A Dynamic Duo - by Doug Gore Technological developments in racing used to be obvious. Engines grew and developed more power, cars became lighter, tires gained width and lost tread, wings sprouted wherever possible. Technology has continued to evolve, but recent developments have been far less noticeable. Take shock absorbers. Once used solely as a dampening device, shocks are now a key factor in chassis setups. They can help loosen up a push just as the car begins to turn in and they can tighten up the same car as it straightens out. How is this possible? Because shocks individually affect wheel bump and rebound rates. To understand this effect, consider a car that is traveling at a fixed speed near its traction limit on a circular skid pad. In this case, the car’s balance will be determined by the relative grip of the front tires compared to the rear tires as weighted by their moments acting on the car’s center of mass. If the car had a 50/50 front-to-rear and side-to-side static weight distribution, the relative moments can be ignored and only the total grip of each pair of tires will be important. But a cornering car always transfers some of its static weight from its inside pair of tires to the outside pair in proportion to its speed, track width and center-of-gravity height. In addition, the end of the car with the most equally loaded pair of tires will have the greatest total grip. Therefore, the front-to-rear distribution of transferred weight from the inside to the outside will determine the car’s overall balance. If a greater percentage of transferred weight goes forward, the front tires will have a greater difference in their loading and the car will tend to understeer. If more weight goes to the outside rear tire, the car will tend to be loose. Since the front-to-rear distribution of the car’s total weight during cornering determines the car’s balance, setup variables that affect dynamic weight transfer distribution partially determine the car’s balance. Taken together, those variables define the car’s front and rear roll stiffness. Individually they include corner spring rates, antiroll bar sizes and the suspension geometry. For example, using stiffer front springs or a stiffer front anti-roll bar increases the front roll stiffness and the percentage of the transferred weight that will go to the outside front tire. Both of those changes increase understeer. These vehicle dynamics apply on race tracks as well as skid pads. But race tracks introduce additional complications because the car is no longer cornering in a steady state. On an oval track there are frequent changes in acceleration, both radially (cornering) and in speed. These accelerations almost always result in dynamic changes in the car’s levelness, both in roll and pitch. For this situation, the front-to-rear distribution of the roll stiffness also depends on the low-speed dampening forces of the car’s shock absorbers since they also resist roll and pitch changes. For example, increasing the low-speed bump stiffness of the right front shock will increase roll stiffness of the front end when the car is increasing the amount of roll in a left turn. That increases understeer, but only during the instant when the car is actually rolling to the right. Once it has fully rolled, the shocks no longer affect roll stiffness distribution. These basic principles help you understand how shocks can affect the understeer/oversteer balance of a race car, in addition to determining how well it will handle when going over bumps. To do that, you need to determine exactly what the shocks’ dampening rates are. That's what shock dynos are for. Most serious racers would not consider changing shock absorbers without having first checked them for rate. Until recently, however, shock dynamometers ran $20,000 and up, well beyond the budget of most Saturday-night racers. That is changing. Several companies, including Roehrig Engineering, are now offering shock dynos in the $10,000-$12,000 range. While that is a lot of money for a small race team, there are at least two other less expensive dynos on the market. One is built by Herb Brinn, the other by ND Tech. Roehrig Engineering has set the standard for racing shock dynos. They currently produce five models priced from $11,000 to $55,000. Their smallest is powered by a three horsepower motor, their largest uses a 30 horsepower motor. All five are computer controlled and each will tell the serious racer almost everything there is to know about shock absorber forces. We used a mid-priced Roehrig dyno as our standard of comparison for evaluating the less expensive dyno we tried. If you can afford one, it is hard to improve upon the performance of Kurt Roehrig’s machines. Herb Brinn collaborated with the late Howard Millican and Millican’s wife Anita to design a less expensive shock dyno. Like the Roehrig, Brinn’s dyno uses an electric motor (two h.p.) to stroke the shock under test. A precision load cell is used to measure the forces produced and two digital meters display the compression and rebound forces with a stated accuracy of ±0.1%. The peak speed of the shock is easily selectable between either 4.7” per second or 13.3” per second by changing the stroke of the crankshaft on the output of the gear motor. The two stroke speeds provide data points for both low-speed dampening and moderately high-speed dampening. Since this dyno will continuously cycle a shock, it is easy to evaluate the effects of increasing temperature on dampening forces. When cycled at the dyno’s higher speed, out test shocks became hot enough to boil spit within a couple of minutes. The elevated temperatures significantly changed the dampening forces produced, just as during a race. This sturdy benchtop dyno has a heavy welded steel frame and an aluminum cabinet. We found it quick and easy to use. It is highly repairable and as the data from our evaluation shocks indicates, its test results are in excellent agreement with Roehrig dyno measurements. This machine has a base price of $5,975 as shown here, and can be upgraded for computer data collection and variable speeds at extra cost. This is a good, solid machine that can provide many years of continuous use. ND Tech’s Leo Anderson has taken a totally different approach to building a shock dyno. Instead of an electric motor, he uses an air cylinder to apply a known force to the shock and then measures the time it takes the shock to move 2”. Its velocity in inches per second can then be computed by dividing the measured time by two and reciprocating the result. Adjusting the precision gauge and regulator varies the force on the shock. The tests can be quickly repeated in five-pound pressure increments, or any other increments, and the stroke times recorded on the handy pad provided. We found this unusual shock dyno to be a little slow, but it produced very consistent results. Doug Gore is the Technical Editor for Stock Car Racing and Open Wheel Magazine. This article is reprinted from the May ‘98 Open Wheel.