Lessons from A Shooting Machine: Part One. (Center Shots).
Recently, I had the pleasure of interviewing Steve Johnson, inventor of the Spot Hogg Hooter Shooter shooting machine. I've never met anyone with clearer insight into the many factors that contribute to consistency and accuracy. Steve opened my eyes to facts about gear that I had never considered. What he's learned from shooting thousands of arrows from different bows will give you the knowledge to become a better archer.
Because of all the subjects we discussed, I'm turning this topic into a two part series. This month I'll look at forgiveness and what the shooting machine can teach us about the subtlest changes in draw length and anchor point. Next month, I'll look at tuning and arrow flight, specifically how they are affected by bow design.
Draw Length Forgiveness
Every bowhunter has a zone in which he anchors. In bows with a wide valley and soft back wall it may actually be more than 1/2 inch long. In bows with short valleys and hard walls it is less. On days when he is feeling strong, the archer pulls the string until the cams roll hard into the wall. On days when he is distracted or tired he may ease into the middle of the letoff valley. A small change in form doesn't seem like a big deal. We all do it, right? Well, the best shooters don't. They've learned through actual shooting the same things a machine now teaches. How far you pull the string makes a huge difference.
"We've tested many bows by using the machine to draw them back various distances within what we feel is a reasonable error range for many bowhunters," Steve said. "Assuming that's about a half-inch range, we found that some bows are much more critical of this slight change than others. As we varied the draw length, the worst bow we tested had an impact that moved more than four inches on a target 20 yards away."
In all cases, Johnson used the bow's peep sight and sight pins to aim at the center of the target, so the test perfectly simulated actually shooting conditions. When you pull the string back slightly farther or shorter than normal, you change the relationship between your eye, the pin and the target. "When you vary the draw length the riser tips a little," Johnson said. "The sight is not accurate anymore."
Next time you're shooting on the range pay particular attention to exactly what part of the valley you are anchoring. Unless you are a highly skilled competitive shooter, I bet you'll be surprised by how much your exact anchor point has been fluctuating. One simple change in bow design will also help eliminate this problem. Bow's with very solid back walls at full draw are easier to shoot consistently than those with soft back walls. You can pull up against it gently and the wall stops the draw in the same place every time.
Johnson further told me that he was able to "fix" the bow with the four-inch error by simply changing the bow's timing (it was a two-cam bow). When it was timed perfectly the bow was very unforgiving of changes in draw length, but when the top cam was set to arrive at full draw slightly ahead of the bottom cam the bow adjusted automatically and the pin followed the impact point perfectly. "In most cases we set two-cam bows so that the harness attached to the bottom cam is about one twist shorter than the top one," Johnson said.
"With single-cam bows we first set the tiller equal and then experiment with different settings (adjusted by turning limb bolts in and out) until the bow's impact tracks the sight through small changes in draw length and anchor point. This procedure is accomplished with a shooting machine because all human errors are eliminated. You know immediately after making a change whether the bow is more or less forgiving."
Arriving At Your Anchor Point
You may be surprised to learn that the way you reach your anchor point also affects your accuracy. "We set up two different tests," Johnson said. "In the first we pulled the string back to a specified draw length without letting it ease forward. In the second test we pulled the string past the proper anchor point and then let it move forward until it was at the exact same draw length as the first test.
"I'm not sure exactly why, but we found that at 20 yards the arrow impacted as much as 1 1/2 inches above or below the aim point depending on how the anchor point was reached."
The lesson is obvious: you have to approach your anchor point from the same direction each time. If you like to ease into your anchor point in order to take advantage of the bow's effective letoff or pull into the back wall, it doesn't matter, you just have to do it the same way every time.
All Bows Are Not Created Equal
I've always felt that all bows were accurate in and of themselves. You could put any bow into a shooting machine, regardless of brace height, axle-to-axle length, weight, cam style, etc. and they would all shoot exactly the same once tuned and sighted in. I was wrong.
Through his studies on a bow's inability to forgive changes in draw length, Johnson opened my eyes to the fact that a bow's forgiveness can be separated from the shooter's form. His testing has further shown that there are other ways in which one bow can be designed to shoot better and tune better than another.
Using the machine, all bows can be made to shoot arrow after arrow into the same hole even at 20 yards. The problems arise when things change from one shot to the next. The most common source of inconsistency is the string and harness system. As they stretch they change the draw length and possibly the bow's timing and nock travel. Any changes in draw length that occur without the bow being re-sighted can result in major accuracy problems. Serving separation is another result of string stretch. As the serving separates, the nock point moves up.
To combat these problems, Johnson encourages every bowhunter to use a high quality string and harness system. It has been my experience that many of those offered as standard on factory bows are prone to stretch dramatically. There are several custom shops making pre-stretched, high quality, strings and harnesses. I've had good luck with products made by Winner's Choice.
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|Date:||Mar 1, 2002|
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