Shock Tuning
In the mid 80’s shocks were becoming better recognized by professional race car teams to be very important. By the mid to late 90’s some had gone too far and believed springs were only necessary to keep the car off the ground in the pits, shocks did the rest. Today, based upon greater knowledge from aids like data acquisition and shake rig testing, there is a much better understanding of the role of the shocks.
Good racecar suspension design and setup minimizes tire force variation (keeps the tire quiet on the track) to maximize grip (bite, traction) and provides tuning for drivability and balance (under-steer, push, tight, handful, over-steer, loose). The best role the shock absorber can play is to assist in controlling the un-sprung weight to aid in minimizing tire force variation.
Sprung weight includes everything 'above the springs', ie - chassis and everything it supports. Un-sprung weight is everything 'below the springs', ie - tire, wheel, spindle, rotor, caliper and upright. One half of the weight of the spring, shock/strut and control arms is added to both sprung and unsprung weights.
Teams should not rely upon shocks to tune balance. If one attempts to control the balance of a car with shocks, one is attempting to control the sprung weight transfer. If the shock is 'stiff enough' to do this, it is too 'stiff' to properly control the much lighter un-sprung weight. Handling (understeer / push / tight / oversteer / loose, etc.) can be achieved with other adjustments in the suspension including spring rates, roll center heights, cross weight, bar rates (where used), etc. Certainly shocks can affect balance but should only do so as they help to achieve grip. That said, there might seem to be exceptions. Wet track and tuning tools limited by rules come to mind. At times slight deviations from the norm might be necessary. But recognize that even for a wet track the masses do not change significantly. In the end, it is important to keep in mind that the shocks can best be used to significantly affect tire contact patch behavior.
A good example of Shock Shop employing this philosophy was the winning car at the 2009 Trans AM race at Mid Ohio. It not only had obviously superior cornering and exit speed but upon returning to the garage after the awards ceremony the driver looked at the tires and exclaimed "we could run another race on these"! Excessive tire wear and conditions like graining can be an indicator of improper shock valving.Shock adjustability is over-rated, over-sold and completely misunderstood by most everyone who think it is needed. One cannot simply purchase a damper because it is adjustable and expect it will work for their car. A shock needs to be custom tuned to suit the needs of a specific car. Provided with data which includes spring rates, motion ratios, bar rates, tire rates, sprung and unsprung weights, Shock Shop utilizes a mathematical tool called Critical Damping Analysis to determine damper performance aimed at minimizing tire force variation in an effort to maximize grip/bite/traction. Shocks are then re-valved to provide forces that closely match the predicted values.
One can download Critical Damping Analysis data sheet in PDF or Excel format as well as Instructions. Although folks sometimes complain about the effort required to gather the data, in the end they always recognize the effort was worthwhile because they learned so much more about the car. Shock Shop provides dyno sheets with every set of dampers. On that dyno sheet are recommended settings for the adjuster(s) along with the damping curves achieved from those settings.
On Track Tuning
1. Help to determine if the spring rate is correct.
2. Help to determine if the damping rates (build of the shock) are correct.
3. Achieve a slight improvement in grip via different shock settings for certain track conditions if one does not have a separate set of springs and dampers specially tuned for these conditions.
4. Make it easier for the person assembling the shock to set certain parameters of the 'build'.
Note that there was no mention of adjusting balance (understeer / oversteer) of the car. Until recently many racers wanted to make a change of a few 'clicks' in the dampers and realize a change in balance. For this to work, the shocks must be stiff enough to control cross-weight (sprung weight) transfer. It is well known that minimizing tire force variation is important for grip. Tire force variation is an un-sprung weight issue. If the dampers are stiff enough to control the cross-weight (sprung weight), they are too stiff to properly control the unsprung weight.
Shock Shop recommends that one begin by adjusting the dampers to the recommended settings and then seek balance/handling from the numerous other tuning tools (springs, bars, rake, alignment, etc.) available. Always work on one end of the car at a time. This provides a yardstick by which to measure the affect of a change. Survey adjustments to find the best handling. When a gain is achieved at one end of the car, start working on the other end. Don’t be forgiving of the car or driver. A couple of laps to warm up car and driver should be enough. Then one or two laps at most should be enough to evaluate each change. This helps maximize the gain from limited time on a test day or race weekend. If an improvement is found, theoretically it should be backed up by returning to a previous setting for a lap to be sure the gain was real.
Lastly, after the car is well sorted utilizing chassis tuning options, one should survey shock settings, one at a time at one end of the car at a time. For example, increase front bump by 5 clicks. Run a lap or two, come in and decrease bump by 10 clicks and begin to hone in on the 'best setting for front bump. Then work on front rebound. Then survey the rear settings. If best shock settings are found to be significantly different from what Critical Damping Analysis recommended, a through review of all inputs to the routine should be conducted. Twice in the last 8 years the best settings were found to be considerably different than recommended. In both instances it was discovered that the wrong motion ratios had been provided. After re-running the Critical Damping Analysis calculations, tweeking the build of the shocks and re-testing, both teams found the recommended settings to be best.
How should spring rates determined?
Before shocks can be tuned, one should have a reasonably good idea of spring and bar (if any) rates. Though ultimate fine tuning of these rates is best determined from on-track testing, how does one know where to start and, if after testing, whether the results are in the ball park of where they ought to be? As testing is very expensive both monetarily as well as time, and can lead to total confusion, it would seem prudent to utilize well proven tools that are available for determining setup before arriving at the track. There are several sources who suggest selecting a 'ride frequency' to start with then proceed through the calculations to ultimately determine spring rates. Some automotive manufacturers start here, but racers aren't so concerned about ride comfort. We want grip/bite/traction! Further, this method results in such a wide range of possible spring rates that one would be about as well off doing nothing but track testing. There is a better option which can not only conserve resources (cost is often less than one set of tires!) but more fully educate a driver / team about their race car. One can not know too much about how the numerous variables affect handling, grip/bite/traction and drivability. Shock Shop recommends using WinGeo3 along with software provided on the CD that comes with Race Car Engineering to evaluate weight transfer, roll and pitch angles based upon measurements taken from the car. There is considerable work involved in gathering the data but the more one knows about the car the lower the lap times!
What to Avoid
Everyone should work very hard get good car balance. In recent times it was popular to valve shocks with lots of rebound and very little compression to allow a team to make small shock adjustments that the driver could feel and thereby, supposedly, achieve a good balance. Unfortunately, this approach is sure to take away grip. Simply put, compression helps keep the tire on the racing surface, rebound keeps it off the racing surface. If the shocks are so stiff as to significantly affect balance, they are too stiff to accomplish their primary job: assist in minimizing tire force variation. Further, lots of rebound does not allow the suspension to droop. In cornering, high rebound and low compression forces move the roll center toward the inside tire contact patch causing undesirable if not unpredictable changes in dynamic cross weight (balance). Most often, minimizing dynamic suspension rate changes will make the car easier to balance. Only if a balance problem can be fully described, and then only with a thorough understanding of the suspension can one utilize dynamic suspension rate changes to advantage. Once again, shocks should not be used to tune balance.
SHOCK DYNO
Shown below is a sample graph from the test of a shock at shock settings from full stiff to full soft. Velocity is shown along the horizontal axis and force is shown on the vertical axis. Positive numbers on the vertical axis represent compression (bump, jounce) forces. Negative numbers represent rebound (shaft extension) force. Several traces are shown, each representing a different setting on an adjuster. In this case, the peak velocity attained was 5 inches per second. The adjuster on this shock is of the open bleed type which allows oil to flow from one side of the piston to the other through an orifice in the shaft. With this type of adjuster we see changes in both low speed and high speed, compression and rebound damping.
As can be seen in the picture above, a shock can be mounted between the shaft on the base of the dyno and the load cell on the crossbar above. The shaft moves in a sinusoidal displacement pattern, compressing then extending the shock, commonly through a one inch stroke. At each end of the stroke the shock motion stops then starts in the opposite direction. At half the stroke distance the shock achieves its maximum velocity. Normally the peak velocity on a dyno run is 5 inches per second for road race cars and 10 or 12 inches per second for oval track. A stroke of 1 inch is generally enough to characterize the performance of the shock. The shock dyno is extremely useful for checking every shock after service to be sure it is operating correctly and to see that a desired change was actually achieved from re-valving. However it does not tell us everything about the performance of a shock on a car.
Maintenance!!
Over heated or worn out shock oil can smell as bad as over heated rear end oil. Yet on the shock dyno the performance has not changed that much from when the oil was new. Just like an engine, the pressure may look great but the problem is loss of lubricity resulting in wear on the engine bearings and rings, shock body and seals. Shock manufacturers are reasonably proud of their parts so it’s much easier on the racing budget to do routine oil changes than replace parts. Establishing a maintenance schedule involves changing oil at frequent intervals to begin with until one can determine the number of sessions the oil can stand before needing to be changed. One might think of simply checking the oil periodically but by the time the shock is open, the oil might as well be changed. And while open, it is also easy to check the shims to be sure none have begun to crack. Some shock companies suggest changing the oil once per season. However, especially on dirt cars or closely cowled formula cars where the exhaust passes near a shock or canister, the oil may need to be changed as frequently as after every race! On the other hand, if not raced in dirt or frequently in rain and located in the air stream on the front of an open wheel formula car, every two years may be adequate.