Guidelines for Making Accurate Crossbow Arrow Speed Measurements with a Chronograph
These guidelines are meant to be an aid to help you make accurate arrow speed measurement using a “home” chronograph. In particular, they are tailored for the F1 Chrony made by Shooting Chrony, as this is a popular, inexpensive model used by many archery enthusiasts.
The first key to making accurate measurements is to understand how the chronograph works. This understanding will help you pay attention to the small details necessary for accuracy. The second key is to understand where errors in the measurement can occur so you can minimize or eliminate them.
How It Works
A chronograph measures the speed of an object using two (2) photo sensors that are spaced a specific distance from each other and a clock. When the first sensor detects that an object has passed over it, it triggers the clock to start counting until the second sensor detects that the object has passed over it. The chronograph then makes a simple calculation of distance divided by time and reports the resulting speed. The distance is the fixed distance the two (2) photo sensors are from each other, and the time is the number of seconds counted by the clock.
Areas for Error
Distance Errors
In the case of the F1, it is designed on a folding platform. While this is great for storage and transportation, it also creates an area for inaccuracy. If the chrony is not folded out completely flat, the fixed distance the chronograph was designed to use will be off (shorter) and the resultant speed calculation will be too fast.
Photo Sensor Errors:
Light Intensity Errors - Photo sensors work by detecting small differences in light intensity hitting the photocell. So ideally you want light of a consistent intensity hitting the photocell, and preferably to have the photocell looking at a white background. The best light to use is natural sunlight, with clear, blue sky days being best. On those days, you will want to install the white diffusers on the rods of the chronograph. That way the photocells will see a completely white background and easily detect when an object passes over them. You can use the chronograph on a cloudy, overcast day, where the clouds are a consistent white, but on these days you should not use the diffusers as the clouds supply the white background. In both cases, you must use the chronograph in an area where no shadows (from tree branches, leaves, etc) fall on the chronograph, photocells or diffusers.
Indoor Use Error
You can also use the chronograph indoors, IF you can ensure that only incandescent light (from a filament type light bulb) is hitting the photocell. Fluorescent light, although it seems consistent to the human eye, “flickers” at a very fast rate (50 or 60 times a second), and the photocells are sensitive enough to sometimes pick up this change in light intensity causing erroneous readings.
Arrow Tip Induced Error
A sometimes overlooked area of error is in the type of arrow point used on the arrows being measured. It is best to use a “bullet type” field point, or even a blunt point when making speed measurements, as these will create a consistent “shadow” on each photocell, regardless of arrow rotation. Using a broadhead can add error to the measurement as it can cause the photocells to detect the arrow at different points due to the pointed style of the broadhead. For example, if one were to use a long, pointed, 2 blade type of head, the head could be verticle when the arrow passes over the first photo sensor, and may not cause enough “shadow” until the arrow shaft itself was over the sensor. But due to arrow rotation, the head could be horizontal when it passes over the second sensor and cause enough “shadow” with the first 1/4” of the broadhead. In this case you would get an erroneously, fast measurement. However, if the broadhead was in the opposite positions than described above when passing over the sensors, the measurement would be off in the slow direction.
Distance Above Photocell Error
All photocells have a “sweet spot window” where they work best. When an object passes directly over the center of the photocell through this “sweet spot window”, you get your most accurate results. For the F1 Chrony, this “window” is an area between 4” and 6” above the photocell. It is critical to have the arrow pass through the window of each sensor, as well as directly over each sensor for accurate measurements.
Distance From First Sensor Error
It is also important that chronograph not be placed too close to the front of the crossbow in an attempt to get the highest “muzzle” velocity possible. The arrow must be completely clear of the bow string before any part of it passes over the first sensor. A good rule of thumb is to have the first sensor no closer than 24” from the front of the riser of the crossbow (where the riser starts and the rail ends).
Steps for Making An Accurate Crossbow Arrow Speed Measurement
1) Assemble the chronograph, and wire rods according to the manufacturer's instructions. For the F1 that means, fully opening the unit, and fully inserting the rods in the proper holes on the black plastic parts of the units that hold the photo sensors.
2) Mark the “sweet spot window” on the wire rods. Using a ruler, measure straight up from the photo sensor (not along the rod) the distance to the bottom of the “window” (4” in the case of the F1). Use a small level to transfer this measurement to the wire rods. Repeat this for the upper “window” distance (6” for the F1).
3) Use a marker, or tape to color the rods between the two measurements.
4) Finish assembling the chronograph, installing batteries, and light diffusers if required.
5) Identify and area where you can shoot, such that no shadows fall on the chronograph, photocells or light diffusers.
6) Setup your target, and a rest to shoot from that will fully support the crossbow – so that it can be aimed at the target without you supporting it.
7) Setup the chronograph in front of the crossbow using a camera tripod or separate table or bench from the one supporting the crossbow.
Setup the crossbow so that it is fully supported and aiming at the target exactly where you want the arrow to hit.9) Line up the chronograph so that the arrow will pass exactly over both photocells. Stand back from the crossbow and sight down the rail, making sure both photocells are exactly in line with the rail.
10) Measure the distance from the first photocell to the point on the crossbow where the riser and rail meet. Make sure this distance is between 24” and 26” Repeat Step 9 if you had to move the chronograph.
11) With the crossbow fully supported and aimed at the exact spot on the target, place a laser pointer or laser bore sighter on the rail of the crossbow. These are close in diameter to a crossbow arrow and thus will give a good approximation of where the arrow will fly when it leaves the crossbow. Carefully move the crossbow left and right at the same elevation as the intended point of impact on the target and make sure the laser hits the rods of the chronograph in the center of the marks you made in Step 3. Adjust the chronograph, not the crossbow if you are off. Repeat Steps 9 and 10 if you had to move the chronograph.
12) Cock the crossbow and install an arrow.
13) Set the crossbow so it is fully supported and aim it at the target at the same intended POI as in step 8.
14) Step back from the crossbow and from behind, sight down the rail to ensure the rail is in line with both photo sensors.
15) Step back to the crossbow and fire the arrow at the target.
16) Record the result from the chronograph.
17) Retrieve the arrow. Shoot only 1 arrow at a time so you can use the same point of aim for each shot. Move the target if necessary so as not to “punch through” from shooting the same hole over and over.
18) Repeat steps 12 – 18 for every shot.
