Adams/Car

"The total cornering power of two equally loaded tires is slightly greater than the cornering power of one tire with twice the load" ("Race Car Engineering and Mechanics", Paul Van Valkenburgh, page 34). This is the basis for understanding weight transfer of a race car in transient and steady state cornering. The end (front or rear) that transfers the most weight will have the lesser traction (assuming a static weight distribution close to 50/50 for the front and rear). This weight transfer can be very different between transient and steady state cornering. In static cornering, with a smooth road, the shock absorber settings will have no effect on the handling capability of the car. This is because a shock absorber is designed to dampen oscillation created by spring compression and elongation.

Looking at Figure (1) it is clear that in order to receive a variation of force from the shock absorber there must be velocity. This is not present in steady state, but will be very evident in transient (initial turn into a corner). An anti-roll bar will have an effect on both steady state and transient cornering. This is because it acts as a secondary spring that only effect the chassis in roll.

In this research the aim was to verify that the program Adams/Car 12.0 would accurately model what has been described. This was done with a model of a Formula SAE (Society of Automotive Engineers) provided by MSC (creators of Adams). A semi-accurate geometric model of Colorado State University's FSAE race car was also provided by students from the spring 2003 semester.

Figures (2) and (3) are shown below with rollover. Both figures are based off of a constant radius corner simulation in which the car is slowly accelerated to its limit. This limit is easily seen by the sudden/large change in each curve. The y-axis on both Figures is lateral acceleration in g's. Figure (2) shows how the ultimate cornering capability of the car changed when the stiffness of the rear anti-roll bar was changed in lbs/deg. Figure (3) shows the same curve when the shock absorber settings were changed with the front anti-roll set at zero and the rear at 50lb/deg.

The curves in Figures (2) & (3) have distinct differences. First, looking at the top left, each curve enters the figure in a slightly different location. This is due to the initial turn in onto the circle that most of the simulation was run on. This initial turn causes the shock absorbers to move and thus provide a damping force. It also puts a force on the anti-roll bar. This combination of effects caused the car to transition into the circle differently. Now, looking at the bottom of the U-shape of each curve. This is the point at which the car could no longer continue around the circle and still increase velocity. In these cases the car had a severe push condition (understeer). On Figure (2) each curve has a different maximum lateral acceleration. This results from the effect anti-roll bars have on steady state cornering. In this case the stiffer the rear bar was made the larger the lateral acceleration capable. It is important to note that this is not always true, but because the car had a severe understeer problem a stiffer rear anti-roll bar helped transfer more weight to the front wheels. In an inverse case, if the car were to have a oversteer increasing the front bar would help. This same change in lateral acceleration capability due to anti-roll bar settings is not seen in Figure (3). The limit for each curve is nearly identical. This is because in steady state the shock absorbers have zero velocity and therefore provide no force. The initial turn in into the circle is what caused these maximums to occur at different times.

This is a front and rear view of Colorado State University's newest chassis designed by graduate Lucas Wiedner. These screen shots were taken from Adams/Car and are nearly 100% geometrically accurate of this generation of car. Future work will include simulation of this car, and comparison of the theoretical expectations presented by Adams to those found through testing the actual race car next year. This will require, at minimum, shock velocity sensors, and the use of a combination of accelerometers. The race team has a product called the G-Cube that can measure acceleration in all three dimensions. This in combination with the velocity sensors would allow for some good comparison. It would also be helpful to have a measurement of force on the shock absorbers.