Crossbow Technology: Rise of the Machines

Crossbow Technology: Rise of the Machines

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Deer hunters using crossbows should understand some of the technical aspects of the bow, arrows and how they work to understand why they hear noise and feel vibration. (Photo: Dr. Todd Kuhn)

As crossbow manufacturers continue to push the speed envelope, trade-offs and compromises emerge. And the question arises: How fast is fast enough?

The fundamental design of modern crossbows is millenniums old. The ancestry of these “super-bows” dates back to around the 4th century, B.C. Recently, crossbows have enjoyed a new found relevance as a weapon of choice for legions of hunters who have discovered their utility as a hunting tool.

As such, horizontal bow manufacturers have ratcheted up their game, building progressively more advanced offerings. These bows ostensibly bend the laws of physics, generating warp-like arrow speeds. However, is raw speed the only variable to be considered when solving the crossbow equation?

THE PUSH FOR ARROW SPEED
America is obsessed with speed — so are archers. For decades, compound bow manufacturers have chased the chronograph, tweaking eccentric systems and shoring up draw force curves in a quest to increase arrow speeds.

First, it was a foot race to 250 fps. Then the sprint to 275 fps. Then an all-out blitz to the mythical 300 fps. Today, engineers have set their sights on the unheard of “400,” having recently eclipsed 370 fps. Likewise, crossbow companies have joined the fray — this race to the pinnacle of pace.

HOW FAST IS “FAST ENOUGH?
So how fast does a crossbow really need to be? According to premium arrow maker Easton, a 400-grain arrow needs to travel only 170 fps to kill a whitetail. Needless to say, you’d be hard pressed to find a crossbow incapable of achieving this lethargic speed. So if 170 fps is fast enough to harvest an animal, will thumping it at 400+ fps render it “more dead?”

In their quest for outlandish chronograph readings, both hunters and manufacturers have gotten wrapped around the axle. Sadly, some hunters make buying decisions based solely on arrow speeds, with many manufacturers catering to this primal desire for raw speed.

However, at some relatively definitive point, from an engineering and arrow flight perspective, you begin to compromise performance for blazing arrow speeds.

WATCH: HOW TO WAX AND LUBE A CROSSBOW

ARROW SPEEDS CAN BE A STICKY SUBJECT
Crossbow speed ratings are a sticky subject, because there is no “standardized” method of reporting them. Compound bows have their International Bowhunting Organization (IBO) and AMO (Archery Manufacturers Organization) ratings, by which their speeds are uniformly calculated using a formula (e.g., IBO = 70 pounds of draw weight, 30-inch draw length and 350-grain arrow weight).

However, the crossbow industry hasn’t come to a consensus on a speed calculus for their collective products. As such, consumers are left to fend for themselves when making heads-or- tails of crossbow speed ratings.

Draw weights vary from model to model, so do power stroke lengths and arrow lengths, as well as methods used when calculating speeds. Manufacturers vary test arrow weights (from 350 to 430 grains), arrow configuration (e.g., fletched versus unfletched) and draw weights (varying as much as 100 pounds). So it is a case of buyer beware.

THE LAWS OF THERMODYNAMICS
Mechanical machines (i.e., crossbows) are capable of achieving only a certain degree of mechanical efficiency. They are, after all, limited by the laws of physics. No matter the machine, none can be 100 percent efficient. Mechanical efficiency is lost due to friction during the shot cycle.

Energy robbing and ultimately speed robbing friction is found throughout the crossbow. Most notably, it is found in the cam bearings, cam axles, inside the limbs (internal friction), string-to-arrow-track contact, arrow-to-arrow track contact, string-to-cam groove contact, trigger latch to string interface, string (internal strand-to-strand friction), aerodynamic resistance to cam rotation and limb-release flexure on the shot and so on. These sources of friction slow the machine down, for lack of a better explanation, diminishing mechanical efficiency and robbing arrow speed.

THE TRADE-OFFS
When crossbow engineers design dizzying fast crossbows, they expect trade-offs and compromises. When blistering fast arrow speeds are achieved, typically the crossbow gets louder, with more hand shock and vibration.

Interestingly, both of these are manifestations of unused/excess energy (i.e., energy that was not used to propel the arrow) being dissipated. Any energy not used to “push” the arrow must be released, because it cannot remain stored in the bow. The result is the audible crack and jolt of an uber-fast crossbow.

Additionally, from an arrow flight perspective, when crossbow speeds eclipse 360 fps (+/- 5 fps) hunting arrow flight can become suspect. Modern broadheads (no matter their claims of “field point accuracy”) are affected by the laws of aerodynamics (i.e., thrust, drag and lift).

No matter their shape or size, none offers a true, aerodynamic profile and eventually becomes unstable and wind-plane due to excessive aerodynamic instabilities at exaggerated speeds. This makes “tuning” broadhead arrow flight very difficult because the average crossbow consumer isn’t capable of making minute adjustments to their equipment to achieve consistent arrow flight (i.e., tweaking buss cables).

— Dr. Todd Kuhn has been writing about outdoor technology for 25 years. He has a Ph.D. in engineering and worked on NASA’s Space Shuttle and International Space Station programs.

 

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November 2, 2017 at 02:02PM

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