Search Results

Automotive suspension geometry describes the kinematic movement of a car's suspension based on a theoretical analysis of measured or designed points. This geometric analysis can be done with pencil and paper on a drawing board or with simple physical models, but is more commonly analyzed by the means of specialized software. By contrast, Kinematics and Compliance (K&C) machines measure suspension parameters as loads are applied to an existing chassis and vehicle suspension. This paper will compare the two approaches as applied to a stock car racing chassis. Significant agreement between the theory and measurement will be demonstrated. Significant differences such as tire deflection will be explained. Additional differences due to compliance of “solid” parts will be described. The paper will also describe suspension parameters which can be measured directly, such as camber and toe, and those which must be calculated, such as caster, kingpin and, instant centers.

Roll Centers are an important tuning tool but their importance is often misunderstood. The roll center has too often become a mysterious concept rather than a simple descriptive parameter. Roll Centers may be determined from forces (the Force-Based Roll Center) or the more familiar kinematic method. This paper will explain and reconcile the difference between these roll centers. That explanation will show how the roll center should be used in understanding the behavior of a vehicle. The most familiar roll centers are for independent front suspensions with two a-arms or wishbones, also known as SLA or Short Long Arm suspensions. For these suspensions this paper will derive a method to construct a more accurate kinematic roll center which explains the differences between the Force-Based Roll Center (FBRC) and the Kinematic Roll Center (KRC). The common kinematic roll center is based on four links between the upright/spindle/wheel/tire and the sprung mass of the chassis.

Ackermann steering geometry relates the steer angle of an inside tire to that of the outside tire. When turning the inside tire travels a shorter radius than the outside tire and thus must have a greater steer angle to avoid tire scrub. Classic Ackermann minimizes scrub by positioning both tires perpendicular to the turn center. It can have a significant impact on tire wear [1]. Ackermann analysis can also be used as a tuning tool in cases where classic Ackermann may not be the objective. Ackermann has been around longer than the motor vehicle - over a century - but there is little rigorous analysis in the literature. There are two common measurements of Ackermann which give very different results. Both are used in texts and computer programs[8] Yet a literature search revealed only a couple sentences discussing the relationship between the two. This paper presents a mathematical analysis of Ackermann which explains the two measurements and develops a formula relating the two metrics.

It is time to introduce a new component to motorsports engineering - the driver. SAE papers rarely deal with the cognitive control system which fills the space between the steering wheel and the seat. It seems that only the safety papers admit the presence of a driver, and they treat the driver as a passive object to be protected. It is the driver who controls the race car. It is the driver who utilizes, or misuses, the capabilities of the car. It is the driver who chooses a path for the vehicle. It is the driver who decides how to use the traction circle to negotiate a turn in hopes of optimizing lap time. The traction circle is a G-G diagram of longitudinal acceleration as a function of lateral acceleration. It defines the capability of a vehicle to combine acceleration with cornering while exiting a turn or deceleration with cornering while entering a turn.

Test-driving is a specialized art. Automotive manufactures, parts suppliers, and tire manufacturers employ test drivers to evaluate their products in a variety of circumstances. But Honda and some other firms prefer the automotive engineer test his own product. This gives direct feedback and provides a better “feel” for how the vehicle reacts. It produces a better car and a better engineer. Some Formula One teams send their race engineers to a racing school. Test drivers can be trained at commercial racing schools. These effectively teach students to drive at high speeds near the limit of the vehicle. The test driver must have the skills to perform a test with minimal danger to the driver and the vehicle. But the demands of a test driver are not the same as a racing car driver, though many test drivers also race. The test driver must evaluate the vehicle as well as drive fast. The test driver must faithfully execute a test plan while observing vehicle behavior.

The roll center is an important analysis tool for vehicle dynamics. But most analysis of the roll center is based on production cars, which usually have symmetric suspensions and a center of gravity near the centerline of the vehicle. Racing cars, particularly oval track stock cars, often have asymmetric suspensions and usually have a weight bias. For a car that is only going to turn left there is no reason for the left side front suspension to be anything like the right side. Oval track cars usually have as much weight on the inside as the rules allow. Analytical tools adapted from the standard industry texts or production car use do not properly address asymmetric suspensions. This paper will analyze the asymmetric suspension and discuss the role of the roll center. It will begin with a theoretical analysis of the roll center and the underlying assumptions.

Engineering in the Motorsports arena of ten involves the detailed comparison of two laps of data. Plots of data acquisition channels as a function of distance allow analysis of driver and vehicle performance at the same point on the track. Distance is computed from the Start/Finish line or wherever a timing beacon is placed. The distance function is usually defined as the integral of speed, which is calculated as the sum of discrete speed values. Speed is usually measured as the RPM of a wheel, usually a non-powered wheel, such as a front wheel for a rear-drive race car. The calculated distance function is subject to errors that can harm the analysis. For the four-mile Road America track this means counting the revolutions of a wheel from the timing beacon to a location up to 6,400 meters away. A skilled driver considers a variation of 3 meters a large change in braking point. This represents rolling a 27.5-inch diameter tire 2,900 times and defining a point within one revolution.