Parallel Limb Bow 101

TALK ABOUT TERMS

Not since High School Geometry Class have we all heard the word “parallel” so often. “Parallel” is the new million-dollar word in the archery industry. In every archery magazine, commercial, talk-forum, and publication, the word parallel commands attention. It’s a full fledged industry buzz-word. The phenomenon has even spawned a number of new parallel phrases, such as….. parallel geometry, a parallel configuration, and parallel technology. Strangely, a few archery marketing gurus have even managed to quantify the parallel ideology, as in….. near parallel, or highly parallel, and the popular virtually parallel. So whatever a highly parallel geometric bow configuration is, we must desperately need it. So perhaps it’s time to take a closer look at this innovation and to develop some way of measuring just how much Parallel Force Quantum Alignment we archery enthusiasts are actually getting.

PARALLEL LIMB ORIENTATION

Of course, the parallel reference is to the bow design phenomenon we commonly know as Parallel Limbs. Over the last few years, the Parallel Limb bow has risen to become the apex predator in the compound bow market, and every bow manufacturer now wants their piece of the kill. But What qualifies a bow as Parallel Limb?

What is a Parallel Limb Bow?

Let’s start at the top.

Basically, a Parallel Limb Bow is a bow that has a long riser and short swept back limbs which are oriented horizontally (more or less) at full draw. Unlike the limbs of a conventional bow, which are oriented more vertically, the limbs of a parallel limb bow angle back directly towards the shooter. So from a side view, the limbs appear to b…well..virtually parallel to each other.

Hence the name of course.

GRADUAL CHANGE TOWARDS TRUE PARALLEL LIMB ORIENTATION

Parallel limb bows didn’t just suddenly appear on the market like a new cam system or camo pattern. The development of parallel limb designs has been gradual. As materials and machining technologies have improved, some bow manufacturers have steadily tinkered with longer riser lengths, shorter limb lengths, higher limb-pocket angles, and increased limb pre-load. So particularly during the last 5 years, we’ve seen an increasing degree of parallel limb orientation in the new bow designs.

PARALLEL SELLS

Anyone who has shot a parallel limb bow can tell the obvious difference immediately. Compared to standard bows with upright limbs, parallel limb bows have very little recoil (and less noise). And the flatter the limb orientation is, the smoother they seem to shoot. There’s no wonder why so many bowhunters (especially) are opting for the newest parallel limb bow designs. Virtually all of the hottest-selling bows on the market are now parallel limb style bows. Unfortunately, the success of the parallel limb bow has created a number of parallel wanna-be’s out there – bows which are advertised as parallel limb but clearly are not…..at least not close enough to be a genuine member of the club.

THE NEED FOR MEASUREMENT

It seems that any swept-limb bow can be adorned with the label parallel limb. However, the various designs certainly don’t share the same limb angles. Some designs are obviously much closer to being truly parallel than others. So are consumers really getting what’s being advertised? Or is the label parallel limb being used so loosely that any bow with a 70º+ limb angle is now being sported as a parallel limb bow? Since all parallel limb bows are obviously not created equal, we think it’s time for some reasonable standards of measurement on this issue.

1. PRACTICALLY PARALLEL TECHNOLOGY

Before we start talking about varying “amounts” of parallel measurement, we should note that the word parallel isn’t a marketing term. It’s a mathematical term that describes the relative orientation of two lines. By definition, parallel lines extend in the same direction, they share a common perpendicular, they are equidistant at all points, and they must never converge or diverge. So two parallel lines would never intersect, even if you extended them forever in both directions like a set of perfectly straight never-ending train tracks. Simply put, either two lines are parallel or they’re not. However, for the purposes of melding the marketing with some basic measurement, we should define a true “parallel-limb” bow as a bow with limbs swept back at 90º or greater (with respect to the vertical axis of the bow) when the bow is at full draw.

2. MEASURING CHALLENGES

So, are there any bows on the market today that actually have limbs that are parallel?

Maybe. That’s what we’re going to find out. But before we can declare any particular design as a truly parallel limb, we first have to decide on a method of measurement. Compound bow limbs aren’t straight. They’re installed with some degree of pre-load (bend), such that a bow’s limbs predominantly have a curved or arched shape even when the bow is at rest. When the bow is at full draw, the curvature of the limb increases. So the angle of the limb, with respect to the bow’s vertical axis, will be different at the limb pocket than near the axles, and the limb angles change as the bow is drawn back. So this could get tricky.

3. MEASURING THE LIMB ORIENTATION

To get the best measurement of the limb angles, we are going to focus only on the last 4″ of the limbs, where there is little to no limb curvature. By carefully measuring the distances between 4 points along with the last 4″ of limbs, we are able to gather enough data to compute the limb angles.

We begin by measuring and marking the limbs at exactly 4″ from the outside corners of the limb tips. We then measure the distance between points A & B and then the distance between points C & D. As you can see, the resulting ÐABD certainly isn’t a 90º angle, but this gives us a nice regular trapezoid, and a basis to begin our calculations.

In this example (2007 PSE Brute), line AB was 32.77″, and line CD was 30.19″. Of course, we already know the distances of line AC and BD are 4″. Now, to find the limb angle, we need to make a right-triangle first.

To compute the limb angle (ÐDBF), we need at least two side lengths of the right triangle DFB. We know the length of line BD, and we can find the length of line FB by finding 1/2 the difference of lines AB and CD. Then we can compute ÐDBF with some basic Trig functions.

  • FB = (AB-CD)/2
  • FB = 1.125″
  • BD = 4″
  • Ratio = ADJ/HYP = 1.125/4 = .28125
  • ACOS(.28125) = 73.68º
  • ÐDBF = 73.68º

So we know that the at rest limb position of the PSE Brute is 16.32º short of actually being parallel (90º). So if we had to quantify how parallel the Brute’s limbs are, we can figure it as a percentage and say that the Brute’s limbs are 81.9% parallel (73.68/90) when the bow is at rest. So the Brute isn’t quite parallel when at rest. But will the Brute’s limbs be 100% parallel at full draw?

Using our Apple Force Draw Machine, we brought our PSE Brute to full-draw, and then repeated the same marks and measurements. The outer measurement was 29.19″ and the inner was 28.63″. This yielded a limb angle of 85.99º, which is surprisingly 95.5% parallel.

At Rest L1 At Rest L2 Full Draw L1 Full Draw L2
32.44 30.19 29.19 28.63
  • Limb Tip Travel @ 29″ Draw: 1.63″ p/limb
  • At Rest Limb Angle: 73.67º / 81.9%
  • Full Draw Limb Angle: 85.99º / 95.5%

PARALLEL COMPARISONS

We repeated this procedure for the popular Fred Bear Element and the new Diamond Black Ice to compute their limb angles. As you can see, although each of these bows is advertised as parallel limb designs, only one of them actually reaches a 90º angle at any time during the drawstroke. However, all three bows are within 5% of parallel at full draw.

2007 PSE BRUTE

2007 PSE BRUTE
At Rest
L1
At Rest
L2
Full Draw
L1
Full Draw
L2
32.4430.1929.1928.63
Limb Tip Travel:    1.63″ p/limb
At Rest Limb Angle:    73.67º / 81.9%
Full Draw Limb Angle:    85.99º / 95.5%

2007 FRED BEAR ELEMENT
2007 FRED BEAR ELEMENT
At Rest
L1
At Rest
L2
Full Draw
L1
Full Draw
L2
32.8829.8828.5027.94
Limb Tip Travel:  2.19″ p/limb
At Rest Limb Angle:    67.98º / 75.5%
Full Draw Limb Angle:    85.99º / 95.5%
2007 DIAMOND BLACK ICE
2007 DIAMOND BLACK ICE
At Rest
L1
At Rest
L2
Full Draw
L1
Full Draw
L2
32.2830.4728.3528.66
Limb Tip Travel:  1.97″ p/limb
At Rest Limb Angle:  76.92º / 85.5%
Full Draw Limb Angle:  92.22º / 102.5%

We also tested a random sampling of a few other parallel-limb bows in the shop. We even measured a non-parallel limb bow for comparison. We’ve ordered them here and classified all bows with limb angles 90º or greater as Class AA Parallel Limb Bows. Bows with limbs angle greater than 85º but less than 90º are called Class A parallel-limb bows. Bows with limb angle greater than 80º but less then 85º as Class B parallel limb bows. And any bow with a max limb angle less than 80º is considered a non-parallel limb bow.

Compound Bow
Make/Model
Max Limb Angle
(@ 29″ Full Draw)
Percent of
Parallel
Parallel
Class
 
Fred Bear Truth94.0ºBEYONDClass AACLASS AA: No significant hand-shock detectable at the shot. Bow does not recoil away from your hand upon release. Minimal forward limb thrust. Excellent efficiency and speed.
Parker Frontier 2-Plus92.2ºBEYONDClass AA
Diamond Black Ice92.2ºBEYONDClass AA
Browning Myst91.3ºBEYONDClass AA
Bowtech Allegiance90.9ºBEYONDClass AA
Jennings CK 3.4R86.4º96.0%Class ACLASS A: Very little hand-shock detectable at the shot.  Some forward limb thrust, but bow remains comfortably stable at the shot. Improved efficiency and speed. 
PSE Brute86.0º95.5%Class A
Fred Bear Element86.0º95.5%Class A
Bowtech TomKat86.0º95.5%Class A
PSE Firestorm Lite82.3º91.4%Class BCLASS B: Notable reduction in hand shock and forward thrust compared to bows with standard limb angles. 
Browning MC Adrenaline81.9º91.0%Class B
Diamond Rapture81.6º90.6%Class B
Bowtech Mission OBS79.0º87.8%Non-Parallel 
PSE Triton NRG73.0º81.1%Non-Parallel

PARALLEL LIMB REVOLUTION

Parallel limb bows appear to be here to stay, at least for a while. For 2007, practically every compound bow manufacturer has swept their limb angles back to keep up with the market trends. Of course this is good news for archery consumers. Since the entire market is soon to be flooded with these designs, you won’t need to spend $600+ just to get one. With the majority of the manufacturers restructuring their line-ups, the parallel limb designs can no longer be reserved for the “high-end” bow market only. For 2007, there will be many parallel limb bows in the $299-$499 mid-price ranges – so even more shooters can enjoy the benefits of the quiet low-recoil parallel limb designs.

WHAT’S THE NEXT “BIG THING”?

The parallel limb secret it out, and no single manufacturer can keep it for themselves. In fact, parallel limb bows are now the industry standard, and we suspect the conventional D-shaped bows are soon destined for the bargain bin. But that leaves bow manufacturers, especially the “en vogue” brands, with a new problem. How are they going to entice “high-end” buyers now? What’s the next “big thing” that will demand the premium price tags? Carbon-fiber risers? Mag-Lev Cams? Who knows! Only time will tell. But until the next must-have innovation comes along, it’s a bow buyer’s market.

PERSPECTIVE

ARE PARALLEL LIMB BOWS MORE ACCURATE?

Probably not. While the parallel limb bow is notably smoother and quieter at the shot, there’s no direct evidence to suggest a parallel limb bow is inherently more or less accurate than a standard D-shaped bow. In fact, target archery professionals still prefer the more upright standard limb designs. Of course, we must also consider that parallel limb bows tend to be short. Target shooters generally prefer longer axle-to-axle designs. And since target shooters aren’t usually concerned about noise, or recoil for that matter, the parallel limb bow isn’t as much of a phenomenon on the competition circuits. But for bowhunters, it’s a different story. Within a typical bowhunter’s range, a parallel limb bow is likely to provide the same consistency and accuracy as any other style bow.

DO ALL BOWHUNTERS LOVE PARALLEL LIMB BOWS?

Certainly not! PLB’s don’t appeal to everyone, as they have a few drawbacks of their own. While the generation-x buyers tend to love the radical batwing shape of parallel limb bows, we hear a number of more traditional buyers comment that parallel limb bows are “ugly”. And perhaps it’s a fair criticism. For those who appreciate the sleek lines and traditional appearance of a bow, the parallel limb bow is no beauty queen. But the criticisms aren’t just about appearance (although we hear PLB’s have nice personalities).

PLB’S PRESENT EXTRA BOW MASS

As we mentioned earlier, parallel limb bows start with a very long riser. Since most of the bow’s weight is in the riser, the parallel limb bow is typically 1/2 pound heavier than standard D-shaped bows. In 2001, the average compound bow weighed just 3.6 lbs. Today, that average is well over 4 lbs. You may in fact notice that the term “lightweight” has largely disappeared from bow advertising. The dilemma is balancing weight against structural stability. The longer a riser is, the stronger it must be. So there is only so much material the manufacturer can machine away (cut-outs in the riser) to reduce the riser’s finished mass. So for now, parallel limb bow buyers will simply have to live with the extra weight.

BOW MASS PREFERENCES

Interestingly, adding some mass to the bow isn’t automatically a bad thing. Some shooters actually prefer a little more weight, particularly competition shooters. A heavier bow tends to be more stable at full draw and easier to hold steady while aiming. So in that respect, PLB’s aren’t necessarily a step backwards. On the other hand, some shooters clearly prefer the lightest bow possible – particularly hunters who hike long distances. Perhaps there is no right or wrong here, as this is clearly a matter of personal preference. But let’s keep things in reasonable perspective. Less than 2 lbs. separates the very lightest from the very heaviest bows on the market. And among popular men’s hunting bows, the difference from the lightest to heaviest is about 12 ounces (the weight of a can of soda). So even considering the extra mass of the PLB riser, your fully-accessorized bowhunting rig will still tip the scales well under the weight of your hunting rifle.

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