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Crossbows Gear & Accessories Hunting

Understanding Crossbow Arrows

by Brad Fenson   |  July 23rd, 2012 0

A machine at Carbon Express tests arrows for proper spine. Static spine is the measure of how much the arrow bends at rest. Dynamic spine is how much the arrow flexes in flight. Photo courtesy of Carbon Express.

We snuck to the edge of a large grass field and peered through limbs, grasses and shrubs. A small group of white-tailed does was moving toward us. My hunting buddy looked back at me with a huge grin and shouldered his crossbow. He pointed it in their direction and waited for them to appear in a clear lane in front of him.

Seconds later, a young doe stepped into the sight window. I watched the arrow disappear into the deer, just behind her front shoulder. The flight was perfect. It hit exactly where it was aimed. The carbon shaft passed completely through the animal for maximum effectiveness and a quick, efficient kill.

This is the way it’s supposed to happen. The hunter had the right gear for the job. Strings weren’t frayed, cams weren’t bent and the arrow was the weight and type the manufacturer recommended.

Crossbows have been used for centuries and the technological advancements over the years have made them powerful. Modern crossbows harness an incredible amount of power. Engineers have ratcheted up the speed and stored energy. Most designs producing speeds between 300 to 400 fps. For these reasons, shooters need to pay close attention to the arrows they select. You will maximize the benefits of your xbow, and ensure only the game animal will take the brunt.

Spine is the measure of rigidity of the arrow from tip to nock. Too little spine could mean too much flexibility and the arrow might not be able to effectively handle that sudden transmission of energy from the limbs. An arrow could shatter. The bow’s energy would have to go somewhere, and there is no other good options, as far as the person pulling the trigger is concerned. At best, it would equate to a dry fire, and you crossbow would probably be a worthless mess after that. At worse, you’d be in the emergency room. There is a lot of power in a crossbow. Similar to a gun or a vehicle, it must be treated with respect.

In the industry, the term static spine is used to refer to the stiffness of the arrow and its resistance to bending. Engineers and designers check the level of static spine in their arrows by supporting the arrow at two points a known distance apart, and applying force in the center of the arrow. The amount of bend at the center point determines the spine. Arrows with high static spine will not bend as much as arrows with low static spine.

Geometry of the arrow’s shaft, the materials used and how much of it, are factors that determine the spine. In multi-layered arrows, like carbon and aluminum, the bonding materials of the layers also contribute to the stiffness.

Dynamic spine is the term used to measure how much an arrow bends when fired from a crossbow. This is influenced by factors such as static spine, string force, fletching weight and nock weight. An arrow should bend and flex to some extent during flight. That flexing and bending influence accuracy and consistency.

Crossbow manufacturers have recommendations for the amount of spine for their models. You can usually find a handy chart on the crossbow box or instructions to help you determine the required stiffness of an arrow for specific draw weights with a given bow.

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