In the area of maintaining conventional motorcycle stability, engineers have established a series of tests that measure the wobble, weave, and kickback of every new model before it is released for production. While physical testing provides a nearly perfect measurement of a prototype or production motorcycle, the number of design alternatives that can be considered is limited due to their high expense and time-consumption.

Alternatively, utilizing computer simulation allows engineers to precisely determine the range over which desirable stability characteristics are maintained, and investigate a significantly wider range of potential dimensions. Design parameters under evaluation can also be changed with relative ease (e.g., lengthening the wheelbase or reducing weight). Simulation results are then validated on the test track before a motorcycle is brought to market.

BMW engineers employed LMS DADS mechanical system simulation software, a multi-body dynamics simulation software, to create virtual prototypes of vehicles and other mechanical systems with general constraints, gravitational forces, and forces due to contact. Although the program includes a tire model, motorcycle tires can roll up to 50 degrees, creating forces that cannot be captured in a conventional model. Thus, BMW engineers took advantage of the program's open architecture and wrote two subroutines to measure wobble, weave, and kickback.

The first subroutine models the driver's actions by controlling the velocity of the motorcycle, moving the handlebars, and operating the brakes. It consists of two separate elements comprising the upper and lower body that are connected with a spring damping mechanism that allows the driver to swivel its upper body. The second subroutine models tires by determining the force applied between the tire and the road. This dynamic subroutine can capture a phenomenon called relaxation in which tire forces do not appear instantaneously, but rather develop over a short period of time. The number of degrees of freedom of the motorcycle tire model can vary, but is typically about 26. The major elements of the model include the mainframe, lower and upper fork, telelever, front and rear wheels, swing arm, and several smaller components. Mass properties, including the center of gravity and moments of inertia are added to the model. Stiffness issues are addressed by incorporating springs and shock absorbers and structural flexibility into the model.

Results

This model has proven the ability to simulate common stability tests with a very high level of accuracy. Engineers consider the simulation results at least as accurate as the measurements they are able to take on the test track. For example, in the weave and wobble test, the simulation provides a damping measurement that is accurate to approximately one second.

The models permitted engineers to measure far more product variations then would be possible if each one had to be built in the prototype shop. As a result, BMW is now able to specify structural design requirements, such as stiffness and mass distribution, that will ensure the stability of the end product. BMW has several projects underway to improve the effectiveness and breadth of their simulation models. They are developing the capability to determine the performance of a design in driving over a particular road profile. They are also improving the accuracy of their models by incorporating flexible elements that more accurately simulate the response of the frame. <

More Information Contact Dick Kading, senior engineer - Multibody Simulation, for LMS CADSI (Computer Aided Design Simulation, Inc.) located in Coralville, Iowa, at This email address is being protected from spambots. You need JavaScript enabled to view it. . Visit LMS online at www.lmsintl.com. Visit BMW online at www.bmw.com.