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Speed limit.

Citius, altius, fortius--faster, higher, stronger--is the Olympic motto, but the crossbow industry seems to have adopted it as well. Every year, crossbows get a little faster, with higher speed and energy ratings, accomplished in large part by stronger components. I took a closer look at how much things have changed in terms of speed and kinetic energy, and which might be more important.

It should be noted some figures provided are based on information compiled during annual crossbow reviews, and thus includes only those models that were part of our analysis--typically, one flagship crossbow from each manufacturer. They are intended to show a trend rather than represent an accurate analysis of all crossbows in any year.

Citius

The first crossbow I owned, a Horton Legend, was rated at between 250 and 300 fps, over a decade ago. By 2011, the average speed for new crossbows from 16 manufacturers was 336 fps. Jumping ahead to this year, the average from 18 different crossbows was 370 fps. That includes three models rated at over 400 fps, but does not include the Scorpyd Orion, which is currently the fastest-rated crossbow at up to 440 fps. The "slowest" model in this year's class was 320 fps.

Fortius

From 2005 to 2011, kinetic energy ratings went from a high of 76 foot-pounds to an average of 110 foot-pounds. By 2016, the average was 122 foot-pounds, with Scorpyd's Ventilator Extreme topping the list at 172 foot-pounds. That's nearly 100 foot-pounds of kinetic energy gained in a decade.

Speed vs. Energy

Archers are forever debating the advantages of speed versus KE. More speed means flatter trajectory and more margin for error in range estimation. It also slightly reduces the time between the shot and impact. More KE means more energy transfer at the point of impact and greater penetration.

The mechanics of a crossbow--draw weight and power stroke--are fixed. So, if you want to gain speed, you must reduce bolt weight. To illustrate the effect, we'll use specifications for TenPoint's Carbon Nitro RDX (Table 1). By reducing bolt weight by 50 grains, from 420 grains to 370 grains, we gain 20 fps of speed but lose 3 foot-pounds of KE.

Based on a bolt trajectory chart from BowReport.com, that speed difference will amount to a difference in bolt drop of roughly two inches at 40 yards, but close to 10 inches at 60 yards. Keep your shots inside 40 yards, and you won't notice much difference, as long as you're shooting at big game. But when you air it out, you have far less margin for error in your range estimations. Meanwhile, a loss of 3 foot-pounds is negligible.

Now, let's look at it the other way. We'll use Barnett's Ghost 410 as an example. Its 410 fps is based on a 400-grain bolt. If you go to a 425-grain bolt (switching from a 100- to a 125-grain head), you gain 10 foot-pounds of KE (from 149 to 159), while losing about 10 fps of bolt speed.

In terms of trajectory, that speed loss is negligible inside 30 yards. We're only losing half as much speed as in our previous example, so let's assume the result is half as much difference in drop, or five inches at 60 yards.

But what about the energy gain? Bullets kill through both blood loss and trauma, a combination of extensive tissue damage and hydraulic shock caused by the transfer of KE upon impact--typically thousands of foot-pounds. Broadheads are designed to kill by causing extensive hemorrhaging from slice wounds. Therefore, a bolt needs only enough KE to penetrate the hide, ribs and flesh and enter vital areas.

Now, consider how many times your bolt has not only done that, but then passed completely through and stuck into the ground. A complete pass-through makes for a better blood trail and faster recovery, but adds no more lethality to the wound.

Easton put together a kinetic energy chart that illustrates how much energy you need to get the job done (Table 2). With small game, those 10 foot-pounds could make a difference, if you're talking about the difference between 0-10 or 10-20 foot-pounds of KE. But you only need 25-41 foot-pounds of KE to kill a deer, and 80 is the max for the largest game. Going back to our example above, switching to a 125-grain head on our Ghost 410 gives us double the necessary energy to take down a Cape buffalo, and four times that needed for a whitetail. Now, consider how long people have been killing deer with bows and arrows and how much KE those bows had 50,100 or 10,000 years ago.

In the final analysis, adding five or 10 foot-pounds of KE to a bolt fired from a modern crossbow designed for hunting big game is not going to make much difference. There may be some slight advantage gained from the physics of FOC and a heavier front end. But when it comes to hunting just about anything in the Lower 48 with a crossbow made in the last five years, I know which side of the debate I'm on.

Tip of the Month:

The reticles on many new crossbow optics, and most crossbow speed ratings, are calibrated based on using a 400-grain bolt. If you change your bolt or broadhead weight, you should either shoot through a chronograph to determine actual speed or do a lot of practice shooting at longer ranges to dial in your scope to the proper setting.

Caption: Most modern crossbows produce far more than enough kinetic energy to push a bolt through a whitetail. I know, because I once fired a bolt that passed through two deer and buried itself halfway in the ground.
TABLE 1

ARROW WEIGHT   SPEED      KE

grains          fps    ft. lbs.

370             385      122
420             365      125
425             364      125
435             362      127

TABLE 2
Kinetic Energy Recommendations

KE (FT. LBS.)                         SPECIES

15-25                  Small Game (rabbit, groundhog, etc.)
25-41                   Medium Game (deer, antelope, etc.)
42-65              Large Game (elk, black bear, wild boar, etc.)
65-80           Toughest Game (Gape buffalo, grizzly, moose, etc.)
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Title Annotation:CROSS BOWS
Author:Humphrey, Bob
Publication:Petersen's Bowhunting
Date:Aug 1, 2017
Words:1015
Previous Article:Part 2 of 3: spot-and-stalk shooting skills.
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