Electronic point for arrow flight: Barton 2011 study

The authors, a group from the Tyndall National Institute in Cork working with an American company in the field, present an electronic point that screws in place of the normal point and records the arrow's flight. A cylinder 9 mm across and 40 mm long holds a three-axis accelerometer, a shock switch that starts the recording, a microcontroller and memory for four shots. Once the distance and arrow weight are entered, the system works out launch and impact speed, kinetic energy, momentum, time of flight and drag coefficient.

The main comparison took place at the ballistic range of NIST, the American standards institute, against its reference chronograph and three commercial chronographs. Over four shots the point gave a mean launch speed of 82.09 m/s against 82.575 for the reference, 0.587% lower, with a standard deviation of 0.067 m/s against 0.448. The commercial chronographs, which gave whole numbers, read between 1.5% and 1.9% below the reference. Only the point could also measure arrival: 78.193 m/s at impact and 173.926 ms of flight over 13.94 metres.

Before designing the housing, the authors had estimated the accelerations it would have to withstand from high-speed footage of seven shots from a compound bow: about 1,100 g at launch and about 3,700 g on impact with a bag target.

To estimate the accelerations, two cameras at 5,000 frames per second filmed launch and impact, using a Mathews Drenalin compound set at 64 pounds and a 29-inch draw, with a light 23.2 g arrow marked at known intervals; position over time was fitted with a polynomial and differentiated twice. This gave the design limits: under 1,200 g at launch and under 5,000 g at impact for most compound bows. The aluminium alloy housing, with the standard 8-32 insert thread, was drop-tested at 5,000 g.

The accelerometer (a 10-bit ADXL345 with a 16 g range) records 3,200 samples per second for 800 ms once the shock switch passes 150 g. Launch and impact push the sensor off scale but remain recognisable in the signal, while free flight stays within range. The point weighs 6.48 g. For the NIST comparison the authors used a Spot-Hogg Hooter Shooter machine with a pneumatically triggered release, an Elite Hunter compound at 60.6 pounds and 29 inches, and 27.5-inch Easton ST Epic arrows weighing 24.753 g including the point, at 13.94 metres. The authors say they built up to 70 points and used some for more than 100 shots with no loss of performance.

The stated accuracy, within 0.59%, comes from four shots with one bow, one arrow and one distance: a promising indication rather than a validation. The point read slightly below the reference on three of the four shots (82.00 against 81.97 on the third), and its much smaller spread compared with the NIST chronograph may mean that the calculation smooths out differences between shots; the paper does not discuss this. Impact speed and time of flight have no independent reference, and there are no data on the drag coefficient or on shaft vibration, which the abstract lists among the quantities the system can measure.

The system is designed for compound bows and hunting. One of the authors belongs to the American company that developed the product, and the paper is a short contribution to a sensor engineering conference, with no statistical analysis.

For an archer the most useful finding concerns commercial chronographs: in this test the three models read between 1.5% and 1.9% below the reference, more than a metre per second out of 82, and gave only whole numbers. Anyone comparing a reading from one of these instruments with one from another chronograph, or with a manufacturer's stated speed, carries an error of more than a metre per second. With the same instrument the systematic offset should matter less, but the rounding to whole metres per second remains: a small difference between two set-ups, say two points or two arrow weights, only shows up over many shots and in the same light.

A point that measures the whole flight shows how much speed the arrow loses before the target, which depends on weight, fletching and shape: here from 82.09 to 78.193 m/s over 13.94 metres, a figure that applies to one set-up only. For recurve archers this remains theoretical, because a 6.48 g electronic point is not the point they shoot in competition, and fitting it would change the arrow's balance and dynamic spine.