Measuring pivot points as accurately as possible is essential in order to acquire reliable information about mass distribution.
Pivot points occur in pairs and are an effect of the rotational inertia of the sword. Any given point along the length of the grip has a corresponding point in the blade. A method to observe these, called the waggle test, is to let a sword hang perpendicularly, pinched between two fingers. If the hilt is moved sideways back and forth a place in the blade will remain stationary. Depending on the action point (the point gripped), the pivot point in the blade will be found in different locations.
There are several mistakes that can disturb this measurement. First, if you pinch too hard and apply torque on your gripping point, it will unsettle the oscillation. That is why the grip must be as light as possible. Second, if the motion of your hand is not quick enough, there will be a motionless point on the weapon, but it will not be the actual pivot point that unlocks the properties of mass distribution. It is only when the frequency becomes high that the motionless point converges to the pivot point. Third, the motion of your hand should be as horizontal and straight as possible, otherwise the observation of the fixed point becomes very imprecise. The Dynamics Computer includes a correlation procedure that helps detecting measuring errors and mitigating their effect. We have found that a simple aid such as wrapping a hair tie or rubber band around the blade and adjusting its position until it seems to lie at the fixed point can increase precision.
When a sword strikes or is struck it will vibrate. There are two main places in a sword where vibrations are cancelled out. These are known as vibration nodes. The weapon appears to be stiffer when manipulated from these locations. In the majority of swords from the high Medieval period to the Renaissance that the authors have studied and documented, the location of the vibration nodes of the hilt and the blade are also nearly corresponding pivot points. This makes the blade node suited for especially powerful blows, creating a “sweet spot” there. Therefore the effective mass at the blade node gives a good idea of how much mass the sword would hit with.
The other measurements needed to fill the Dynamics Computer are straightforward and well known. Here is an example of a complete set of measurements.
They are further explained in the accompanying Walkthrough. We hope you will enjoy building graphs for the swords you come across, and sharing them with the world!