July 18, 2011
I love those old movies where Davy Crockett and Daniel Boone are running around the wilderness shooting charging bears with their old flintlock rifles. Growing up, I wished I was them. One of my lasting memories from watching those shows was the click, pause, boom that occurred every time those great mountain men pulled the trigger on those guns. It was a romantic image of early woodsmanship, but I bet it wasn't so romantic for the men of the day when they flinched during that pause between the click and the boom and that grumpy wounded bear then ripped their arm off!
I think nostalgia would become less interesting for me about the time that hurried shot went astray. Good form is required to hold a gun's aim steady for a long period. The longer it takes between the initiation of the shot and the actual departure of the bullet, the more time the shooter has to influence (harm) the outcome. Gun buffs call this lock time. A short lock time means a more forgiving rifle. In this column, I am going to look at the same subject but through the eyes of an archer. My goal is to find ways to reduce the "lock time" on a bow, and thus make that bow more forgiving.
Lock Time and Brace Height
Brace height is the distance between the back of the grip and the string. Typically, we look at brace height as the sole way to change the amount of time the arrow is attached to the bow and thus under its influence. This is only part of the story, but I'll come back to the rest of the story later. First, let's discuss brace height.
Consider this example: Suppose you are prone to snapping your bow hand closed the instant you trigger the shot. Many people do this, and I am guessing most never realize it. This causes a slight movement of the bow that can potentially move the arrow off target. If the arrow is still attached to the bow -- still under its influence -- when this movement occurs, accuracy will suffer.
If you can get the arrow off the bow sooner, there will be less time for the impulse created by your bow hand to do its dirty work. A reduced lock time translates into a more forgiving bow.
That is the argument for a higher brace height -- it has a shorter lock time than a bow with a lower brace height. That makes some sense, but we also need to look at the numbers.
If the arrow is moving 275 feet per second by the time it leaves the bow, that means that it takes just a bit more than .0003 second (let's say .0004) to travel one more inch. Come on, there is not much we can do in .0004 second that will influence the course of the arrow.
- When considering how forgiving a bow may be, you have to look at more than just its brace height. String speed must also be taken into account. Lock time is a better measurement of forgiveness.
But that only tells half the story. Let's keep bumping up the brace height. If we compare a bow with a six-inch brace height to one with an eight-inch brace height, the arrow on the low-braced bow is still picking up speed when it goes past the eight-inch point. So, it is not traveling 275 fps at that point, but something less. So, that means the arrow will stay attached to the low-braced bow more than .0008 second longer. Maybe it is actually something like .0012 (or thereabouts).
You are only looking at a little more than a millisecond of increased lock time -- a millisecond! We're making all this fuss about higher brace heights being more forgiving just to give us a millisecond less lock time? No thanks -- I'll take the speed. I am suddenly much less enthusiastic about "forgiving" bows.
Lock Time and Arrow Speed
Brace height is only part of the equation to determine the lock time on a bow. In fact, I think I can prove it is the least significant part. Blake Shelby at PSE first got me thinking about this when PSE came out with the X-Force a few years ago. His point was that since the string on this bow was moving so fast, the arrow got off the string sooner on the X-Force than it did on other, slower bows, even though they had higher brace heights. His point was that a faster string creates a more forgiving bow.
It is time to look at the numbers again. Let's say you shoot an arrow at 300 fps while your buddy shoots his at 275 fps. You both have bows with a seven-inch brace height, but you are shooting a lighter arrow than he is. The arrow accelerates from zero up to its maximum speed in a non-linear fashion (the force curve is not a straight line). Because of that, it isn't simple to figure out exactly how long each is on the string, so I am going to ballpark it. If you are a college physics major, please turn away now. Assuming constant acceleration, I can come up with an average velocity over a typical force curve. That might get me a C- in class, but it is close enough for this study.
Following this route, I find that the 300 fps arrow is on the string for roughly 13 milliseconds, while the 275 fps arrow is on the string for roughly 14 milliseconds. Again, we are dealing with a small difference. Big deal.
But what if you are shooting 340 fps and he is shooting 275 fps? Now your arrow is off the string 3 milliseconds sooner. That is likely to make a difference.
Bringing it Together
When you start looking at these numbers, you realize that sometimes, we as archers are really splitting hairs to find something to argue about when comparing bows. Yes, if you make a significant upgrade to your bow speed, or a big increase in brace height, I honestly think the bow will be more forgiving because the arrow is gone sooner. I think that might be a noticeable change, because you are dealing with lock times that are reduced by several milliseconds.
But small changes in arrow speed (+/- 25 fps) and small changes in brace height (+/- 1 inch) are not really noticeable. Literally, you are dealing with a millisecond of lock time here and a millisecond there. It is not worth worrying about.
Big changes are noticeable; small changes aren't. I hope that will help you stay sane while deciding whether you should get the bow with the seven-inch brace height or the one with the six-inch brace height. I don't think you will notice much difference.
But before I let you off that easily, I will plant the seed for another debate I can come back to in a few months. A faster arrow can make your system more forgiving in two ways. I talked about reduced "lock time," but I didn't get into the benefits of a flatter trajectory. When you start to look at the big picture, speed really is king. All other things being equal, a faster bow is a more forgiving bow. I stand behind that.