The goal here is measure something that is invisible, to create a picture of something that cannot be photographed using evidence based on a series of tests. My personal goal is to do the work in the backroom so to speak, testing, crunching numbers, developing tests etc. that can be easily converted to workbench techniques requiring nothing more than good work habits and some understanding of theory.
I am looking at ways to identify and minimize losses in efficiency of wood bows due to hysteresis. What we will not deal with here is bow design, excess mass, strings or any other factor that reduces efficiency. We are only dealing with hysteresis.
For some time my hypothesis on hysteresis has been counter to many of the experts who have chosen to simply accept that hysteresis is inherent in wood and that wood can never measure up to synthetic materials. I refuse to accept this and have significant evidence that the great majority of hysteresis present in wood bows is due to damage done to the bow during the tillering and shooting in process. Thousands of bows built and countless of hours of seemingly fruitless testing have in recent years started to produce undeniable patterns that have led me to a series of tests that could verify my suspicions.
The only way I can figure to make this post of any value to anyone is to explain the nature and logic behind the various test procedures.
Measuring stored energy in a bow:
With a fiberglass bow this pretty straightforward we draw the bow back slowly and measure the draw force in 1” increments, we might even slowly lower the bow back down re-measuring each increment to see if it recorded the same on the return stroke. We total up all these measurements for a stored energy value of inch pounds; we divide that by 12 and convert it to foot pounds of stored energy.
Now on a wood bow it is not so simple, we often do use the same procedure but we get a false reading because of hysteresis. To understand hysteresis in this context we need to think of a bow that was just unstrung, let’s say you took a measurement within 1 second of unstringing and found you had 2” of set, you took another measurement 2 seconds later and it was now 1 ¾”, 10 seconds later it might be 1 ½” now 10 min later it might go to 1 ¼” and after 2 hours may settle in at 1” set. After a few days it may even pick up another ¼”. The limbs never stopped moving they just slowed down dramatically as time went by.
What I have essentially done is to find a way to take this measurement into the bow while it is strung and actually into the shot itself using performance based evidence and comparisons as proof of theory instead of a tape measure.
My method of measuring a wood bows stored energy involves using the virtual mass of the bow. Depending on the length and design of a bow most bows will have a virtual mass of about 200 grains. I will explain virtual mass here and why it is so important to these calculations.
Virtual mass is the difference in the amount of weight an arrow would have to be to shoot at exactly the same speed as a tested arrow and be 100% efficient. An example would go like this, a stored energy measurement shows a bow stores 50# of potential energy. We shoot a 500 grain arrow through a chrono and find it is shooting at 188 fps. This represents about 39ft#’s of energy. If the bow shot a 640 grain arrow at 188fps it would represent about 50 ft#’s and be 100% efficient. I subtract 500grains from 640 grains and I get a virtual mass of 140 grains. If no hysteresis is present this 140 grains will stay relatively consistent regardless of the arrow weight I shoot and how fast the arrow is going. If hysteresis is present the faster the arrow goes (lighter arrows) the higher calculation I will have for virtual mass.
Because hysteresis is a bi-product of speed by measuring speed of various arrow weights I can track the presence and effects of it fairly accurately.
Back to measuring stored energy in a wood bow. Now that we understand virtual mass I will explain how I use it to more accurately weight the potential energy with hysteresis present. I do my first shot through the chrono with as heavy an arrow as possible. Because this arrow is moving so much slower I will minimize the hysteresis in the shot. The arrow I chose is 4,000 grains. Most of us understand that heavier arrows are more efficient, by going to an extreme here my 200 grains of virtual mass only account for about 5% of the total energy in the shot so I should produce a shot reflecting about 95% of my stored energy, if I have 5% hysteresis it will reflect about 90% of my stored energy.
As I continue to shoot lighter and lighter arrows though the chrono and calculating virtual mass after each shot I will see a pattern develop. If my virtual mass stays relatively consistent I know I have a bow with low hysteresis, if the virtual mass continues to climb with each lighter arrow I will be able to calculate my actual losses due to hysteresis in real time speeds.
Just having this knowledge does me little good if I can’t do anything about it, it does let me know how much room for improvement I have. This is where no set tillering comes in.
No set tillering is simply a method of monitoring the condition of the wood throughout the tillering process. Very easy, a little tedious at first but low tech. If the wood is starting to break down during the tillering process the first indication of this will be a weakening of the woods resistance to bending. An easy way to track this is to carefully measure the weight of the bow at a specific draw length early in the building process. I usually take my first measurement at around 14” draw. Lets say I get a reading of 20#@14”. Each 1” further I draw the bow I will always go back to my 14” measurement to see if it has changed. If it has changed it means the wood is starting to breakdown, if it has not changed I can feel safe to continue drawing further 1” at a time rechecking after each additional draw. Depending on how your tillering is advancing you can make decisions on how you want to continue, you may want to get a little more limb working in some areas or you may want to start narrowing areas as you approach final draw. The condition of the wood will steer you in the right direction as long as you are monitoring it.
Some guys have a natural gift and do exactly this without even being aware of it, some of us don’t have that gift so a little knowledge can be one more tool in our tool box. If you successfully manage to bring a bow out to full draw with no loss of weight you will be amazed at the performance and how close it can compare with a well made class bow of similar design. You may also decide that you might want to start making the working parts of your bow considerably wider once you realize how much you are loosing. Once I started doing this I started making my bellies flatter and less rounded, I also increased the width of my bows considerably and surprisingly enough I added no mass! Dead wood is dead weight!
I know I left some things out so feel free to ask any questions.
