Roll Axis - Roll couple distribution...
Roll Axis - Roll couple distribution technology was primarily developed for symmetrical suspension systems such as those found in production automobiles. This method involves calculating the resistance to roll of each suspension system. It does not take into account all of the dynamic effects associated with a race vehicle. An example is the additional roll angle/anti-roll angle created as a result of compression caused by downforce vs. spring split.
Let's go back for a minute and see how chassis technology was progressing through the 1960s to the early '90s. Much of the research was undertaken by the engineers for the automakers, with a heavy influence 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 for the sprung mass. The front and rear roll centers were depicted as the points of resistance to lateral forces and were connected by an invisible line or axis and a right angle line between the CG and the "roll" axis. This was the vehicles moment arm.
A moment arm is much like a pry-bar or shovel handle. The CG is equivalent to the end of the bar we hold on to, and the roll centers and axis is the opposite end, where we are applying a force. 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.
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 spring base among other factors. In subsequent skid pad testing, a neutral handling car was found to have significantly different roll couple numbers at the front and rear with a heavy emphasis on the front.
The roll angle analysis method...
The roll angle analysis method allows us to predict how each end of the car wants to react to the force created by cornering. If we know what each end wants to do and can alter the suspension components such as spring rates and roll center locations, then we can design our car to have a balanced setup prior to testing or racing. This is exactly what we shoot for by trial and error guessing. This method saves a lot of time and frustration.
The handling characteristics of the car could be altered by changing the "couple" at each end, but the handling still could not be predicted and was arrived at through trial and error methods. It soon became obvious that the "roll couple distribution" thread of vehicle dynamic analysis did not present a viable method that could accurately predict a stock car's handling performance. Something was obviously missing.
Because technology is universally shared in most cases among automotive engineers in general, we can be fairly sure that up until the early '90s, there did not exist a method that could be used to accurately predict the handling of either an F1 car or a stock car. In my discussions with some of the top race engineers I was led to the same conclusion, a definitive method and associated software did not exist at that time to accurately predict a stock car's handling characteristics.
Then in the early '90s a new method was developed that differed from all previous threads of vehicle dynamic research. It involved treating the vehicle as two separate masses, each with its own separate suspension system and desires.
This method made a lot of practical sense because in a stock car we have two "axles", each supporting their own weight, and each resisting the lateral forces created by cornering. In physics, we are taught that everything is fluid, or somewhat flexible, and dividing the car into separate halves allows us to analyze each one independently to determine its desire.
Using circles to represent...
Using circles to represent the amount of load on each tire, we can visualize weight distribution. With a low static distribution of weight (low cross weight percentage of 48-52 percent), after the weight transfer that occurs during cornering, an ideally balanced setup will have equal load on the left side tires and equal load on the right side tires.
The review of early notes kept by a select few of the top stock car crew chiefs of the '80s and early '90's indicate that they were thinking in the context of a balanced setup and thus the seeds were sown. The goal was to arrange the setup so that the two ends of the car would be balanced in each one's efforts, thereby providing a perfectly balanced and handling car.
Based on these early considerations, in 1994 a method was developed and algorithms (equations and mathmatical calculations) were written that would provide the answers needed to achieve that goal. Weights and measurements from the car and race track were entered into a computer program, and through a series of calculations, a prediction for what each of the two suspension systems desired to do was arrived at. Although the calculations were very complex, the two answers were presented as simple front and rear roll angles.
By treating each end of the car as an independent system removed from the other, we could now accurately calculate and predict what angle each wanted to roll to. The key to developing a balanced setup is to match the desires (roll angles) of each end. So, if the two roll angles are the same, we will have a truly balanced setup.