USAR Pro Cup Race Car - The Seven-Post Rig

"In order to make the car more consistent lap after lap, the focus of the test was to create a more stable platform out of the vehicle under transient conditions. Using the drive file created to replicate the track, overall grip and body motions were measured throughout the whole lap as well as an isolated area through the dog leg. This allows focusing attention on the most difficult part of the track compared with the complete lap and evaluating the different effects of the changes made on the dog leg versus the whole track.

"Body roll and pitch were considerable issues when the vehicle first came off the track and onto the rig. Through a number of setup changes on the rig, the body motions were reduced significantly by the end of the test. Figure 3 presents the body roll and pitch from the first run to the last run on the seven-post rig."

Editors Note:
It may be debated as to the effectiveness of the goal to reduce body roll in the pursuit of performance on a short track. Some argue that the improvement in aero efficiency necessitates this goal. Others argue that the methods needed to achieve this goal, i.e. very stiff antiroll bars, stiff rear springs, and so on, cause the setup to be very hard to maintain and in many cases not very consistent. We will leave that decision to the individual team.

"The first run is in red while the last run is in green. The overall roll is significantly less as seen in the graph (Figure 5) while the nose stays down better through both corners in the pitch graph. This optimization also includes the evaluation of tire grip by evaluating the variation of the contact patch load. The more this signal varies, the lesser grip the vehicle will experience on track.

"In order to evaluate the compromise between grip and body control, RaceWorks Inc. presents the information with a graphical approach as well as in a spreadsheet that highlights the optimum setup. As it can be appreciated in Table 1, the best grip and body control numbers can be easily spotted. The green cells represent the lower values and hence, the best configuration, while the red cells represent the worst result. These results are evaluated alongside the graphical results so that all parameters are considered before assuming the best setup. Compromises are often unavoidable."

Frequency Sine Sweep Results
"Frequency sine sweep tests allow the analysis and characterization of the suspension system of a vehicle. This is a standard test on seven-post rig facilities and consists of exciting the vehicle with a sinusoidal motion with all wheel actuators moving in phase. This input signal starts at a low frequency (i.e., 0.5 Hz) and ends at a higher frequency value (i.e., 25 Hz) while maintaining the maximum stroke velocity constant.

"By analyzing the vehicle's reaction to the sine sweep, it's possible to find out where each corner of the car was stable and where it became unstable. The way this is accomplished is by analyzing the input versus the output of each corner of the vehicle. A graphical representation of this comparison for the left front wheel output vs. the left front input can be seen in Figure 4."

"The red trace in the graph is the setup straight from the track test while the green line shows the final setup at the end of the seven-post test. The large variation in the red line shows that the grip is inconsistent throughout the frequency range, thus the vehicle will be inconsistent to drive. The green line was from the last run of the seven-post test where it was possible to create a much more consistent response.