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Why Are Some Bows Faster? – December/January 2010

by Bill Winke   |  October 28th, 2010 0

Question: Can you explain bow speed as it relates to the physics of the bow? What are the biggest factors in helping today’s bows achieve the speeds they have and do you think they will get faster in the future? — Steve Novak, Midland MI

Arrow speed is measured using a standard method so that we can get an apples-to-apples comparison. The most popular current standard is what is referred to as IBO speed. This is the recorded speed when shooting a 350-grain arrow from a 70-pound bow with a 30-inch draw length through a certified chronograph. IBO speed is not the actual speed you can expect to shoot the bow unless you have those exact specifications, but it does permit us a standard for comparison. A fast bow is one that has an IBO speed over 320 fps. Typically, most hunting models have IBO speeds over 300 fps.

Arrow speed depends upon three factors: the design of the cam, the distance from the string to the back of the grip with the bow undrawn (this is called brace height) and the bow’s efficiency.

Cam shape: As I mentioned earlier, designers can set up the lever arms in the cams any way they want. They can design the cam so that it causes the bow to store a lot of energy. Or, they can design the levers so the draw is softer and smoother and more comfortable.

High-energy bows always feel like they draw harder than bows that store less energy, even though the peak draw weights are the same on both. The more energy stored, the harder the bow feels and the faster it shoots, all other things being equal.

Brace height: The shorter the brace height (again, the distance from the back of the grip to the string) the greater the potential for speed. When the bow has a short brace height, you must draw the string farther to reach the same draw length. Potential energy (the fancy name for stored energy) is defined as force times distance; so anytime you increase the distance of the pull you increase the energy that the bow stores.

Efficiency: Efficiency is the ability of the bow to give back the energy that you put into it. It is actually a comparison of energy: divide the amount of kinetic energy in the arrow as it leaves the bow by the amount of stored energy in the limbs at full draw. This ratio will always be less than one. The lost energy is due to friction in the eccentrics, cable guard slide/harness system and any kinetic energy contained in the limbs and string and harnesses after the arrow is gone. This leftover energy, by the way, creates vibration that causes bow noise.

The most efficient bows on the market today have efficiency ratings in the range of 80-85 percent. The more efficient the bow, the faster it will be for the same amount of effort from you. Obviously, a bow with high efficiency is good. Efficiency drops off as you increase draw weight or decrease arrow weight. That’s why a lighter arrow doesn’t penetrate quite as well as a heavier arrow — the light arrow has soaked up less of the bow’s energy on release and there is more energy left behind for the bow to dissipate through vibration – that creates noise. The lighter arrow produces a noisier shot.

Sure, you can drop the brace height a bit more to store more energy and you can make the cams a bit more aggressive so the bow comes up to maximum draw weight faster and holds it longer. But those two routes have been pushed nearly to the point of unacceptability. Bowhunters are only going to tolerate a brace height so low or a draw cycle that is so aggressive. There is a point where the bow won’t sell.

The real opportunity to increase speed without any negatives is to increase its efficiency. That is, to increase the return of energy by reducing the weight of moving components, the responsiveness of the limbs and by reducing the speed at which everything but the string and arrow move. I don’t know how much we have yet to gain in this regard, but at least any gains we make in efficiency make the bow quieter and more pleasant to shoot, not less so, as with dropping brace height and making the bow harder to draw.

We may also be able to make bows faster if we figure out new ways to store energy more efficiently (other than the flexing of a fiberglass limb). Who knows what form that may take, but with the current definition of a bow, we have pushed speed nearly as far as it will go.

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