I have just bought myself a chronograph. Specifically, a Chrony F1 with lighting set. The lights let you use it indoors but I have no intention of so doing. It’s just that it was on sale and getting it with lights worked out cheaper than getting it without.
For those who don’t know, a chronograph is a device for measuring the speed of an arrow (or bullet, pellet, paintball etc). It’s basically a long box with a laser at each end that beams upwards. It starts an internal stopwatch as the front of the arrow goes through the first beam and stops it as it goes through the second beam. Using the recorded time and the distance between the beams, it works out the speed of the arrow. This is a brilliantly useful tool: every time you change something – fletchings, brace height, string material, arrow shaft etc – you can shoot before and after through the chrono and see what effect the change has on your arrow speed (and therefore on your trajectory and point of aim: see earlier posts on arrow speed). I explained all this to Claire, my loving and long-suffering wife. She replied “you want it because you’re an archery geek and it’s a cool new toy”. She was, of course, entirely right; but it is also a really useful piece of kit.
The chronograph is famous among archers for its disappointments. We all expect our favourite bow to be whipping arrows out at 200 fps or faster. Then we get to a chronograph and discover that we are barely touching 180. Like all good science, it can be a cruel destroyer of our cherished illusions, but one that should lead us to make informed changes to improve our situation.
Today the chrono arrived and I took it down the woods for a play, together with my Border Ghillie Dhu and a variety of arrows to test. I ended up comparing the following (all made by Easton and listed: type; spine; mass in grains; grains per pound of draw weight (gpp)):
XX75 Platinum Plus; 2016; 431gn; 11.65gpp
XX75 Tribute; 2016; 429gn; 11.59gpp
X7 Eclipse; 2014; 372gn; 10.05gpp
X7 Eclipse; 2114; 390gn; 10.54gpp
A/C/C; 3-39; 384gn; 10.38gpp
Note: I have assumed for these purposes that I was drawing 37lbs. This is the marked weight of the bow at 28″. I did not measure the draw weight or my draw length for these purposes, since the aim was to compare arrows rather than necessarily give accurate gpp readings.
Judging purely by arrow mass, therefore, we would expect the Platinum Plus and the Tribute to be of similar speed, with the Tributes maybe a tiny bit faster. We would then expect the 2114 Eclipse to be faster than those two, with the 2014 faster still. We would expect the A/C/C to be somewhere between the two sizes of Eclipse. Sounding a note of caution here, I will say that I had limited numbers of the X7 Eclipses and had a few error readings (basically I missed the beam). This means that the speeds for the X7s may be less reliable than the others.
There are other variables beside mass. One is the nocks. Nocks can be tremendously important to arrow speed. The nocks on all of these arrows were the same, except for the A/C/C, which has a rather better (and more expensive) nock. The remainder all used the same nocks. The situation is complicated a little by the fact that the nocks on the aluminium arrows (i.e. all except the A/C/C) have been splayed and this can mean that they vary slightly in fit. We shall keep this at the back of our minds while remembering that I shot at least 3 of each type of arrow and then took an average, which should mean that differences in nock splaying cancel each other out.
Air resistance can be a big factor in arrow speed. It is determined by a variety of factors including thickness or shaft and size/setup of fletchings. The aluminium shafts all have the same fletching but in any event we would not expect resistance to make a significant difference at the short range here (I was standing about 3ft from the chronograph).
The average speed of the arrows is noted below (in feet per second):
Platinum Plus: 173.53 fps
Tribute: 177.59 fps
2014 Eclipse : 188.16 fps
2114 Eclipse : 182.38 fps
A/C/C: 194.42 fps
As we expected, then, the Tribute marginally outshoot the PP. Both are outshot by the Eclipses, with the lighter Eclipse faster than the heavier one. The anomaly is the A/C/C. This went faster than expected from the simple masses. One explanation may be the nocks. Another can be seen when the raw data is examined. On one shot I really went for it, nailing the release, pushing the bow hand forward and probably drawing quite a bit further than usual. The result was an arrow that went at 208 fps! If you remove that arrow and stick with a fair comparison of regular draws, the A/C/Cs drop to 187.65, which is in the expected range, slightly higher than we might expect, but that’s likely to be the nocks. This removal of the faster arrow is not some form of special pleading, by the way: it is perfectly sound to remove an anomalous result from a set of statistics. This result varied from the mean by more than twice what any other arrow did and produced a result at odds with all of the rest of the data. Keeping it in would be poor science.
The final adjusted results, therefore, are:
XX75 Platinum Plus; 11.65 gpp: 173.53 fps
XX75 Tribute; 11.59 gpp: 177.59 fps
2114 X7 Eclipse; 10.54 gpp: 182.38 fps
4/40 A/C/C; 10.38 gpp: 187.65 fps
2014 X7 Eclipse; 10.05 gpp: 188.16 fps
This is in accordance with our expectations of reduced mass bringing greater speed. What lessons have I learned? Well, for starters I shall stop buying Platinum Plus and go back to Tributes as my basic arrow. I shall also look at investing in another set of X7s. Quicks Archery very kindly sponsored me and Claire by giving us reduced rates on X7s before Korea last year. These have gradually vanished into the undergrowth or been bent around trees or target frames and the time has clearly come to replace them!
I do not claim that this is anything approaching a perfect experiment. There are various things that should be (and will be) addressed. One is shooting more arrows so as to get a better idea of the average speed. Another is getting somebody else to shoot it, to try to obviate any unconscious bias (although I did this to some extent by not weighing the arrows until after I had shot). My draw length is not as consistent as it should be, although this should have evened out between the different arrows.
This is the first in what will be an ongoing series of articles where I get to play with the chronograph. I shall set out more detailed and scientific findings in future posts, in which I hope to deal not just with the effect of changing other variables such as brace height but also to try a variety of bows and do some kind of comparison (I am intrigued by the number of relatively inexpensive bows out there that advertise speeds over 200 fps without giving a draw length, draw weight, arrow weight etc. After reading this post, I hope that you will share my cynicism of such claims).