Tuesday, February 4, 2014

A Tale of Two Pipe Diameters - Comparing the Efficiency of Two PVC Bows

There have been a lot of questions regarding the speed of PVC bows. There seems to be a huge misconception, largely based on my own videos, that PVC bows are really slow. It's my own fault. In an effort to be fair and conservative, almost all of the speed tests done of my channel have been with 10gpp (grains per pound) arrows.

While most archers shooting selfbows and some shooting longbows use this measure, the rest of the archery world tends to use lighter arrows to measure bow speed. This usually results in higher speeds. For example, a bow's AMO (Archery Manufacturers Association, now Archery Trade Association) speed is based on a 60 pound (maximum) at 30 inch bow shooting a 9gpp arrow. I won't get into it now, but those extra two inches of draw will raise a bow's efficiency (a 60 pound at 30 inch bow will be faster than an identical bow that's 60 pounds at 28 inches if both shoot the same arrow). The IBO (International Bowhunting Organization) method measures speed based on a 70 pound (maximum) at 30 inch bow shooting a 5gpp arrow.

Compared to my tests of bows drawn to 28 inches and shooting 10gpp arrows, the AMO and IBO speeds will be much, much higher. Most times manufacturers of bows will put speeds on bows based on using a lighter than 10gpp arrow. All of this combines to make the speeds I get from the bows I've tested seem really slow. In an effort to help dispel the myth that PVC bows are slow by default, I'm going to start testing not only by my 10gpp standard, but also with lighter and heavier arrows to show what PVC bows are capable of.

So all of this brings us to the topic of the day. Two bows, each built by an independent PVC bowyer, two different pipe diameters, both 45 pounds at 28 inches, and both have a similar unstrung and braced profile (if you don't count the snakey curves). Which will reign supreme as most efficient? Will it be the lightweight 3/4" bow built by Bruce Laberdee? Or will it be the heavyweight 1" bow built by Ken Nyholm? We shall see.

The first step to finding a bow's efficiency is to first figure out how much energy it is storing in its limbs. The easiest way to do this is to measure the bow's weight at every inch of draw. The resulting measurements, when laid out on a chart, create what is known as an F/D (force/draw) curve. By taking the numbers under the curve, or all the weights at every inch combined, you get the bow's energy storage in inch-pounds. Divide that by 12 and you get the bow's storage in foot-pounds.
Here are the F/D curves for both bows. Both bows store about the same amount of energy, though the 1" bow stores slightly more.
After measuring both bows, it turns out the 1" bow actually stores a little more energy. The 3/4" bow stored 37.25 foot-pounds, while the 1" bow stored 39.16 foot-pounds. Now comes the true test: speed. In order to figure out how efficient both bows are, we need to measure speed and then figure out the energy output, or how much energy is actually translated from potential energy to kinetic energy.

After shooting both bows through the chronograph, the 3/4" bow ended up with an average of 164FPS with a 10gpp arrow and the 1" bow ended up with an average of 145FPS with a 10gpp arrow. Already we can see that the 3/4" bow is faster with the same arrow. Now that we know the speed, we can figure out the bow's kinetic energy ((Speed(FPS) X Speed(FPS) X Mass(grains)) / 450,240 = KE(foot-pounds)). The 3/4" bow comes out to 27 foot-pounds, while the 1" bow comes out to 21 foot-pounds. Divide this number by the potential energy and you get the dynamic efficiency. The 3/4" bow comes out on top at 72% efficient, while the 1" bow comes out to only 54% efficient.

So what does this all mean? Basically, it means that the 3/4" bow turns 72% of the energy it stores into kinetic energy to propel an arrow. The 1" bow only converts 54% of its energy, so even though it stored more energy it still ends up with a very low output. But why is that? Why is the 3/4" bow more efficient?

One of the biggest factors here is mass. The 3/4" bow has much less mass than the longer and heavier 1" bow. That means the 1" bow needs to put a lot of energy into moving heavy tips and limbs rather than moving an arrow. Because 1" pipe needs to be made long to achieve the same weights as shorter 3/4" bows, it is already at a disadvantage. While the very short bows do succumb to inefficiency due to being so short and excessive limb travel, they come out on top because a comparable 1" bow will be almost double the physical weight.

The simplest way to make a PVC bow more efficient is to find ways to cut weight. Wooden siyahs, tapered tips, and shaped tips all help bring tip and limb mass down. Another way to make a bow more efficient for its draw weight is to increase early draw weight, which can be done by reflexing and recurving a bow. Making a PVC bow more efficient is a whole post in and of itself.

For those that are looking at speeds, here are the recorded speeds for both bows with different arrow weights (both bows drawn to 28" AMO). One thing to add is that these arrows are at least the weight labeled or higher so if my numbers are off, they are on the slower more conservative side. My lightest arrows were 300 grains, as I don't have anything lighter that I would like to shoot out of these bows. The rest are a little heavier than labeled by a few grains or so.

3/4" Bow - 196.6FPS (6gpp), 183FPS (8gpp), 164FPS (10gpp), 157.7FPS (12gpp), 140.9FPS (14gpp)
1" Bow - 168.5FPS (6gpp), 149.9FPS (8gpp), 145FPS (10gpp), 137.9FPS (12gpp), 128.8FPS (14gpp)

So what do all these numbers mean? Just to give a little perspective, a good rule of thumb for selfbow speed is that a bow should shoot at least 100FPS plus its own weight with a 10gpp arrow (100 + Bow Weight = Speed in FPS) to be considered a decent performer. So a 45 pound bow should shoot at least 145FPS. So by those standards, even the 1" bow isn't doing too badly. The 3/4" bow is doing very well by those standards, shooting over its weight by about 19 pounds. Most would consider shooting over by 15 FPS to be very good for a selfbow/backed bow. With a lighter arrow, the speeds are higher because the bow's energy is moving less mass. One interesting thing to note is that KE can often be increased slightly by increasing arrow weight, as KE is both a function of speed and mass. Most times I will shoot 10-12gpp arrows because of this.

A bow can be fast but not comfortable to shoot. Personally, the 1" bow is easier to shoot if I'm using fingers. The longer bow has less string pinch as well as virtually no stack at full draw. These make the bow a joy to draw. This 1" pipe bow has some handshock, something that is tricky to get rid of in 1" pipe. On the other hand, the 3/4" bow requires a release of some sort to be accurate because of its string pinch, and the large amount of stack makes bringing the bow to full draw and holding it a struggle. It does, however, have very little handshock with a 10gpp arrow and almost none with a 12gpp arrow.

Ultimately, both of these bows are good bows for different reasons. I believe that what you shoot really should be what feels good to you. It's not all about speed and efficiency. If it were, there would be no selfbowyers, no traditional bowyers at all. Sometimes it's less of what a bow can do, and more of what a bow does to you. It's how it makes you feel and how it fits you. PVC bows aren't right for everyone, but for those that catch the bug, there's nothing better than teaching a plastic pipe to bend with power and grace.


P.S. - I'd like to add that you should take my speed tests and any speed tests as they are: tests of speed in a particular situation with certain variables and conditions. The speeds I got in this test only apply to the bow set-ups I tested. If you were to change the bowstrings used, the results would change. If the draw lengths were changed, the results would change. If the arrow weights were changed, the results would change. Even the type of release and how smooth your particular method of releasing is will change the results.

While speed is important, don't rely only on speed as a measure of a bow's capabilities. Also don't rely on manufacturer rated speeds as true speed. If speed is important to you, have your bow tested with your particular set-up and and your comfortable draw length. Be honest and the test won't lie. I've seen guys overdraw their bows to get a nice speed rating and then underdraw their bows when actually shooting. For example, I can get this bow to spit out a 650 grain arrow around 180FPS and a 550 grain arrow at over 190FPS if I draw clear to 32-33 inches. Bring that down to my comfortable shooting length (27.5) and I get more modest speeds of 155FPS and 165FPS respectively. While I can say the bow shoots 190FPS, truth is that any shot I do under normal or stressed conditions will be closer to 165FPS (which I am happy with!). Also be wary of speed ratings without arrow weight or draw length specified.
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