# Golf Ball / Baseball Spin

Last Reviewed: 03/16/01
Article ID: 3502
DESCRIPTION:
Problem Summary:

How to calculate the spin of a golf ball ? The ball is moving and rotating
as the real thing. Ball is meshed with hexes.

Resolution/Avoidance:

A little tricky, but here it goes. Mesh six bar2 elements , two in each
of x, y, and z directions so that the 6 beam elements join at the center
of the ball. The outer node of each bar2 should be equivalenced to a
node in the ball but the center node should not necessarily be
equivalenced to the ball center. The six bar2s should be connected
at the center to a single node though (so translations of the beam ends
would produce rotations and average translations at the center.

Create one explicit MPC for each bar2. They will all have as independent
DOFs the rotations of the center node (the one all bar2s are connected to).
The other end of each bar2 will have as dependent rotation DOFs .
These MPCs prevent a rigid body torsion of the beams which would
otherwise create singularities.

After you run your analysis the spin of the ball will be given by the vector
rotation of the center node connecting the six beams.

Notice three beams could have been enough for an estimate but since
they may deform this six-beam solution will provide a better estimate.
It is important that the stiffness of the beams be small enough that would
not affect the deformation of the ball in any significant measure but big
enough that the system of equations would not become ill-conditioned.