Let's start with skidpad numbers, how to measure them, and what they really mean. The skidpad is generally a 200- or 300-foot circle that the vehicle negotiates as quickly as possible. The lap time is then converted to a lateral acceleration number that is commonly referred to as the g number. Keep in mind that this is an average g number. Typical vehicle-mounted accelerometers will produce much higher peak numbers. These peak g numbers can be far above an average g number created on the skidpad. We've seen street cars that can pull a peak g number well above 1 g, but it's tough for that same car to come close to that on an average g. When testing on a given circle, it's important to straddle the circle with the center of the vehicle. Testers will often cheat by placing the outboard tire on the circle, reducing the diameter of the circle by a couple of feet, which improves the lap time and increases the average g number. Here's the formula: lateral acceleration = 1.227 x (radius of circle/time2). Let's say we have a 200-foot circle and our best lap is 11.5 seconds. We plug these numbers into the formula: lateral acceleration = 1.227 x [100/(11.5 x 11.5)] = 0.927 g.
The skidpad is an indication mostly of tire traction since the suspension is already loaded, so the shocks are not contributing much to vehicle performance. This is more a test of spring rate, front or rear suspension design, and tuning and how well each end of the car works in relationship with the other. The g-force is limited to the speed at which either the understeer or oversteer is manageable. Ultimately, a better suspension and stickier tires will increase the speed and therefore the average lateral g number.
A better evaluation of the entire suspension is the slalom test in which several cones are spaced a given distance apart and the vehicle is driven as quickly as possible through them to achieve its highest average speed. Our slalom tests are done with seven cones spaced 70 feet apart with the course length at 420 feet. The higher the average speed through the course, the better the car handles. Testing with a stopwatch is a low-tech way to accomplish this but is fraught with inconsistencies because the timer must be accurate with his reactions. We use a Brower Timing Systems portable, wireless system with sensors that trigger when the car enters and exits the slalom and send the data to a handheld receiver. This is a relatively simple, wireless system that eliminates timing inaccuracies. The biggest drawback to this particular test procedure is that there is no standard distance or number of cones that must be negotiated. Each magazine or test organization has its own procedure and slalom distance, which makes comparisons difficult.
However, this type of test gives us a great baseline from which to do evaluations on modifications to the suspension and is a great tool for vehicle response to dynamic suspension loading. Basically, it consists of a series of aggressive lane changes, which loads the suspension harder as the car nears the end of the course. Often, this results in severe oversteer or understeer situations. Of course, hitting a cone results in a time penalty and negates the run. An onboard electronic accelerometer can accompany this type of testing to evaluate instantaneous g-loads and would be of some benefit, but the elapsed time is the most valuable test, since that represents that car's ability to maintain a high average speed throughout the entire course.