Do we have any engineers in the house?

[Badger]

What I wanted to show actually is that this quote

"it would seem to me that none of the forces that are present during the draw have any relation to the forces present during the shot"

appears to be flawed

BenBen

      I don't think it is flawed, the presure point on your bow hand disappears when you let go of the string, also the limbs are responding to the point on the string where you are pulling, the instant you let go of the string the limbs will start responding at the point of your arrow knock.

    As I said earlier I don't have a problem with the split finger method of tillering, there are several good reasons to tiller that way. But when I hear that this method of tillering is how it should be done for proper limb timing or for proper arrow flight thats when I say hogwash. Tillering for how we hold the bow is just another example of many of the tradeoffs we have to use when building bows. Most of us tiller so when we take a photo at full draw it looks ballanced and perfect. But the fact is the instant we let go of the string it is no longer ballanced and perfect because the pressure points all change.

    Kind of like carrying a bucket of nails up several flight of stairs and then dropping them off the balcony, it doesn't matter to that bucket how you got them up to the balcony, they are going straight down regardless.

[KS51]

From an engineering perspective, there is one difference between the draw (tillering) and the release.  Specifically, the tillering process is a function of work, force over a distance.  This is essentially the storing of potential energy where time is not a factor (it can happen relatively slowly).  Upon release, the system becomes dynamic and time is critically important.  Now we are talking energy, work over time.

Now, when we tiller and because of two dominant factors: 1) strings can only be in tension 2) the system must be statically balanced (the tension and lever arm must be equal across the pivot point otherwise the system wants to rotate)  we are, generally, constrained to making the limbs the same length or very close to it.  This has the effect of defining the limb geometry given that we are attempting to store the most potential energy is the least amount of wood. 

Though, thinking about the Japanese bows, the off-center balance point is compensated by there being a corresponding amount of material in the lower limb so that the lever-arm/energy storage equations would balance out.  The lower limb must also dictate the geometry and the upper limb is lengthened but bends less.  I can only speculate, but the arrow position on the string should follow a straight and level line from release to brace point.

I have probably generated more questions than I answered and I am not entirely sure I addressed the question you posed, but there is a difference between work (tillering) and energy (release) and that is time.

Ken

[Badger]

 Ken, the question is not addressing work and energy. Only the issue of ballance in the limbs that is not the same when drawn as it is during the shot.

[bradsmith2010]

ok I get it,, but give me an example of how to tiller with your theory in mind,, makes since to me,, I just need a practical application,,,,I agree with you,, once you let go of the string the bow doesn't know how it was held,,,especially with a  light grip,, ,, making a bow look right,, does not always translate into best tiller,, I have always felt that when the bow is shooting great and not taking set,, there is no need to "Look" at the tiller,, looking at the tiller was just part of getting the bow tillered,,

[Cameroo]

  Kind of like carrying a bucket of nails up several flight of stairs and then dropping them off the balcony, it doesn't matter to that bucket how you got them up to the balcony, they are going straight down regardless.

The bucket wouldn't fall straight if you put some sideways momentum into it as it was released. Wouldn't that be what the string would do to the arrow, just as it leaves the nock, if the bow is simultaneously rattling in your hand due to handshock from poorly timed limbs?

[KS51]

From my perspective, the work-energy aspect is important and a part of the balance of limb-string question.  The wood that the bow is made of can only transfer work at a certain rate (this may tie back to your hysteresis hypothosis) and under a dynamic situation, each moment in time will have an instantanious state where the system could be evaluated.  At each state, we would idealy want the nock of the arrow and the arrow pass to always follow along the original straight line intended, regardless of where we held the bow and pulled the string from.

Since you asked for an engineering answer, the tools generally used are to break the problem down into discrete instances in time.  The things that must "add up" at each instance are, 1) the geometry must balance i.e. tension in string and effective lever arms must be equal around the pivot point 2) energy state must balance i.e. change in limb energy must equal change in arrow energy.

The differences between tillering and shooting the bow is time and possibly gripping the bow (conteracting torque intentionally or not).

To get a better idea of the release when tillering, making the tiller tree so that the saddle is on a pivot may be useful. (Like a flange bearing with a saddle mounted on a shaft) Then tillering can be done so that both even bending and no rotation occurs during the draw.  Set the bow and draw point where ever you want, just tiller for both rotation and even bending.  You would have to make sure the bow statically balances on the saddle in the first place by changing limb geometry on either side of the saddle or having tension on the string while holding the bow level, before letting go.

Ken

PS - one of the reasons I stopped approaching bow making from the engineering perspective is because you wind up having to deal with all the engineering required and very lttle of it it can be broken out independantly without ignoring something significant.  This quickly leads to dealing with tensors or finite element analysis or both and takes a lot of fun out of it.

[Badger]

    Brad, I think tillering for split finger is about the best compromise. I think tillering for 3 under is pushing it too much. The main issue I have is some of the explanations that go behind the logic for tillering, I think it makes newcomers believe it is more complicated than it is. Bottom line is we should always tune the bow to the arrow and adjust nock height accordingly. As I said earlier it is some of the reasons given for tillering bows to the way we hold them that bug me more than the act of actually tillering them that way. There are plenty of good possible reasons to tiller a bow to how it is held so we don't need to make up some extra ones just to make it sound absolute. There is a little bit of a range in the middle we have to work with that will still allow us to properly tune our bow. It is not all that precise.

   If presicion were needed I would choose supporting the bow at the arrow rest and pulling it down from the arrow nocking point.

[Badger]

  Kind of like carrying a bucket of nails up several flight of stairs and then dropping them off the balcony, it doesn't matter to that bucket how you got them up to the balcony, they are going straight down regardless.

The bucket wouldn't fall straight if you put some sideways momentum into it as it was released. Wouldn't that be what the string would do to the arrow, just as it leaves the nock, if the bow is simultaneously rattling in your hand due to handshock from poorly timed limbs?

  One thing all springs have in common is that they build energy as they compress, if one is stronger or weaker it will instantly ballance with the other limb because of the string keeping them together. The handshock happens after the arrow leaves and you have a moment of slack in the string. As far as torquing goes that goes along with shooting tecnique and doesn't relate to the post.

[DC]

     My logic is that the  bow hand is no longer relevant and the point on the string where we draw the bow is not relevant.

I'm no engineer but I'm not sure I agree with this. If you imagine a full drawn bow and arrow, it is being held in position by your bow hand and your arrow hand. Now if you could let go with both hands simultaneously, the arrow nock would go forward and the bow grip would come backward as the bow straightens. The mass of each part would control how much. Since the grip is trying to come back it is putting a force on your hand which makes your hand relevant.
Don

[bubby]

I think this is one of those nachos and beer topics, going to be a fun read😉

[Badger]

     My logic is that the  bow hand is no longer relevant and the point on the string where we draw the bow is not relevant.

I'm no engineer but I'm not sure I agree with this. If you imagine a full drawn bow and arrow, it is being held in position by your bow hand and your arrow hand. Now if you could let go with both hands simultaneously, the arrow nock would go forward and the bow grip would come backward as the bow straightens. The mass of each part would control how much. Since the grip is trying to come back it is putting a force on your hand which makes your hand relevant.
Don

     There is some small forces on the bow hand but the bow is still free to rock and bend where it wants. A one ounce arrow under accelration suddenly weighs about 1 1/2 pounds so has significant influence on the bow limbs at this point.

[DC]

The bow is also trying to accelerate but in the opposite direction so it's effect is also increasing. This is when we need an engineer. The bow out weighs the arrow by a factor of 15-20 so the arrow is going to move much more. I'm thinking that subtle changes in grip are going to come into play here. Although, decent archers seem to agree that this bow is a sweet shooter and that one is a POS so there is definitely something inherent in the bow that makes it sweet. That's all I got

[Dances with squirrels]

I'm not an engineer, but this is my take on it. There is a LOT of energy stored in the limbs at full draw. When we drop the string, they expend that energy, most of it into the movement of the arrow. It is their energy that controls the arrow, not the other way around. If relative limb strength isn't balanced, the arrow nock doesn't come straight back relative to the shelf/handle during the draw, and then upon release, the opposite, or nearly so, happens and some of that energy, proportional to the degree of unbalance, is 'misdirected' and sends the arrow's nock end in a direction other than perfectly straight ahead.

If, as you contend Steve, the weight of the arrow balanced the energy of the limbs the split second the string was free, arrow flight would always be perfect, no one limb could ever be too strong, and handshock would never be increased because of it.

[Badger]

  Arvin, I mostly agree with what you are saying. I do agree we need to be ballanced, my only contention is that tillering for the way we hold the bow durring the draw is not ballanced for the way it will shoot. Obviously it is close enough as great bows are made every day using this method, including myself. I said both limbs will always have identical pressure on them but that doesn't mean they will travel on the right path. This is why we tune our bows and adjust arrow nock height to make up for subtle differences we might experience and it works fine. I don't even suggest anybody change anything, I just suggest when someone is explaining how it works not to say a perfect full draw tiller shot is needed to ballance the limb timing because limb timing is different when the string is released. 

[DC]

https://www.youtube.com/watch?v=ir5GKAQG14g

At about 1:20 min in this video Mike is shooting the longbow. Right after he released you can see the nock end of the arrow drop a bit. At the same time the bow rotates counter clockwise(top come back) a bit. Is this what we are talking about?