and rebound numbers would change to reflect the spring change. We might use a pair of 6.5/4.5 shocks on the front and a pair of 5.5/3.5 shocks on the rear. That is because of what we said before about the higher rating in the springs promoting rebound and therefore we increase rebound resistance. The higher spring rates also resist compression and so we can reduce compression resistance in the shocks.
When the banking of the racetrack is very high, 18 degrees or more, we need split valving, but we would go up on both the rebound and compression rather than up on rebound and down on compression. The reason is that the turn speeds are much higher on the very high banked tracks and the shocks will move much faster in both the transitional areas of entry and exit, and also as the car goes over bumps and dips in the racetrack. Without the higher level of control, the suspension might well bottom out with obvious negative results.
For a shock installation such...
For a shock installation such as this coilover design, the shock/spring combination will move at a slower speed than the wheel. If the speed of the movement of the wheel were 5 inches per second, then due to the motion ratio and the shock installation angle that the shock/spring is mounted, the shock speed would be only 3.82 inches per second.
A more realistic layout for a very high banked track might be a pair of 6.5/5.5 shocks on the front and a pair of 5.5/4.5 shocks on the rear. We have increased the resistance for both the rebound and compression as well as introduced split valving to a degree.
Low Banked Race Tracks
For low banked racetracks, we would necessarily spring the car much softer to slow down all of the movements in order to help maintain traction. One of the basic ingredients needed to maintain grip at low banked tracks that have less grip is to soften the whole setup including spring rates as well as shock rates. As we soften the springs, we would also soften the front shocks to 5/4 valving and the rear shocks to a pair of 4/3 shocks. This is the general way shocks can be matched to the stiffness of the spring setup.
Here is where we might go back to the true 50/50 shocks. Instead, many teams want to promote left front travel with Pro-dive, soft compression numbers and a high rebound rate at the left front corner. They are trying to tie down that corner to reduce the height of the left front valance. This reduces the amount of air that travels under the car and helps promote downforce. Most of the gain from this arrangement is not more traction, but making the car turn better, which can be achieved using better front geometry at less expense.
In the Part One story, we talked about how the shock regulates the speed at which each corner of the car moves as load is transferred on to and off of the four corners. If we split the rates between the four corners, we can effectively change the load distribution on the four tires while the suspension is in motion. So, to utilize the resistance of the shocks to redistribute load, we must have load transfer taking place and that in turn causes the needed suspension movement. If the suspension moves, so do the shocks.
On many big spring stock clip...
On many big spring stock clip stock cars, the speed of the wheel, the shock and the spring are all different. If the wheel were moving at a speed of 5 inches per second, the shock would be moving at 3.82 inches per second and the spring would move at 2.65 inches per second. If the shock were mounted farther from the ball joint, then it would need to be designed with a higher rate of resistance for both rebound and compression.
Compression rate differences between a pair of shocks helps to redistribute load between the four corners of the car where load is being transferred onto a particular end of the car. At the front of the car, compression rate differences can change the load distribution on entry to the corner while load is being transferred to the front due to deceleration. At the rear, compression rate differences can help to redistribute load that has transferred onto that end from load transfer caused by acceleration off the corners.
Rebound rate differences help to redistribute load between the four corners of the car when the suspension is in motion as load is being transferred off of a particular end of the car. In the front of the car, rebound rate differences can change the load distribution on exit while the suspension is in motion when load is transferred off of the front to the rear. At the rear, rebound rate differences between the LR and RR shocks can help redistribute load on the four corners of the car, again while the suspension is moving and adjusting to the load being transferred off of the rear onto the front due to deceleration of the racecar.
We need to think out the motions of the car related to how it is driven and how the track is configured, as well as the desires we have for improvement. Tuning for corner entry can affect corner exit. Do not tune with shocks for problems that can be fixed with basic setup parameters.
A car that is tight on entry should not be fixed with installation of a high rebound LR shock that will reduce the loading on the LR tire on entry. A car that is tight on entry most likely has geometry problems. The rear is out of square, the front-end geometry is not designed correctly, or the setup is just plain tight.
Remember, the more you have to trick up your shocks and shock selection corner to corner, the harder it is going to be to be consistent. We are getting many reports of teams going to more basic designs of setup and shock selection and winning races due to consistency.
One exception is when we go beyond the normal softening of the springs into what has become known as the big bar and soft spring (BBSS) setups that have become popular in short-track asphalt racing. These setups often use super soft springs in the front and, to help control dive on entry, the teams must increase compression resistance or Anti-dive characteristics to overcome the tendency for the car to bottom out as we brake and turn left on entry into the corners.
AFCO Racing Products
QA1 Racing Shocks