|
 While
modern compound bows are quieter, more accurate, and
more vibration free that we could have imagined just 5
years ago, it seems as if compound bow speeds may have
hit a technical rut. Today's IBO speeds aren't so
different than those of 5 years ago. A soft-cam
target bow shoots about 280 or 290 fps. A basic
hunting bow shoots roughly 300-310 fps. An "en
vogue" high-performance compound often squeezes into the
315-325 fps range, and a few radical designs venture
into the 330's and 340's. So what's the problem?
Why aren't bows getting faster and faster as technology
continues to improve? Will we ever see a 400 FPS
IBO Speed compound bow, or is that goal simply unattainable from a
hand-drawn weapon?
Can any bow shoot 400 fps today? Sure! It
just can't be done safely or within accepted industry
standards. If you shoot a light enough arrow, your
high-performance compound bow can surely make 400 fps.
It just won't do it for very long. Shooting
underweight arrows puts enormous stress on a bow's
components - much like dry-firing the bow. If you
shoot dramatically underweight arrows, you may
eventually warp the cam tracks, bend the cam axles,
damage the string and/or buss cable, or get
catastrophic limb failures (just to name a few potential
hazards). So it would be particularly unwise to
"cheat" your way to faster arrow speeds by ignoring
industry standards. While every expert doesn't
agree on this issue, the most commonly accepted arrow
mass standard is the IBO (International
Bowhunting Organization) Standard which
states that minimum arrow mass must be at least 5
grains per each pound of draw weight. So to
meet the safety standard, a 70#
bow must shoot an arrow that weighs at least 350 grains
(70x5).
The IBO also has a standard for speed-testing bows.
Since it would be confusing for one manufacturer to
test and rate their bows using one standard, and another
using a different standard, manufacturers generally rate
their bows using the same IBO method. To get an accurate
IBO Speed rating, manufacturers must test their bows
under the same preset conditions: setting the bow for
exactly 70# Peak Draw Weight, exactly 30" Draw Length,
and they must shoot a test arrow that weighs precisely
350 grains. This way consumers have a fair
apples-to-apples method of comparison.
OK, so let's start at the top. First we need to
know, is it theoretically possible for a bow to shoot 400
fps at IBO test standards? Surprisingly, the
answer is yes. However, a compound bow engineered
for the "theoretical maximum" would have a totally flat
powerstroke which pulled at peak weight from the first
inch to the last - with no let-off. And to
actually reach the theoretical performance limit, the
bow would need to be 100% efficient (today's bows are
typically 75-85% efficient). But let's take a look
at the on-paper possibilities.
Using the archer's Kinetic Energy formula, KE =
(mv²)/450240, we can compute that a 350 grain
arrow traveling at 400 fps would carry 124.38 ft-lbs of KE. So the first step is to see if it's
mathematically possible to store that much energy during
a 70#/30" powerstroke. After all, if you can't put
that much energy into the bow (with muscle effort), you
could never hope to get that much out (energy into the
arrow). So let's start by looking at energy input.
|
Brace Height |
Powerstroke |
Max
Storage |
A bow with a 6" brace height and a 30" draw
length will have a 22.25" powerstroke. If
the draw weight remains constant at 70 lbs. for
the entire powerstroke, that bow could
potentially store up 1557.5 in-lbs (22.25 x 70)
or 129.79 ft-lbs (1557.5/12) of energy. So it
is possible for a powerstroke to store the
minimum 124.38 ft-lbs. In fact, any bow
with a brace height up to 6.93" could
theoretically store the required energy to
achieve 400 fps with a 350 grain arrow.
However, note again that this is assuming a
totally flat powercurve (pulling 70# all the
way back with no ramp-up or ramp-down of draw
weight during the cycle). This also
assumes 100% efficiency (more on this in a
moment), such that the bow's output precisely
equals it's input.
|
|
5.0" |
23.25" |
135.63 ft-lbs |
|
5.5" |
22.75" |
132.71 ft-lbs |
|
6.0" |
22.25" |
129.79 ft-lbs |
|
6.5" |
21.75" |
126.88 ft-lbs |
|
7.0" |
21.25" |
123.96 ft-lbs |
|
7.5" |
20.75" |
121.04 ft-lbs |
|
8.0" |
20.25" |
118.13 ft-lbs |
|
8.5" |
19.75" |
115.21 ft-lbs |
|
9.0" |
19.25" |
112.29 ft-lbs |
|
|
So it
is theoretically possible. But could it be
done while still maintaining let-off and a tolerable
powerstroke? And could bow efficiency be boosted
high enough to get the required output? To understand this issue, we'll first
need to understand the basics of the Force Draw Curve.
The force draw curve is a simple graphic representation
of how a compound bow's draw weight changes as it is
drawn back and then let back down (draw cycle). Careful examination of this graph can tell us a lot
about how a compound bow will feel and perform.
|
Take
a look at the graph on the right. This graph
represents drawing-back a compound bow and then letting it back down.
Length (distance) is plotted against weight. At
marker
the bow is at rest. At marker
the bow has been drawn back about 4 inches and the draw
weight has increased to roughly 40 lbs. As the
shooter continues to draw back, the weight gradually increases
until reaching the bow's peak weight (roughly 67# in
this example)
during the 10th inch of the powerstroke at marker
.
Then the draw weight begins to decrease
until finally reaching full let-off at marker
representing the end of the powerstroke (full draw).
The spike in the middle of the graph represents forcibly
overdrawing the bow (pulling against the wall).
Some shooters tend to hold hard against the wall, others
don't. So the spike in the middle could be
different (taller/shorter) depending on the shooter.
The second half of the graph represents letting the bow
back down (or firing the bow). Notice that the
graph is almost a reverse of the powerstroke, except all
the draw weight values are slightly lower. |
The "V" shape formed between the two halves of the graph
is commonly referred to as the "valley", which
represents how quickly the bow transitions to and from
full let-off. A bow with a narrow valley doesn't
allow you to creep forward before the bow begins to
aggressively pull forward. A wide valley allows a
little more leeway for shooters who tend to creep.
|
The sample graph above is taken from a moderate bow (PSE
Bruin) with a relatively smooth-drawing cam.
Notice that the overall shape of the graph is a smooth
bell-shaped curve with a gradual rise and gradual
decline. Interestingly, the general shape of the
curve is a good estimate of how aggressive the draw
cycle will feel to the shooter. And as you might
expect, all cams are NOT created equal. Some cams
are specifically engineered to produce a smooth feel.
Others are made for best possible performance.
The actual geometry of the cam system determines how
soft or aggressive the powerstroke will be. Take a look at the additional sample graphs
below, taken from
bows with different types of cam systems. |
|
 |
ROUND WHEEL: As
you can see, a Round Wheel style bow has a very smooth
bell-shaped curve which rises to peak weight for only a
moment then gradually descends to full let-off.
This cam style will feel very smooth and easy to draw,
but will store the least amount of energy and shoot the
slowest. Although this type of cam has been around
for decades, some shooters still prefer the soft feel of
this style cam - particularly instinctive-shooters and
finger-shooters. So a number of manufacturers
still offer bows with traditional round wheels or cam
geometry ground to replicate the round wheel powercurve.
|
 NOTE:
Since many shooters associate a "round" cam
with a "smooth" cam, a few bow manufacturers
deliberately machine their cams to be round
in shape, although the cam generates a very
aggressive hard-cam cycle. Don't be
fooled by this little mind-trick. The
"power side" of a single cam is the center
groove - NOT the outside track. So
don't automatically assume a round cam is a
smooth cam with the same draw
characteristics as a round wheel.
They're not the same thing. |
MEDIUM CAM: The Medium Cam
graph is typical of today's basic single and hybrid cams. These cams are more
aggressive, ramping to peak weight more quickly and then
coming to full let-off more abruptly. So they tend
to store up more energy than a Round Wheel bow,
and shoot notably faster. However, a Medium Cam
is sure to "feel" a little heavier than a Round Wheel
bow of equal peak weight. This type of cam
geometry suits most shooters well, offering a reasonable
blend of feel and performance.
HARD CAM: The last example is a
Hard Cam system, optimized for maximum energy
storage and speed. Notice how quickly the bow
ramps up to peak weight and how quickly it transitions
to let-off. Also notice the distinct high-plateau
on the graph where the shooter must draw the bow over
several inches at peak weight. This type of cam
geometry will store dramatically more energy, and will
usually have an IBO Speed of 320 fps or more. The
downside is that
Hard Cams feel harsh and heavy compared to other
bows of equal peak weight. So they certainly
aren't for everyone. But for shooters who want the
hottest possible arrow speeds, the
Hard Cam is the way to go. |
 |
This is a comparison graph, taken from a popular
manufacturer's catalog. Notice how this
particular manufacturer offers 4 distinctly
different cam grinds. The pink line
represents the least aggressive grind
(smoothest) and the blue line represents the
most aggressive grind (fastest). The white
line indicates the standard (most popular) grind
which offers a moderation of the two extremes.
Of course, there is no right or wrong. But
manufacturers are smart to offer buyers a
choice, as each shooter has different
expectations. So most bow companies offer
both soft and aggressive cam grinds to try and
suit everyone.
NOTE: This graph looks a little different
because it only shows the front half of the draw
cycle (which is typical). Our in-house
graphs are generated with Easton's Bow-Force
Mapping System which can show both halves of the
draw cycle. |
|
|
So
what do all these lines and curves really mean when it
comes to getting to 400 fps? Everything! As
cam aggression increases, so does stored energy.
On the Force Draw Curves, the input energy is
represented by the areas under the curve. The more
area, the more energy stored, and the more speed
the bow can yield when fired. As you can see on the graphs below, the
Hard Cam Bow has the largest area under the curve
(represented in blue). So any design that's going
to make 400 fps will have to take the Hard Cam
concept to the extreme. |
|
 |
 |
As the Hard Cam bow becomes more and more
aggressive, the graph begins to look less like a
curve, and more like a rectangle. In fact,
at the theoretical maximum, the draw force curve
wouldn't be a curve at all. If a bow
maintained a constant 70# peak weight throughout
the entire powerstroke, the resulting area under
the "curve" would be a 22.25" x 70# block
(assuming a 6" brace height and a 30" draw
length). But of course, the bow would have
no let-off, rendering it virtually unshootable.
|
 |
Without at least some let-off, developing the
400 fps super-bow wouldn't be very realistic.
So we'll need to incorporate some minimal
let-off into the super-bow draw cycle to make
the bow usable.
In our optimized example, we've adjusted the
curve to incorporate 65% let-off during the last
2" of the powerstroke, reaching 65% relief at
exactly 30" draw. This reduces
our energy input by 45.5 in-lbs or 3.79 ft-lbs.
|
 |
The other obstacle would be how quickly the bow
could ramp-up to peak weight at the start of the
draw cycle. During the
initial pull, you effectively change a straight
string into a bent string (at the nocking
point roughly), but the angle begins at 0º.
So until the angle increases enough to rotate
the cams, the weight cannot ramp-up. Even
today's most aggressive cam designs require
about 4" of drawstroke before coming up to peak
weight. But on the 400 fps super-bow,
we're going to assume engineers have managed to
get that down to an insanely abrupt 1".
So we've adjusted the optimized curve to allow
for some ramp-up time. Of
course, this reduces our energy input by another
35 in-lbs or 2.92 ft-lbs.
|
The powerstroke on the 400 fps super bow would
represent the most aggressive possible curve,
while still maintaining at least some
field-utility and shootability. So before
we discuss bow efficiency, let's take a look at
the numbers and see if we theoretically still
have enough energy to get the job done.
|
|
|
Brace Height |
Powerstroke |
Theoretical
Max
Storage |
Super-Bow
Max
Storage |
|
5.0" |
23.25" |
135.63 ft-lbs |
128.92 ft-lbs |
|
5.5" |
22.75" |
132.71 ft-lbs |
126.00 ft-lbs |
|
6.0" |
22.25" |
129.79 ft-lbs |
123.08 ft-lbs |
|
6.5" |
21.75" |
126.88 ft-lbs |
120.17 ft-lbs |
|
7.0" |
21.25" |
123.96 ft-lbs |
117.25 ft-lbs |
|
7.5" |
20.75" |
121.04 ft-lbs |
114.33 ft-lbs |
|
8.0" |
20.25" |
118.13 ft-lbs |
111.42 ft-lbs |
|
8.5" |
19.75" |
115.21 ft-lbs |
108.50 ft-lbs |
|
9.0" |
19.25" |
112.29 ft-lbs |
105.58 ft-lbs |
|
If you remember, we're going to need 124.38
ft-lbs. to push a 350 grain arrow at 400 fps.
So even after making changes to incorporate
let-off and limited ramp-up length to the
super-bow powerstroke, it is still
possible to hit the 400 fps mark.
Unfortunately, the 400 fps super bow will now
require a wrist-blistering sub-6" brace height
to hit the energy minimum. However, it is
to be expected that the ultimate speed-bow would
have a short brace height. So the 400 fps
bow is still a possibility at 100% efficiency. |
|
 |
You may remember noting that the second half of
the Force Draw graph is almost a reverse of the
first half (powerstroke), except all the draw
weight values are slightly lower on the way back
down. This is one of the unfortunate
realities of any machine. No matter how
good a machine is, you can never get more energy
out than you put in. In fact, you always
get less.
Of course, the energy isn't lost, it just gets
converted into other things we don't necessarily
want (heat, vibration, noise, etc.). For a
compound bow, the goal is to essentially convert
muscle energy into propelling an arrow forward.
And if the system were 100% efficient, then the
arrow would leave the bow with the same amount
of energy used to draw the bow back.
Unfortunately, the reality is somewhat
different. Even the best bows on the
market are well shy of being 100% efficient.
|
|
As you draw a bow, some of your muscle energy is
used to overcome friction in the system rather
than just to compress the bow's limbs.
Friction in the cam axles, string surfaces,
cable slide, etc. all add a little draw weight
to the cycle. So when you draw your 70#
bow, you're expending a few pounds of effort
just to make things "turn". Sadly, you
don't get this energy back when you fire the
bow. This is why the output side of the
Force Draw Curve is always a little lower than
the input side. This degradation or loss
of effective draw weight due to friction forces
is called hysteresis, and it's something
that even the 400 fps super bow will need to
contend with.
So even if we manage to get the 124.38 ft-lbs of
input energy into the bow, we're going to get
some lesser amount out. The question is,
how much will we lose? To help visualize
the concept, the graph at right is the same as
the system efficiency graph above, but it has
been folded in-half to allow you to compare the
two curves. The area between the two
curves (in dark blue) represents the energy that
is lost to hysteresis.
While a good high-performance bow is around 80%
efficient, there are a few bows that really lead
the field. The new Bowtech Guardian, for
example, boasts efficiencies in the 86-88%
range. So this is clearly an area where
all manufacturers can improve. We submit
that a 95% efficiency goal is attainable before
the end of the decade.
|
 |
|
So how
close are we now? Over the last few years,
several bow manufacturers have laid claim to
having the "World's Fastest Bow" with 340+ IBO
Speeds. So we're realistically within
50-60 fps of the theoretical Super-Bow.
And it does seem that average IBO Speeds are
slowly creeping up each year. So bow
manufacturers are making some progress.
However, 50-60 fps would represent quite a quantum leap.
| |
Brace Height |
IBO Speed |
|
Industry Average
IBO Speeds |
|
Mathews Black Max II Turbomax |
5.50" (2004) |
340 fps |
|
|
Bowtech Black Knight II |
5.75" (2005) |
350 fps |
|
2004 |
300.5 fps |
|
APA Black Mamba Extreme |
6.63" (2006) |
345 fps |
|
2005 |
301.1 fps |
|
APA Black Mamba X1 |
5.50" (2007) |
353 fps |
|
2006 |
304.7 fps |
|
APA Black Mamba X2 |
7.06" (2007) |
340 fps |
|
2007 |
306.9 fps |
|
PSE X-Force |
6.00" (2007) |
346 fps |
|
2008 |
??? |
So, can we get realistically there? Can there be a 400
fps Super-Bow in our near future? Take a
look at the chart below. If we use the
Archer's KE formula, with known values for KE
and arrow mass, we can solve for arrow velocity
and estimate the IBO Speeds of our theoretical
Super-Bow, given a particular mathematical
efficiency. As you can see, the answer is
yes - but just by a fine serving thread.
With an ultra-aggressive cam system, an anorexic
brace height, and a wildly improved total system
efficiency, a 400 fps Super-Bow could
someday be built. |
Brace
Height |
Power
Stroke |
Theoretical
Max
Input |
Super-Bow
Input |
IBO Speed @
85% Effic. |
IBO Speed @
90% Effic. |
IBO Speed @
95% Effic. |
IBO Speed @
100% Effic. |
|
5.0" |
23.25" |
135.63 ft-lbs |
128.92 ft-lbs |
346.1 FPS |
366.5 FPS |
386.9 FPS |
407.2 FPS |
|
5.5" |
22.75" |
132.71 ft-lbs |
126.00 ft-lbs |
342.2 FPS |
362.3 FPS |
382.5 FPS |
402.6 FPS |
|
6.0" |
22.25" |
129.79 ft-lbs |
123.08 ft-lbs |
338.2 FPS |
358.1 FPS |
378.0 FPS |
397.9 FPS |
|
6.5" |
21.75" |
126.88 ft-lbs |
120.17 ft-lbs |
334.2 FPS |
353.9 FPS |
373.5 FPS |
393.2 FPS |
|
7.0" |
21.25" |
123.96 ft-lbs |
117.25 ft-lbs |
330.1 FPS |
349.5 FPS |
368.9 FPS |
388.4 FPS |
|
7.5" |
20.75" |
121.04 ft-lbs |
114.33 ft-lbs |
326.0 FPS |
345.2 FPS |
364.3 FPS |
383.5 FPS |
|
8.0" |
20.25" |
118.13 ft-lbs |
111.42 ft-lbs |
321.8 FPS |
340.7 FPS |
359.7 FPS |
378.6 FPS |
|
8.5" |
19.75" |
115.21 ft-lbs |
108.50 ft-lbs |
317.6 FPS |
336.2 FPS |
354.9 FPS |
373.6 FPS |
|
9.0" |
19.25" |
112.29 ft-lbs |
105.58 ft-lbs |
313.3 FPS |
331.7 FPS |
350.1 FPS |
368.5 FPS |
But who would shoot the Super Bow? No
matter how it was marketed, the Super Bow would
ultimately be an uncomfortable ultra aggressive
bow with a wrist-popping short brace height.
So even if it was achieved, it's unlikely the
Super Bow would be more than a novelty in the
archery market? In fact, the 340+ fps
super bows of today tend to be slow sellers that don't attract much attention
after the chronograph is switched-off. So
while everyone appreciates good performance,
it's clear that shooters generally prefer more
moderate designs with 7"+ brace heights and
comfortable powerstrokes. So the Super
Bow's future may be more about the possibilities
of research and development than actually trying
to market and sell the first 400 fps bow.
 Since
shooters tend to turn their backs on
ultra-aggressive drawstrokes, developing hotter
powercurves may not be the answer. We
submit that the real future of high-performance
bow technology will be about tweaking
efficiencies. Just a 5% increase in
efficiency would add over 15 fps to the average
IBO Speed - without making drawstrokes
unnecessarily uncomfortable. This 5%
increase would make today's popular bows all
seem like super bows. And should
efficiencies ever squeak into the 95% range,
even a moderate bow may someday shoot well into
the 340's or 350's. But for now, the 400
fps Super Bow will just have to remain a
speed-junky's fantasy.
|
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