What Makes a Fast Bow?
When you walk into an archery shop to talk bows, the term “fast” is bound to enter the conversation. Of course, “fast” means the ability to cast an arrow at greater speed.
A fast bow can be a true godsend, particularly when hunting and, suddenly, a shot at a nice buck or bull presents itself, but you aren’t quite sure of the actual shooting distance. In this scenario, an experienced guess of how far the shot is (given the distance is moderate — less than 40 yards), coupled with some potent arrow speed, and you’ll likely deliver that lethal hit needed to put the animal down fast. This is all due to the straight-line path a fast arrow follows compared to the arching line of a slower one.
This is the essence behind shooting a fast rig. However, if there’s one thing I’ve learned in 20-plus years of shooting bowhunting gear, it’s this: for every gain there is usually an equal loss or sacrifice. So, like everything else, speed does have a tradeoff. Let’s take a look at what makes a bow fast, and what tradeoffs might be required of shooting a fast bow.
There are two basic ways to make a bow faster. The first is to shorten the bow’s brace height. This factor affects the bow’s overall draw stroke, which essentially is how much you have to pull to get to full draw. The shorter the brace height, the longer the draw stroke. For example, if your draw length is 29 inches and the bow’s brace height measures 7 inches, then your draw stroke (the amount you pull to reach full draw) is 22 inches long. If the brace height is only 5 1/2 inches long, then the draw stroke would change to 23 1/2 inches long. This extra 1 1/2 inches of draw stroke will increase the bow’s stored energy capability, allowing greater force to be transferred to the arrow during the shot phase, or what is known as the powerstroke.
The brace height (measured in inches) is simply the distance between the bowstring and the grip’s throat in the undrawn position. One inch of brace-height reduction, generally speaking, yields about 10 extra feet per second of arrow speed.
Another factor that affects this powerstroke is draw length. The longer the draw length, the longer the effective powerstroke. This factor, as well, will make the bow propel the arrow faster and more powerfully.
However, the downside to a short brace height (5 to 6 3/8 inches) is that it can be more difficult to shoot. This is due to the arrow’s increased “string time” with the bowstring during the shot phase. Why this longer string time becomes a problem boils down to bow-hand torque. The longer the arrow stays on the string, the greater the chance the shooter will torque the bow. This factor creates a magnified effect and causes less consistent shooting.
In the backyard, this low brace-height factor might not be very noticeable. But shoot from a cold treestand or a crouched position on the ground and this feature can produce a total accuracy disaster.
Another torque concern comes from simply drawing the bow back. From an engineering standpoint, it’s impossible to design a bow with the grip’s throat in line with the arrow and the centerline of the string. Therefore, all bows must be drawn by pulling “up” on the string somewhat. This creates “natural vertical” torque. This factor changes based on how far the arrow passes above the hand and the brace height of the bow. But, in every case, the lower the brace height, the greater the vertical torque. This component almost always makes the bow less smooth and accurate to shoot.
Lastly, a short brace height places the bowstring closer to the path of the forearm. This creates a problem since the bowstring will often collide with a baggy shirt or jacket on a cold day, causing errant shots. A good string stop minimizes this effect, but it won’t eliminate it completely.
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The second way to add speed or power to a bow is at the cam system. By using cams of a more aggressive design, increased energy storage is transferred to the bow’s limbs, resulting in greater performance.
However, getting back to the “no-free-lunch” philosophy in bowhunting tackle, this sort of geometry creates a harsher draw cycle that’s less smooth from start to finish.
Archery engineers use what is called a draw-force curve to compare the “pulling action” of different cam systems. This chart calculates the pulling force required at every inch of draw length.
When describing the draw-force curve of a mild cam system, the charted line will follow a gradual upward pattern — with a moderate plateau at the peak drawing weight — followed again by a gradual descent into the draw valley.
In contrast, an aggressive cam system’s draw curve will be much more abrupt. It will climb quickly, plateau longer at peak weight, and then fall sharply into the draw valley. This “abrupt” line pattern means faster “weight buildup” and a less fluid-like bow pulling action — something aging shoulders or a cold body in a treestand might not like. This is yet another strike to the ultra-fast bow design.
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Maximizing Draw Return
So far, I’ve painted a pretty lousy picture for the fast bow. But this is where that changes, thanks to something known as dynamic efficiency.
Without getting too complicated, dynamic efficiency basically relates to the force you put into something and the amount of force you receive in return. In other words, when you pull a 70-pound bow back, is all of that stored energy completely transferred to the arrow?
The answer is a resounding no, as working friction and dynamic weight does occur throughout the bow’s system. This friction is found in the cams’ axles, the cable-slide unit, and how the bowstrings and cables interact with the grooves in the cams. The physical weight of the components (cams, limbs and bowstring and cables) and even how they react physically also degrade a bow’s dynamic efficiency.
But, thanks to refinements in all these areas, bows are becoming ever more dynamically efficient. This is a huge plus, as now you can have greater arrow speed without sacrificing shootability or draw smoothness.
A few years ago, a bow yielding 80 to 85 percent of dynamic efficiency was considered exceptional. Today, we have bows producing efficiency levels approaching 90 percent! (Note: Arrow weight has a direct influence on bow dynamic efficiency. Therefore, for consistency, bow efficiency levels are calculated using a 540-grain, industry-standard weight arrow.)
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Finding the Sweet Spot
Now that we know where a bow’s speed and energy comes from, what makes for the ultimate setup?
This is never an easy answer, as the best bowhunting bow revolves heavily around the individual. Specific draw length, shooting form and hunting application are all important factors that change what is considered the “best.” But, after relentless testing of different bow designs, these are my recommendations based on a wide spectrum of hunting conditions:
- Brace Height: Stay away from 6-inch-or-shorter brace height bows unless you are a highly experienced shooter. Even then, I’d still be careful. My preference is for bows with 6 3/4-inch brace or higher. These are simply more user friendly in the field.
- Draw Cycle: Next, draw smoothness does count for a lot when you’re in the deer woods, so be cautious about overly aggressive cam designs. Many of today’s fast bows have been engineered with a stronger pull at the beginning of the cycle in favor of an easier-to-handle, more gradual descent into the draw valley. This is a good tradeoff, as such bows feel more comfortable at full draw, especially when you have to relax and wait longer for the shot to occur. I know many of today’s top-end cam systems were designed with this thinking in mind, including Hoyt’s RKT Series, BowTech’s Experience, and Mathews’ Creed, to name a few.
- Bow Balance: Lastly, don’t underestimate the accuracy of a well-balanced bow. This feature can really make the setup perform better all around, especially in reducing shooter-induced torque and for tightening groups downrange. For bows that tend to be top-heavy, or tip forward excessively, attach a large counter weight below the bow’s grip (opposite of stabilizer hole), or use a V-bar adapter with counter weights on the stabilizer hole, to help offset this imbalance. I like to do this with bows that have a stronger reflexed riser, such as the Mathews Z7 Magnum.
- Draw Length and String Angle: A bow’s string angle is important for two reasons. First, ultra-short speed bows tend to produce a sharp string angle at full draw. This often makes the bow less stable and “twitchy” from side to side, allowing for easy hand torque. This will hurt your 50-plus-yard shot accuracy. Secondly, string angle affects how the bowstring interacts with your face and how close the peep sight sits from your eye. Since I like to use the bowstring to anchor lightly against my nose, I require a certain string angle. Also, this angle places the peep an ideal distance from my eye to create optimum field of view with the peep’s orifice and target. You cannot guess string angle based on a bow’s axle length, as many of today’s short bows have extra-large cams, which lessen string angle. The only way to know a bow’s string angle is to draw it back at your draw length.
There’s no doubt, speed is a great thing to have in a bowhunting rig. But this is only when it doesn’t come at a cost. My advice is to seek a balanced approach, which is going with a moderately fast/forgiving bow design that optimizes dynamic shooting efficiency and overall shootability. Such a bow will simply perform the best when the chips are down.
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Bonus: Arrow Speed and Weight
In order to increase speed, many archers will opt for lighter arrows, but such projectiles can actually degrade shooting efficiency. Sure, lighter arrows come out of the bow faster, but downrange, they shed energy more rapidly, resulting in less punch and penetration on game. This reduced performance can be a mistake when taking longer shots out West, so be cautious.
Secondly, light arrows are less stable in flight because their reduced mass has less inertia. This element makes them exceedingly more difficult to tune using larger broadheads.
Also, and perhaps the biggest concern of all, lighter arrows create more bow noise. Again, this is due to lower mass, which means less efficiency in absorbing the bowstring’s force during the powerstroke. This unused energy is transferred back to the bow’s limbs and riser, adding to shot recoil, vibration and noise.
I’ve always favored midweight arrows since they reap the best of all worlds — great speed, moderately smooth/quiet shot and solid downrange punch. I classify “midweight” as any arrow shaft that weighs 9 grains per inch or more for an average 400 spine arrow.
Editor’s Note: This was originally published June 11, 2013.
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