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The 400 FPS Compound Bow

400FPS Compound Bow?

Can we get there?  Should we get there?
With enough draw length, draw weight, and a light enough arrow, almost any modern compound bow can make the chronograph chirp to 400+ feet per second.

by Michael Blanton

Absolutely, yes! You bet. With enough draw length, draw weight, and a light enough arrow … almost any modern compound bow can make the chronograph chirp to 400+ feet per second. Of course, that’s exceptionally stupid – and dangerous. Compound bows are subject to a number of safety standards, not the least of which is the 5 GRAINS PER POUND rule. Unfortunately, this pesky rule is something of a barrier – a limiting factor.
cliffThe fact is, the top-gun compound bows seem to be stalled-out in the 350-ish fps IBO range. Maximum compound bow speeds haven’t really changed in ten years. Of course, many products have made huge technological leaps in the last decade. Heck, I think I still had a Motorola Flip-Phone and 768 kbps DSL in 2003. I’m not sure how we all survived before 4G and Pandora, and Netflix, and instant Facebook updates on our iPhones. So what are we still doing in the 350’s in 2013? Isn’t it possible to build an honest 380, 390,
or 400 fps IBO bow?

You’re not going to like this, but the answer is yes … and you wouldn’t buy one- you probably couldn’t shoot it anyway – so nobody would waste money building such a thing. The quest for more and more speed has brought the compound bow industry to the precipice of the performance cliff. It’s over. There’s nothing left to do but gather at the edge – or jump. If you’re not sure what that means, join me on my hunt for the next big thing – the real 400 fps bow.

SPEED SELLS: In our industry, speed sells, and extreme speed REALLY sells. But we all have rules and standards to follow – for safety – for market fairness – for truth in advertising. Of course, now and then, the temptation for the extreme speed dollar is just too great. Someone takes a chance and dances with the devil …

400 FPS IN A FISH TANK STORY: I still remember standing among the anxious crowd of archery dealers, industry media, and marketing people gathered in the conference center lobby that January morning, all awaiting the opening of the 2003 Archery Trade Association (ATA) Show. In my hand was a brochure from High Country Archery and a bold-print invitation to visit their demonstration booth to see something special. That morning, HCA was going to be my very first stop. I had already downed my two coffees and studied my event map. I was ready for battle – and I knew just where to go. I was on a mission to see something remarkable.

HCA had just launched an advertising campaign claiming that their bows were now shooting 400+ fps. Wow! Considering that the very fastest bows of 2002 were just barely cracking the 350’s, this was really going to be something to see. How had they pulled this off? HCA has been a stud bow company of the 1990’s, and although their popularity had taken some serious hits post millennium, you can never underestimate the potential phoenix factor in the archery industry. If HCA was really outperforming the next-best competitor by 50 fps, this was going to be a BIG deal. I already had the front sides of my purchase orders filled out and a couple of sharp #2 pencils in my backpack. If HCA was going to have the baddest bow on the market, I wanted to be first in line to get some.

The ATA ShowI looked around to scan the crowd – my competition. Practically everyone had the same brochure in their hands. Clearly, HCA had gotten everyone’s attention. I wiggled my way to a little better starting position. I looked at the clock as the second-hand finally lumbered up to 9:00. 3-2-1-Go! A long line of double-doors simultaneously burst open, and the huge crowd began pouring into the ATA Show.

I wouldn’t describe myself as quick or nimble, but on that day, I left everyone in the dust. Even my own crew couldn’t keep up. I arrived at the HCA demonstration booth, claiming a front-row spot – dead center. Whoo! In just a matter of minutes, the rest of the crowd filled-in all around the HCA demonstration area – hundreds upon hundreds of dealers and industry people were there to see the 400 fps bow. And there it was. Well … I thought so anyway.

HCA had things ready and waiting. Their setup looked quite impressive really. HCA had built a large wooden crate with high Plexiglas sides – it almost looked like a really big fish tank with an open top – maybe 7 or 8 ft.. tall. Inside the fish tank sat a beautiful HCA bow, strapped securely to a metal bracket, and mounted behind the bow was a machine that appeared to have some kind of robotic arm. This was going to be good – whatever it was.

Bow in a FishtankHCA wasted no time getting the show on the road. They began to explain that HCA bows were no longer bound by the 5 GRAINS PER POUND rule. Uh oh! HCA claimed that they had “toughened-up” their bows to such an extent that they could actually withstand repeated dry-firing. The new HCA bows were reportedly so strong, no minimum arrow mass would be necessary. You could hear the crowd murmur and buzz uncomfortably at the idea. Obviously HCA was about to try to pull a rabbit out of their hat. In our industry, tinkering with the safety standards is manufacturing witchcraft – an unspoken taboo. Even so … 400 fps is 400 fps. Everyone held their ground. If this were true, the archery industry might be about to change.

To prove the point, HCA was going to repeatedly dry-fire a bow, right there in front of God and everybody. You must understand, the word dry-fire is a four letter word in the archery business. In our industry – in our sport – a dry-fire is something you just DON’T DO – certainly not on purpose. Dry-fires are an excellent way to destroy a perfectly good bow and perhaps even injure yourself and bystanders – not to mention it’s a sensitive subject. Dry-fires cause a lot of conflict in archery stores. We call the conflict a D&D (dry-fire and deny). At least a few times a year, someone dry-fires their bow, then returns the bow to the store and swears upon the Father, the Son, and the Holy Spirit that their bow “just blew up” (though the damage pattern clearly indicates a dry-fire). This, as you might imagine, puts us in a very awkward spot. You’ve surely heard the phrase, “The customer is always right”? Well … that’s a tough one.

So what if HCA was really onto something? Customers would never again have to sacrifice their souls to avoid paying a dry-fire repair charge. No more arguments or diatribes would be necessary. We would never again be stuck playing referee between a customer and his factory warranty. Dry-fires would be irrelevant – a concern of the past. Glorious! Perhaps HCA was really about to change the rules, but deliberately dry-firing a bow went against everything we had ever been taught. It was as if General Motors had invited all of their mechanics to see them test a new engine with no oil in the crankcase. The combination of fear, excitement, and just morbid curiosity had my shoes cemented in place. Wow! OK! Let’s see it.

The presenter explained that the demonstration bow was a full 70 lb. peak bow with a 30″ draw length and that this particular bow “… already had 6,000 dry-fires on it.” That seemed like a pretty far-fetched claim to me, but I reserved judgement. The presenter continued to explain that the new HCA bows could handle unlimited dry-fires … they were THAT tough. Ladies and gentlemen … behold! Here we go! With the press of a button, the robotic arm went into motion. It reached up, grabbed the string at the nocking point and slowly drew the bow back. Nobody made a sound. Hundreds of people seemed to be simultaneously holding their breath. All you could hear was the sound of the robotic arm’s servo motor. The bow came to full draw and there was a brief silence … then … WHAAAAACK! The bow fired. People looked around as if to check each others’ reactions. But nothing had happened. Everything looked OK. The bow was all right. The machine reached up and drew the bow back again … WHAAAAACK! Again, nothing happened. The bow was OK. Then again … and again ….

You know that look you make when you start to overcome skepticism? It’s the same look you make when you taste something you thought would be terrible, but it turns out to really be OK. You push-out your lower lip a little and make those four or five quick little head-nods? You know the look. As the demonstration continued, a couple of my crew members had shuffled up beside me, and I gave them “the look.” They returned the look – and we all stood there with our bottom lips out – bobbing our heads for a minute – thinking.

The Devil is in the DetailsWhat would this mean? If HCA bows could really withstand dry-firing, then we could safely set them up at 4 grains per pound, maybe even 3 grains per pound – providing we could find arrows that light – with enough spine stiffness. Our wheels were turning. Sure, people would give up a lot of KE with those super light arrows, but for customers who wanted really fast speeds and super-flat trajectory, we could potentially sell them an HCA rig that would be shooting an actual 375, 400, even 425 fps. We would have to let customers know they wouldn’t be legal at IBO shoots but …. The machine kept cycling the bow over and over … WHAAAAACK! Oh, and we should probably call our insurance company and get something on paper. We kept thinking out loud. Wow! What is Bowtech going to say? Will other bow companies relax their arrow mass minimums, you think? We were standing there, practically picking out our dancing shoes… and then … quite suddenly ….


The demonstration bow suddenly exploded! It sounded like a dozen Victor rat-traps snapping closed inside a metal pail. Pieces of hardware and fiberglass smacked the Plexiglas and flew from the top of the fish tank. Most of the upper limb assembly was slung out, hitting the floor just in front of me and coming to rest six rows deep in the crowd. The entire crowd seemed to simultaneously take a half-step back and recoil with the same word, “Whoooooaaahhh!” Nobody cursed or said anything disparaging. They just said, “Whoooooaaahhh!” I guess that’s the word you use when something like that happens. Everyone just stood – stunned for a moment. The robotic arm reached up to draw back the bow again – but the bow was basically gone. Only the riser remained strapped to the bracket. Everything else was either scattered on the demonstration floor or laying in bits and pieces in the fish tank. Someone hit the stop button and the robotic arm slumped to a halt.

Like Wile-E-CoyoteRemember watching those Wile E. Coyote cartoons as a kid? Remember how you were almost rooting for the Coyote, because he worked so hard to assemble his ACME gadgets and rig up his snares, but in the end he always ended-up being smashed under a big rock or falling 10,000 ft. to the canyon floor? This was that moment for HCA. Beep beep! The HCA guys scrambled to clean up the mess and salvage the situation, but the damage was done. The point had been made. Most of the crowd politely shuffled away. It was a brave and bold effort. And I must at least honor their courage. If that bow hadn’t exploded, the standard today might actually be 4.5 or 4.0 grains per pound. 400 fps arrow speeds might be commonplace, and High Country Archery might have put Dunlap, TN on the proverbial map. And for what it’s worth, most bows were lucky to survive one dry-fire in those days … let alone many. So the experiment wasn’t a total failure. That bow was tough. Nonetheless, the plan with the ACME Rocket and the “Free Bird Seed” sign didn’t work out.

HCA later claimed that the failure was due to a broken string – not a failure of the bow – but frankly, that’s an irrelevant technicality. Splitting that hair made no difference. Had that failure happened in the hands of a customer, the scope of and liability for the disaster would have been the same. Bows simply cannot catastrophically disintegrate in the hands of a customer – ever. That’s why safety rules exist. That’s why the 5 grains per pound rule is still applicable today – a decade later. That why we routinely change string and cable sets. And that’s why we never miss an opportunity to remind customers about the dangers of dry-firing a bow. When our industry stops putting safety first, someone gets hurt. We all know it – it’s our business to know. And I admit, I was tempted by the thought – by the extreme speed dollar, but the thought of that explosion happening in the hands on one of my customers put a big lump in my throat. I laid my HCA brochure on one of their display tables, and left with my lesson learned – and a piece of their complimentary peppermint candy.

WE’VE CHANGED THE STANDARDS BEFORE: HCA’s plan was foolish and haphazard, but not entirely crazy. With a few tweaks and strategies, it could have worked. We’ve shifted our testing standards before – quite recently, in fact. Most archery consumers have practically forgotten about the old “AMO Speed” testing standard. Fifteen years ago, bows were routinely tested at 9 GRAINS PER POUND and only 60 lb. peak draw weight. As you might imagine, this resulted in much lower numerical scores (typically in the low 200’s). A bow with a 245 fps AMO Speed was by all rights a very fast bow, but as the lighter-faster IBO standard gained popularity, bows rated only by the old AMO standard seemed to be standing still. Most consumers simply didn’t understand the difference in the rating systems, or the concept that a 245 fps AMO score represented a significantly faster bow than a 300 fps IBO score.

MORE IS MORE AND THAT’S IT: What consumers understood was that more was better. As such, bows with numerically faster IBO scores were considered “faster” – whether they were or not. Once the IBO Speed model took roots, the old AMO rating system basically disappeared (at least in advertising). There are still many inside the industry who believe the old AMO test standard is the more realistic model, but the point is now moot. To customers, an AMO Speed looks too slow – and nobody wants to buy a slow bow. So the 70/30/350 IBO Speed model has endured for the last decade – the only universal rating system customers understand and accept. And it’s probably worth noting that some bow manufacturers resisted the 5 GRAINS PER POUND standard, claiming that a 350 grain arrow was simply too light and that it imparted too much stress on the bows (like a dry-fire). But like it or not, the IBO Speed was here to stay.


So let’s get back to the main question. Can a compound bow shoot 400 fps? Let’s rephrase the question. Can a compound bow, using IBO Speed Testing standards at 70# peak draw weight and exactly 30″ draw length, shoot a 350 grain (5 grains per pound) arrow at 400 fps? Basically, can you shoot 400 fps within current industry standards? The answer, oddly, is still yes. But as we said in the beginning, you wouldn’t like it – you couldn’t shoot it – and nobody would waste money building it.

LIMIT OF THE IBO SPEED MODEL: As a matter of energy storage, the IBO Speed standards (70#, 30″, 5 grains/lb) puts a mathematical cap on the maximum amount of energy that a compound bow can store. I know this sounds anticlimactic, as we’re a culture of consumers who expect products to keep getting better, faster, more powerful, etc. But you must remember, the compound bow isn’t powered by an external fuel , battery, or a computer processor. Despite the high-tech talk and anodized gizmos, your compound bow is still a simple hand-drawn weapon. Its output can never exceed the input it gets from the human body. The human body and the testing model provide hard mathematical limitations. So naturally, we can’t expect compound bows to perform faster and faster forever. At some point, the output of the bows will begin to approach the maximum input of our testing model … and then the speed race is over. This is the performance precipice I mentioned at the top. I regret saying this, but we’re essentially there now.

If you don’t already know the archer’s KE formula, you should get to know it. It’s pretty simple. KE=mv²/450,240, where KE is kinetic energy in ft-lbs, m is the mass of the arrow in grains, and v is the velocity of the arrow in fps (and yes, yes, I know the standard formula is KE=1/2mv²). The archer’s KE formula just tidies-up and converts all the units so we can use grains, fps, and ft-lbs – which makes the formula easier to use. Let’s say that your bow shoots a 400 grain arrow at 300 fps. The KE output of your bow can be found like so:

KE=79.957 ft-lbs.

Now, to create a true IBO legal 400 fps bow, we must first figure out how much energy would be required to get the job done. So if you were shooting your 400 fps IBO Speed bow, at 70# peak draw weight, and exactly 5 grains per pound of arrow mass (350 grains), the output of the bow would be expressed like this:

KE=124.378 ft-lbs.

WE NEED 124.378 FT-LBS AND NOT A PENNY LESS: So in order to shoot a 350 grain arrow at 400 fps, we need the bow to AT LEAST store 124.378 ft-lbs. of PE (potential energy) during the drawstroke. Since we can’t get more out of our compound bow than we put in, in order to hit the 400 fps goal, we must be able to load the bow’s limbs with that much energy. And in order for it to be a true “IBO Speed,” we must do it within the confines of the 70# peak draw weight and 30″ draw length standards. So is it possible … actually, yes.

Before we talk about storing energy during the drawstroke, you should know that a bow’s draw length isn’t the same as the powerstroke length. If you have a 30″ draw length, that doesn’t mean you’re actually pulling the string back a 30″ distance. The official draw length of a bow is found by measuring from the nocking point on the string, in a line perpendicular to the center line of the bow, to an imaginary point above the pivot point of the grip, plus 1.75 inches. What that means is that the powerstroke of your bow (the distance you’re actually pulling on the string) is equal to your draw length, minus your bow’s brace height, minus 1.75″. So if you’re shooting a bow set for 30″ draw length, and the bow has a brace height of 6″, for example, the actual powerstroke length of the bow is 30-6-1.75 = 22.25.

energy storage for compound bowSo if we are trying to build an honest 400 fps Super-Bow, we have to understand that we are limited to 70# peak draw weight, and a powerstroke length of 22.25″ (assuming we want our bow’s brace height to be 6″). The next question is, given those limitations, can we do that much work (force x distance) in just 22.25″. The answer, again, is yes. If your compound bow is set for 70# peak, and there is no ramp-up to peak weight – the bow starts off right at 70#, and the draw weight stays at exactly 70# all the way back, inch by inch, and there is absolutely no let-off when you come to full draw … here’s the potential energy you could store in the bow’s limbs: 22.25×70=1,557.5 in-lbs. or 1557.5/12=129.79 ft-lbs. Hooray! The minimum energy required to go 400 fps was 124.378 ft-lbs, and our 6″ brace height 70# bow makes the cut with more than enough potential energy @ 129.79 ft-lbs. If we are REALLY daring, we could even choke our brace height down to 5.5″ (132.71 ft-lbs) or even 5″ (135.63 ft-lbs). Unfortunately, a nice comfortable 7″ brace height won’t quite get us there (123.96 ft-lbs).

IN THEORY: So, theoretically, an honest 400 fps certainly is possible. But we’re assuming a few things. First, we’re assuming that the bow can operate at near-perfect efficiency. Our energy numbers here are pretty close, we don’t have much margin to spare. So our 400 fps Super-Bow can’t waste much energy on friction, noise, vibration, etc. We need every little ft-lb of energy to be stored by those limbs and then successfully transferred into that arrow. If we lose just 5% in the transfer (6.49 ft-lbs), we won’t have enough energy to make the 400 fps. We also must assume that our customers won’t mind a 0% let-off bow, and the harshest imaginable drawstroke (70# from start to finish). Our 400 fps Super Bow will not be for the timid.

But wait! We want 70-80% let-off. We want a smooth drawstroke with good transitions and a comfortable valley. Nope! Not today cupcake. Moderating the drawstroke to include a gradual ramp-up to peak weight, and a gradual decline to a comfortable let-off and valley will cost us energy storage. We can’t spare it. You wanted a faster bow? Well, you have to make some sacrifices. Here’s where we get to the precipice of the performance cliff – the point where we know that faster bows are still mechanically possible – but we know that you wouldn’t like them – you couldn’t shoot them – so nobody would waste money building them. This is the proverbial cliff I’ve been describing, the fixed-point on the landscape of the compound bow industry. Beyond that fixed point (about 360+ fps) we jump into the abyss of ultra-short brace heights and drastically harsh draw strokes.

UNDERSTANDING THE DRAW CYCLE: Years ago we tested bows on our draw-force machine and graphed their cam cycles. Some bow manufacturers clearly had their engineering down, the draw-force curves were clean and deliberate. Makers like Bowtech and Hoyt were controlling every moment of the drawstroke – nothing was incidental or haphazard. Other makers still had a few humps and ugly spots in their graphs – they hadn’t quite gotten the geometry of their cams just right. But today, virtually every cam on the market has a drawstroke that is computer optimized millimeter by millimeter. Some cams are optimized for speed, some for comfort, and some try to find a blend between the two. There’s no mystery left in cam geometry. The only mystery is in trying to predict what YOU want to buy. At what point will customers say a bow is “too aggressive” or “too harsh”? Where is that perfect blend where customers will think a bow has plenty of speed while still feeling that the drawstroke is smooth and comfortable?

draw force curve

This graph represents drawing-back a compound bow and then letting it back down. Length (distance) is plotted against weight. At marker (1) the bow is at rest. At marker (2) 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 (3). Then the draw weight begins to decrease (4) until finally reaching full let-off at marker (5) 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 sample graph above is taken from a moderate bow with a relatively smooth-drawing cam (305 fps IBO Speed). 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.

rd-forcedraws (1)

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 there are a few traditional round wheels and cam grinds that replicate the round wheel powercurve still on the market, but slow IBO Speeds make them poor sellers. If you’re interested in this kind of bow, don’t expect much of a selection in the new bow market. Instead, consider buying a 20 year old wheel bow that’s still in good condition – there are countless thousands on the used market – and they’re cheap!

MEDIUM CAM: The Medium Cam graph is typical of today’s basic single and softer hybrid cams with IBO Speeds in the 300-320 fps range. These cams are more aggressive than wheels, ramping to peak weight more quickly and then coming to full let-off more abruptly. So they tend to store up more energy and shoot notably faster. However, a Medium Cam is still generally acceptable to most shooters. Most shooters will describe this type of cam as “smooth drawing,” simply because peak weight doesn’t persist throughout much of the cycle. For bowhunting and general purpose use, this type of cam offers a good 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 330 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.

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 green). So any design that’s going to make 400 fps will have to take the Hard Cam concept to the extreme.

force draw curves energy input comparison

NO FREE LUNCH: Before we go through the exercise of mock-designing the 400 fps Super Bow, we have to dispense with the idea that it can somehow be smooth and comfortable. It won’t. It can’t. Even if such a bow were covered in black velvet and Corinthian leather, there will be nothing “nice” about this bow. It will be a snarling pitbull that we can scarcely keep muzzled and on the leash. But let’s work through the numbers and try …

THEORETICAL MAXIMUM: So let’s examine the hard-cam profile more closely. 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 (as described a few paragraphs above: no ramp-up to peak weight – the bow starts off right at 70#, and the draw weight stays at exactly 70# all the way back, and there is absolutely no let-off when you come to full draw), the draw force curve wouldn’t be a curve at all. Drawing the bow back would literally feel like picking a 70 lb. weight 22.25 inches off the floor. The resulting area under the “curve” would be a 22.25″ x 70# block (assuming a 6″ brace height and a 30″ draw length). A cam with this profile would render a bow all but unshootable.

graph of compound bow cams 1

INCORPORATE MINIMAL LET-OFF: To test the feasibility of our 400 fps Super Bow concept, let’s consider some very minor modifications – to at least restore minimal shootability. For illustration purposes, we’ll use some simple lines here and avoid the Calculus. 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 below, we’ve adjusted the curve to incorporate a scant 65% let-off during the last 2″ of the powerstroke, reaching 65% relief at exactly 30″ draw. This mathematically reduces our energy input by 45.5 in-lbs or 3.79 ft-lbs.


RAMP-UP TO PEAK WEIGHT:  We also should be realistic about 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.

compound bow draw cycle - restore ramp to peak weight

WE MUST SHORTEN THE BRACE HEIGHT TO 5.5″: Our IBO legal 400 fps Super Bow is taking shape. With a possible geometry for the ramp-up and a very minimal cut for the let-off, the bow would no longer be un-shootable. The question is, do we still have enough energy? If you remember, we needed 124.378 ft-lbs to get our 350 grain arrow moving at 400 fps. Our original “theoretical maximum” cam could generate 129.79 ft-lbs, but after the modifications to allow for let-off and ramp-up, we lost 3.79 ft-lbs. and 2.92 ft-lbs. respectively. So now the best we can make is 123.08 ft-lbs., just shy of the required 124.37 ft-lbs. So our 6″ brace height won’t get the job done. No problem. We can always shorten the brace height by 1/2″ and pick-up and extra 2.92 ft-lbs. So now our 400 fps Super-Bow has a 5.5″ brace height – pretty short – but maybe we can still pull this off.

compound bow system efficiency graph

EFFICIENCY: Getting energy in the bow is one thing – getting the energy to efficiently transfer into the arrow is quite another. You may have noticed 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 out than you put in. Of course, the energy isn’t lost, it just gets converted into other things we don’t necessarily want (heat, vibration, noise, etc.). 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. If that were the case, then our 5.5″ brace height 400 fps Super-Bow would practically be ready for production. Unfortunately, the reality is somewhat different. Even the best bows on the market are well shy of being 100% efficient.

compound bow hysteresis graphHYSTERESIS, THE ENERGY THAT GOT AWAY: 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. In fact, you get double-charged, as those same forces resist the bow’s motion both directions. 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 two paragraphs above, but this graph has been folded in-half to allow you to compare the two curves. The area between the two curves (in blue) represents the energy that is lost to hysteresis.

EFFICIENCY OF THE 400 FPS SUPER BOW: While a good high-performance bow is around 80% efficient, there are a few bows that really lead the field. A few hot-shots boast efficiencies in the 85-90% range. So this is clearly an area where all manufacturers can improve. But what about our 400 fps Super-Bow? Well, let’s be optimistic for a moment. Let’s assume that the 400 fps Super-Bow is made with extra greasy bearings, and super slick strings, and highly polished cam grooves and the like. Every surface has been optimized to reduce friction like no other bow before it. So, let’s say our 400 fps Super-Bow only loses 5% to hysteresis – twice as good as the best bows today.

SUPER-BOW OUTPUT: Now we can finally predict the performance of our 400 fps Super-Bow. With the cam modifications and shortened brace height, we computed a possible energy storage of 126 ft-lbs. But now we must subtract our 5% efficiency loss, bringing our final total to 119.7 ft-lbs. Not enough. We’re going to miss the 400 fps mark by 4.678 ft-lbs. No problem though, we can always reduce our brace height a little more. Since each inch of brace height reduction yields 5.833 extra ft-lbs, another 13/16 of an inch should do it. So we simply reduce our 5.5″ brace height to 4-11/16″ and were done. We made it. The 400 fps Super-Bow can be born. I told you it was possible.

Surely you might sense the sarcasm and spitballing in this make-believe project. Even if we assume the cam geometry could be manipulated for that insane ramp-up, and efficiencies could somehow be brought up to 95% (and those are some whopper assumptions), a 400 fps IBO compound bow still wouldn’t be realistic. For those of you who didn’t follow my wanderings into Ridiculousville, our modified Super-Bow would still come with a draw cycle from hell, no valley, very little let-off, and a 4-11/16″ brace height. The result would be a total wrist-blistering nightmare. Just looking at it would make grown men cry.

NOT SO SUPER: When we create a bow that nobody would really want to shoot – a bow that’s simply too aggressive for any useful purpose – we have taken the last step off the cliff. The 400 fps Super-Bow doesn’t exist because it shouldn’t exist – and frankly, it’s probably not going to exist. As long as the rules stay the same, 350-360 fps is the performance precipice. Once a cam profile is modified just enough to be realistic, and we get cams rotating as quickly as possible, and setup a reasonable brace height, and we consider efficiencies that are actually attainable with today’s materials and technologies … what we really end up with is bows like the Bowtech Insanity CPX, the APA Mamba M6, or the PSE Omen Max. Those bows are already at the precipice.

PSE OMEN MAX: If there is any bow on the market that qualifies as the current “Super-Bow,” perhaps it’s the 358-366 fps PSE Omen Max. PSE has been a major player in the speed-bow market since the introduction of their X-Force series bows in 2007. Nobody can deny that the X-Force bows are fast – really fast. And PSE is probably the most sophisticated bow manufacturer in the market. So it’s safe to say that the Omen Max represents the last few inches of the performance cliff and the best go-fast engineering the industry has to offer. Since the Omen Max has a 5.5″ brace height, some might even argue that the Omen Max has one foot dangling off the edge. But the point is, the Omen is the last stop. The next step is just a leap into lunacy – to brace heights and cam profiles that no one could effectively shoot – or to the abandonment of our current standards. The next step we take is over the proverbial cliff.

SHALL WE GATHER?: At the top, I mentioned how the quest for more and more speed has brought the compound bow industry to the precipice of the performance cliff, and that there’s nothing left for us to do but gather on the edge – or jump. What I mean is that the entire compound bow market is collectively inching towards that 350-360 fps limit. Ten years ago, only a brave few ventured out into the 340’s and 350’s. The bows were just too radical, and mainstream customers didn’t really like them. And although top-speeds haven’t really increased, a number of innovations have made all bows more civilized. Even our speedy bows have gotten more sophisticated – more refined – more soft and cuddly. So today, a 330 fps bow hardly seems radical at all. Those 300 fps IBO Speeds have all but faded away. Most of the 310’s are fading too. Even the modest midline bows now poke into the 320’s without fanfare, and I think the trend is going to continue. We may soon see those 320 bows start to disappear – maybe even some of the 330 bows. Sooner or later the entire market is going to end-up within 20 fps of the same cliff. There will be nothing left to do. No place left to go. The whole compound bow market will begin crowding the edge of that performance cliff … that is, until somebody jumps!

PLANNED OBSOLESCENCE: There no getting around it – speed sells. But it’s not a game here on our side of the counter. The archery industry employs countless thousands of people. It’s a half-billion dollar industry that many families depend upon. We NEED people to keep buying new compound bows. We NEED customers to be excited about the latest technologies and improvements. But the sad reality is, sooner or later our customers are going to figure out that compound bows really aren’t getting any faster, or smoother, or quieter, or more accurate. When the new bows no longer outperform the old bows – when we lose the advantage of planned obsolescence – we’ll all be in big trouble. A good compound bow should last 10 or 20 years, at least. There’s no reason to replace a perfectly good bow, unless the new models are significantly better. What are we going to do then?

MAYBE HCA HAD THE RIGHT IDEA AT THE WRONG TIME: Strangely enough, the industry will probably just rewrite the rules – taking a page from the old High Country Archery strategy. When sales start to slump, someone will remember that day in 2003 when the bow exploded and a company hung its head in shame. Someone will remind us all that the industry must go on. We must go faster – and faster – as our customers demand. Desperate times will indeed call for desperate measures. One day soon, out of simple necessity, I think the industry will just change standards. And if we’re clever, customers probably won’t notice a thing.

NUMERICALLY HIGHER RATING SYSTEM: There was a good lesson to be learned when numerically higher IBO Speeds became the standard ten or fifteen years ago. Higher numbers are better – especially when we’re talking about power and speed. And we learned that most customers had no interest in dissecting the measurement standards, they simply gave preference to the highest score. When the 350-360 fps bow no longer seems special (and we’re almost there), the industry is going to reexamine that rating system. Something will have to change. Either we will start testing bows at longer draw lengths, or begin correcting speeds for altitude density, or test bows at 0% let-off … something will have to change to push those advertised numbers higher.

FEWER GRAINS PER POUND: More than likely, the solution will just be one little number. The 5 GRAINS PER POUND rule will be modified, similar to what HCA tried to do in 2003. Except this change will be made for the good of the industry – quietly – with no Plexiglas fish tank or bow explosions. I suspect that one day soon, the 5.0 standard will be updated to 4.5 grains per pound – maybe even 4.0 grains per pound. The justification will be simple and palatable. Today’s bows are tougher, so the testing standard is being updated to reflect the changes in technology. And in all fairness, most modern bows would probably survive the reduced arrow mass standards without any significant drama. 5.0 to 4.5 isn’t such a huge change – not like a purposeful dry-fire. It’s only 35 grains. So why not?

LET’S ALL AGREE TO GO FASTER: If the major bow companies and the ATA agree to quietly apply a new standard, say 4.5 grains per pound, few people would even notice the switcher’oo. More importantly, the new standard would rejuvenate those 350 fps bows into 369 fps bows. 330 fps bows would become 348 fps bows. Even the lowly 310 fps bows would become 327 fps bows. Now THAT’S something we could sell. The cliff edge would be temporarily moved forward, and customers would start selling off their slow old 5 grain per pound bows for the numerically faster 4.5 grain per pound models. The vast majority of those customers would have no idea why bow technology suddenly seemed to improve, and they wouldn’t care. After all, speed sells. And who knows … one day we might even get a bow that shoots 400.

Be careful out there – and watch that edge.


1 Comment on "The 400 FPS Compound Bow"

  1. Wonderful article from back in the day that still applies. No matter how hard we try, we’ll never achieve a 100% efficiency system. Now, nearly a decade later, we have seen bows tickling the limits but without some changes to the IBO standards the speed-race seems to have taken a back seat to refinements on other fronts. In the future I expect to see the advancements of the IBO race translate into all around more efficient/quiet bows in the 330fps range.

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