Racing Suspension - Stock Car Dynamics, Part 1

In the May '03 issue of Circle Track, we ran Part One of a three-part story on stock car vehicle dynamics. Since we all tend to forget, we decided it was time to refresh our memories a bit. Since that time, many successful dirt and asphalt teams have applied the technology presented then to become more successful in their racing endeavors.

The early evolution of the knowledge of vehicle dynamics ran directly parallel to the development of the passenger cars and trucks for several understandable reasons. First and foremost, the big automakers had the funds to finance the extensive and expensive research. Also, the need existed to develop better suspension systems for ride comfort as well as drivability. Along with these two driving forces also existed the element of competition to develop more advanced cars than the other automakers in order to enhance their sales.

Early pioneers of stock car dynamic research include Maurice Olley and his group, whose work represented much of the significant progress that had been made in vehicle dynamic research. Among many other notable accomplishments, his work in the early '30s led the industry to adapt the double A-arm suspension system, or short long-arm (SLA) suspensions.

In 1952, Bob Schilling, the head of the mechanical engineering department in the GM Research Laboratory division, and his group met with a group of aircraft engineers that included Bill and Doug Milliken. The aero engineers were then contracted by GM to attempt to apply techniques that had been used in aircraft design to the study of land vehicle dynamics.

A compilation of that work, as well as other research, is contained in a book published by the Society of Automotive Engineers (SAE) titled Race Car Vehicle Dynamics.

During the period between the late '40s and the early '90s, no one had completely developed a way to predict how a stock car would handle and therefore be able to adjust the suspension components to attain that perfectly balanced setup. We knew that if we could ultimately predict the distribution of weight on the tires when the car was executing the turn, we would know how the car would handle.

What has been refined in most top racing series over the years is the art of trial and error. Advanced measuring systems are in use today that not only record movements, pressures, and temperatures, but also the forces exerted on components. These systems have become useful and necessary tools of the modern day chassis tuner and developer. As the teams compile and study all of this information, the fact still remains that many teams tend to react to, and not necessarily predict, the handling nature of their cars.

Technology related to the ability to predict the handling characteristics of your car has now evolved. It is a continuation and refinement of the work that early vehicle dynamics pioneers such as Olley started. Without their efforts, none of what we have learned over the past 10 years could exist.

Let's go back and see how chassis technology progressed through the '50s up to the early '90s. Much of the research was undertaken by the engineers who were either working for or contracted by the automakers, with a heavy contribution by the General Motors group.

The primary thread of their analysis of vehicle dynamics involved a model of a vehicle that treated the body and frame as a single unit with a single center of gravity (CG) for the sprung mass. The roll centers at the front and rear were connected by an invisible line or axis, and a right-angle line between the CG and the roll axis was the vehicle's moment arm. This is basically aeronautical engineering technology that is used to simulate how aircraft maneuver and respond to various rolling forces during flight.

A moment arm is much like a prybar or shovel handle. The CG is equivalent to the end of the bar we hold on to, and the roll centers and axis form the opposite end, which is the object we are trying to move. The longer the bar (moment arm), the more leverage we have, and the easier we can move the object (or in this case, roll the car). The bottom of the moment arm is what we have named the moment center. The older technology refers to this point as the roll center, but because the chassis almost never rolls around this point, we coined a new name for it.

In the roll axis thread of technology, each end of the car was calculated to have a given roll-resistance percentage based on the spring rates and other pertinent information. In theory, a car that had 50 percent front and rear roll resistance should have been balanced. In subsequent skid-pad testing, a neutral-handling car was found to have significantly different roll couple numbers at the front and rear.

The handling characteristics and roll couple of the car could be altered by changing the couple percentage at each end, but the handling still could not be predicted. The cars were dialed in through trial-and-error methods. The roll couple distribution thread of vehicle dynamic analysis did not present a completely accurate model that could predict a stock car's handling performance.In the early '90s, research continued and the early model was refined until an improved method was developed that represented a more accurate model. It involved treating the vehicle as two separate masses, each with its own separate suspension system. The roll couple was now measured as roll angles. This solved basic errors in predicting the roll resistance.