Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on June 13, 2007, 04:12:01 am
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I started using mass on my bows several years ago and just in the last 1 1/2 years started trying to put it to some kind of formula. I had become pretty comfortable building bows in the 60" and 62" range but found when i stepped outside my comfort zone my performance started falling off or at best became inconsistent. Once you understand the logic behind it you really dont need a formula at all. I still prefer to use the formula because I predetermine how much my bow is supposed to weigh when finsihed and it just speeds things up for me a bit getting the mass to the right point before I start serious tillering. I like to know my mass ahead of time for several reasons, #1 if the bow is high in moisture I will find that out quickly as it will be too heavy, I will know to halt work before I have done any damage. If I am working on a backed bow it will tell me if I have a poor match of back to belly materials. It will also help me find the right design for my bows and pretty much assure me consistent performance.
It is not one of those things you just read and you know it, more like work it into your bow building program and simply use it as another tool. Your understanding of it will grow as you use it.
basic formula, bow length- for every 2" past 54" add 1 oz of weight
nock position for every 2" reflex add 1 oz weight
stiff handle and fade area- add 1 oz for every 2"
draw length- a zero value would be draw length equals working limb (handle and fade minus bow length divided by 2) add 1 oz for every 2" draw over that number or subtract 1 oz for every 2" under that number
draw weight 1 oz for every 5#.
side notes, english longbows, subtract 1% from calculation for every 5# over 50#
backed bows with well matched backings, subtract not more than 10% from calculated mass, 5% seems to work pretty well.
example: 74" english longbow drawing 30" and 125#
74" = 10 oz ( 74-54=20/2=10)
nock position 0=0 oz
handle and fade area 0=0 oz
draw length = -3.5 oz ( 74/2=37-30=7/2=-3.5)
draw weight=25oz (125/5)
Total=31.5 oz minus 15% (1% for each 5# over 50#)
total =26.7 oz
example stiff handled bow
64" 55#@26"
length 64" = 5 oz
2" reflex= 1 oz
8" stiff handle and fade = 4 oz
26" draw = -2 oz
55# draw= 11 oz
Total, 19 oz
these are based on normal arc of the circle tillers, in the book variations in tiller are discussed and how to deal with them mainly by manipulating the handle and fade area calculation and also the draw length calculation in the case of stiff tips.
Being exact is not really an issue being close seems to help quite a bit as fas as consistency goes.
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Badger, not to beat a point to death but, suppose the wood is a real snaky piece. Say the nock to nock is 70 inch's as the crow flies. Does the fact that "if" you were to "measure" the center line, along the length of wood, NtoN, and got 80 inch's, would this be a variable for consideration? I keep thinking about it and I want to say no. The action between the nocks in a straight line is what matters. Then again you still have all this mass of wood to contend with. It seems the limbs would be "thinner" than if the bow was a straight piece. BTW I like your formula and will use it on my next bow.
Dick Bernier
Mebane NC
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yeah im interested to try it. i sort of get it although i do see variables for different risers especially seeing as i dont have any real grip on most of my bows. i think for the most part it means getting the outer limb weight as low as possible. am i wrong?
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Jamie, knowing the length of the stiff center area is more for the sake of knowing how much working limb there is. The shorter the working limb the wider it needs to be. Not about low mass bows at all. My bows are much wider than they used to be since measuring the mass. It is about mass placement, the outer limbs have to be narrowed down if they are not bending that much, but not extreme by any means. Long bows with shorter draws, say 72" bows drawing 27", will work best with a longer stiff center or at least semi stiff center area along with the stiff handle. Say you have 8" handle and fades but the bow does just barely bends comming out of the handle for 6" on each side, case like this you split the difference and use a calculation with a 14" handle and fade area. Once you start playing around with the weight of the bow you start to get a sense of where the mass is working best and when to leave it stiff or bending. It tends to give the bowyer a little more lee way also. I will do a build along next week when my staves come in, Steve
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i have one now im getting startied on. typical for me . 62" bend in the handle. roughly 1 1/2 " wide shooting for 55#. tiller would be full circle with last 6" or so bending. can you give me an example of your formula for that and then maybe this will sink in .lol
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Jamie, for a 26" draw it calls for 12.5 oz, so i would try to keep it around 14 or under. If you do the formula it will almost force the bow to be an elb, I doubt if it would work for a bendy handle flatbow unless the wood you are using is very light. It's trying to tell you something LOL. Steve
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As a point of reference, I compared my two osage ABC bows to Badger's boo backed ipe and they were both within .5 of an oz of each other and his. These bows were crafted a year apart with no consideration of mass weight whatsoever. I think it's a pretty remarkable phenomenon and to me validates the wood-is-wood dogma.
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Jamie sometimes a slight manipulation of your design idea is all that is needed to bring a bow right in. A full arc 1 1/2" wide red oak bow might weigh somewhere around 16 or 17 oz. The formula says that is too much ands it will show in the bows performance. Just keeping the mid section a bit stiffer and using a more elyptical tiller will still allow you to use the 1 1/2 wide and bring your target mass up to around 16 oz, here you will notice good performance. Steve
Dave, it is funny that you mentioned that, basically after reading your tillering tecniques and starting to practice them I was able to start using the mass projections with some accuracy. Your tecnique lends itself particulary well to this.
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David,
Any chance you could post a summary of your tillering technique Steve is referring to. It would be great to have that info included in this thread and would make the thread a great reference for future use.
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Marvin, if I remeber right Dave was just giving advice to a new comer in a non descript thread, one of the ose cases where you read the same thing a hundred times and it suddenly becomes clear. I had been rushing my tillering quite a bit up to that point. Dave advised the now comer to never pull a bow any further than what it would take to expose an area that needed work, and he advised never to pull a bow past intended draw weight durring tillering while constantly monitoring the set a bow is taking. They say when the student is ready the teacher will appear. I know speaking for myself I have always had personnal objectives attached to my bow making. These change periodicaly and this is what keeps my interest. For a long time simply making the fastest bow I could make was my objective. I have changed that now to the fastest bow I can make repeatedly that will hold up and maintain it's performance over time. There seems to be about 15 fps difference at this time so narowing that gap is todays challenge. Exploring different designs and different ways to implement existing designs along with the trade offs involved could keep one busy for a lifetime I believe. I always have to go back and look at the straight limbed long bow as the anchor post of it all and when done right is hard to get much improvement on. Steve
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Steve,
I feel this is a very usable model. I also feel that this forum is lucky to have such a dedicated bow building researcher such as yourself
sharing good info like this. There are several bowyers on this site that serve the sport this way.
This idea has met with some resistance by many of the Icons of the trade. I was never sure why, becuase if you push the width to its limmit, the mass relationships are pretty easy to see.
Do you know why it works? What is happening?
I am writing about it, and saw this post. I was thinking of making it's own post on PA, rather than making people wait for the book,
but since this is a related topic, I think your post will help people to understand.
Dan
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Steve,
I asked David to share his tillering approach to lead people to the next step so to speak. It's my opinion that your mass theory cannot be fully realized or seperated from a very carefull tillering methodology.
Dan,
Good to see you here. Did you see my post on the fight archery site about the message board?
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Marvin,
I replied to it today.
I feel that what Steve is doing is different and sepperate from Tillering. Although Steve uses it as a tillering aid, it is really a design technique. It can also bedesign on the fly tillering if the bow is being narrowed.
I believe it is important to understand what Steve is showing us here.
It is also impotant to understand why. It has puzzled me that the "why" has never been addressed. I have seen bowyers walk all around the "why" when writing about the affects, in books, whithout ever standing squarely on the real issue and explaining it.
Dan
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Steve has said that you need a cetain amount of wood to do a certain amount of work. If the bow is under the target mass it will probably degrade. Any more mass and you have essentially an overbuilt bow, not the worst possible situation but not as good from a performance perspective. I like Steve's tillering method of reducing width until you see a bit of set, then proceeding on thickness. I think it gets you to this target quite efficiently, and takes into account the properties of each piece of wood that is being worked.
Didn't mean to speak for Steve, I'm sure he'll add to this. But I wanted him to see I've been paying attention.
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One thing that I suspect dan would agree with along with many other experienced bowyers is that we are often doing things unconciously because we know they work, When it comes to time to explain the how's and why's we tend to overlook what we consider the obvious. Dan has been posting his semi pyramid bows for years, The mass theory when used on a stiff handle bow will almost force you to build that same design. The biggest part of the theory is just basic stuff, mass placement, put the mass where it does the least harm, near the handle, Maintaining the force draw curve, keeping the tips stiff and a minimum of set on a straight bow will do a pretty good job of this with no further ado. The " why" probably the most important part is simple but not easy and will still challenge the best of bowyers to their highest level of skill on every bow, and that is to minimize wood deformation, even the softening of the wood that takes place before set becomes visually evident. In my entire life I could count the bows on one hand that I was able to complete with no perceptable wood defoprmation, 3 of those were somewhat overbuilt. perfection is the goal but not neccessarily the expectation. Getting back to another of Dan's posts where he talks about the relationship of working limb to draw length I believe. This is a very key and very important element of design. It has a major effect not only on performance and speed but also smotthness of draw and handshock. learnign how to manipulate the center area of your bow by using various degrees of stiffness to control not only the working mass but also the force draw curve and the amount of wood deformation. The tillering tecniques used to control mass will monitor these things and tell you when to take your tillering toward the handle or toward the tip. More than you cna really cover in 1 thread, but general ideas can be conveyed. Steve
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Steve, your information is all good. Preventing the wood from breaking down without overbuilding is a fine line for sure.
This certainly is what we are all trying to do. The key to why bows of vastly different woods in the same length and design of bow
will be the same mass just before the wood breaks down, has to do with how a bow works, how it stores energy, and its motion.
I have a whole chapter on it. I think it is cool stuff, because once it is understood, you can manipulate it. Take advantage of it.
It takes a long time just to list the ways you can use it/ exploit it. It is possible to come to the right conclusion by paying attention to detail and observing the outcomes of bows built, without enderstanding fully what is happening and why.
Dan