deflex and reflex theory

[simk]

This is a long and interesting discussion. The straight-limb-fulldraw-theory now really took my attention. Therefore I just wanted to remark that this summer I accidentaly started to do bows with straight limbs at fd. I did a trilam glueup on my old angular-form of ash-ash-yew. actually I wanted the the limbs to form more of an arc a at full draw when ran out of belly lam and created straight limbs at full draw. I just have to say that it really was shooting tremendous (b2w can testify i think...). It finally failed at the fades due to beginners problems - using just white wood glue (all repairs finally failed).

I then did the r/dbirch which shoots well but didn^t hold much reflex.

I'm on another glue up now including enough belly wood for any f-d-tiller. What do you think the final f-d should lookalike for optimized performace and comfort. To where should I aim tillering Off course without overstressing the wood 

My thouhts about straight fd are that it takes a lot of non working wood on the limbs to keep the limbs stable (bad). Having the bend mainly on the inners gives perfect string angle (good). The reflexed unbraced profile gives good string tension at brace. Just my thoughts...Cheers

[stuckinthemud]

Half
Lots of good ideas being explored here, and the concepts are challenging. Since we lost Stuckinthemud after he third post, I don't suppose it would be inappropriate expand the discussion to point out that raw speed does not always equate to long cast or accurate arrow placement.

BTW, are these conceptual bows "geared" towards lighter arrows? Or arrows of any particular type?

You didn't loose me, I'm really enjoying this, but I learned long ago, there's times to speak and times to sit and listen   

[avcase]

Halfbow,
The Hickman extreme Recurve design had a pretty incredible force-draw curve. I believe it may have had a brief moment in the draw where the draw weight decreased as the bow was drawn. The problem was that efficiency was very poor because the string hadn’t no leverage on the unsupported limb and all that’s forward energy remained in the limbs at the end of the shot. This evening symptom of this would be a huge amount of limb flapping post-shot. Modern “Super Recurves” are very similar to this design, except they are using modern composite sizes to keep the limb mass to a minimum. The efficiency is still poor, but the energy storage makes up for it when shooting heavier hunting type arrows.

The idea of using string to prevent this limb flap has been kicked around for years, but I haven’t seen much development work yet to make this work on a real bow yet. I hope you will pursue this!

Alan

[Halfbow]

Thanks Alan. Yeah the flapping is exactly what I was imagining. But I wouldn't have guessed the draw weight actually decreased, that's amazing. But good to hear I'm thinking along the right lines.

[willie]

Think of a cable backed bow. If you put bridges along the back to elevate the cable, the limbs get much stiffer, and not just tiny parts of them. I know this not just because it makes sense (which it does, you're essentially thickening the limb by moving the tension side farther out), I also know it first hand because I've done it. My bow here uses the same concept flipped around.

I can agree with you about the stiffening effects of a cable backed arrangement, as I have had some similar results.
As for the concept flipped around, I will have to take your word for it until I can give it some more thought or see the concept demonstrated better.

Stuck, glad your still here and OK with the "thinking out loud" direction this thread has taken.

[DC]

It's sounding that there are a few things that are good to a point and then start being bad. What you have to a design is a bow that has all these things peak at the same time. So all you have to do is figure out what these things are and when and how they peak. Then assemble a bow that has them all happen at the same time. Sounds pretty straight forward to me. You guys get busy on that and report back.

[Tuomo]

The problem was that efficiency was very poor because the string hadn’t no leverage on the unsupported limb and all that’s forward energy remained in the limbs at the end of the shot.

Alan, could you elaborate that topic? I can see what is happening (have made a few too-much-reflex-*****bows) but I can't get what is science behind low efficiency and flip-flopping. What exactly means "no leverage on the unsupported limb"? Why is so much limb's forward energy remained in the limbs? Every bow has moving limbs, which have some forward (in direction of arrow) kinetic energy and momentum - but what is the process of energy transfer to the arrow?

[Halfbow]

Tuomo, I may be able to help answer. A different context might make the concept more intuitive.

The leaning tower of Pisa is finally falling over. You see it start to slowly lean more and more. Thinking quickly, you get out your rope and manage to lasso the top. You're going to try to stop its fall. Maybe you just barely caught it before the point of no return.

Now would you choose to stand next to it and pull down on the rope to try to stop it?




Or would you choose to stand away from it.




One of these gives you more leverage.

All bow limbs have forward momentum throughout the power stroke. Ideally that momentum is stopped instantly at the end of the power stroke, when the bow hits brace. Any movement in the bow limbs after the arrow has left the string is indicative of energy that could have gone in to the arrow, but didn't. So you really want those limbs to stop. The string is the only thing there to stop them.

But some string angles offer better leverage to stop them. If the string has poor leverage, it will get stretched out as the limbs keep moving forward despite it. In the first pic, imagine the tower falling, lifting the guy off the ground, his weight barely making a difference. The force that would lift him is the same force that would stretch a bow string.

The now longer string means the limbs can travel past the intended brace position (related to: longer string means lower brace). Eventually, assuming the string doesn't break, it will stop stretching, the limbs' forward momentum will finally die, the string will elastically rebound, and the limbs will go back, oscillating back and forth for a bit around the intended brace position, flapping.

[stuckinthemud]

Nice diagram, very neatly explains why I need a much stronger string when I'm tillering reflexed bows at low brace than I do at full brace - thought it was just that by the time I got to full brace I'd reduced the draw weight but I guess not

[Selfbowman]

Good explanation even Arvin understood. Thanks.

[Stick Bender]

 I like the cracks in the tower nice touch 

[DC]

Isn't the arrow gone by the time all this happens? I understand that the limbs will still flap around and all but how would this get more energy in to the arrow.

[Tuomo]

Halfbow - thank you for the nice explanation, figures were nice!

But, as DC stated, "Isn't the arrow gone by the time all this happens?" Before the string is at brace height, energy is transferring to the arrow.

It is about efficiency - what is the physics behind poor efficiency of super recurves, as Alan said?

Bow stores energy and after release some of energy goes to arrow and some dissipates as losses:

-hysteresis - bow related constant, should not depend recurves
-string elasticity - should be not a problem with modern materials (or??)
-recoil - not a problem, if bow is heavy
-air drag - not a problem
-bow limb mass and related forward momentum - not a problem, if limbs are light enough

Any of those losses are not related in any way to recurves. I am not sure is poor efficiency of super recurces related to bow geometry or is it material related? If we would have perfectly rigid (non-elastic) string, would we have a problem?

So, the problem may be in energy transfer. I can imagine, that shooting a bow which has a bit loose string (brace height just under zero) is not nice and efficiency may be poor - I should try. Although the bow stores more energy when it is braced normal way. As Halfbow said "All bow limbs have forward momentum throughout the power stroke. Ideally that momentum is stopped instantly at the end of the power stroke, when the bow hits brace. Any movement in the bow limbs after the arrow has left the string is indicative of energy that could have gone in to the arrow, but didn't. So you really want those limbs to stop. The string is the only thing there to stop them.".

Hickman said (as we know by experience also): "The arrow velocity increases with increase in bracing height up to a certain point, after which is slowly decreases with additional increase in bracing height." So, the efficiency of the bow also increase with brace height.

[Stick Bender]

One thing that's been known about D/R design for years is that in most cases these bows are most efficient when there within a 1/2"-1" of stacking especially with guys making lighter draw weight bows I'm wondering if that plays in to length vs draw length & the limb frequency /vibration theory Steve mentioned but I'm sure each material used has it's variants on the theory to, really only one way to find out  Halfbow I enjoy your drawings !

[willie]

So, the problem may be in energy transfer.

Yes

but what is the process of energy transfer to the arrow?

energy is transferred from the limb to the arrow via the string. the string and arrow slows the return of the limb compared to a "dryfire" without the arrow.

as the sketches above indicate, the force the string exerts on the limb at the end of the power stroke has  minimal effect on the return of the limb, so remaining energy in the limb is not transferred efficiently, leaving excess energy in the limb to be dissipated by vibration.

Remember all that high early draw strength/energy we wished to have?  It wants to return to the string at the end of the powerstroke.
Creating a limb with more high early draw reaches a point of diminishing returns if limb is reflexed too much.