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Computer Simulation Ensures Motorcycle Stability
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.
Setup
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 Dick.Kading@lmscae.com.
Visit LMS online at www.lmsintl.com. Visit BMW online
at www.bmw.com.
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