This article presents a simple method to determine the falling direction of a
motorcycle to the ground after a crash or slight impact, which can be used in
accident reconstruction. The motorcycle is divided into two groups, namely,
front assembly (front wheel, front-wheel cover, fork, handlebar, and rearview
mirror, etc.) and rear assembly (frame, body shell, engine, transmission, rear
suspension system, muffler, real wheel, and center stand). We introduce
gyroscopic moment as well as roll moment and explain their effects on the
motorcycle falling direction. The research shows that when impact takes place in
the front assembly of a motorcycle, the falling direction of the motorcycle
would depend on the gyroscopic moment, which in turn can be judged by the
direction the handlebar was turned to. If the handlebar is turned to the left,
the motorcycle falls to the right. If the handlebar is turned to the right, the
motorcycle falls to the left. For collisions that occur in the rear assembly of
a motorcycle, the falling direction of the motorcycle would be determined by the
roll moment, which is associated with the heights of the equivalent impact point
and the center of mass of the motorcycle. A simple table is included for quick
reference. People can easily determine the falling direction of the motorcycle
after an accident based on the table without knowing the theories of mechanics.
Real traffic accident cases in Taiwan are used as examples to validate the
method. The method to determine the falling direction of the motorcycle can be
used to verify the moving direction of the motorcycle before impact which is
important in accident reconstruction.
