A method for getting the best button spring setting as regards tuning that seems have to appeared in recent times is measuring arrow speed with a chronograph. The basic approach is to measure the speed difference between a fletched and bareshaft arrow using a 'gate' type chronograph. This speed difference is recorded as the button spring tension is varied across a range which includes the optimum spring setting.
If you plot a graph of the bareshaft/fletched arrow speed diffference against spring setting you get
a curve similar to the one illustrated. The spring setting where the curve has its lowest value is the
optimum spring setting as regards tuning. In practice because of various errors involved in the speed
measurement you cannot determine the minimum point from the graph (the grey area) but you can infer its position from
the observed curves either side of the optimum spring setting.
How does this approach work?. Well always remember that with any tuning process what we are varying
is how much angular velocity the arrow has at launch and in the case of the pressure button we are trying
to get the average launch angular velocity equal to zero. If with a given button setting the bareshaft and
fletched arrows are launched with the presumed same initial amount of rotation then over a given
distance, because of the 'braking' effect of the fletchings the bareshaft arrow will rotate more than
the fletched arrow, What gate type chronometers measure is not the linear velocity of the arrow but how
long it takes for the pile of the arrow to pass between the two gates. This is not necessarily the same
thing (which is why accurate arrow speed measurement with a chronometer is quite complicated). As can be
seen from the diagram because the fletched arrow rotates less than the bareshaft one the measured speed
for the fletched shaft is higher than for the bareshaft arrow. The higher the launch angular velocity
the higher this measured speed difference will be. With zero launch angular velocity the measured speed
difference between the two arrows will be just that due to the arrow (fletching) mass difference.
Measured arrow speed will also be affected by the arrow vibration but as the frequency and phase of vibration of the two arrows will be much the same it does not invalidate this method as a tuning procedure.
Last Revision 1 July 2009