deflex and reflex theory

[stuckinthemud]

Instinctively, I have always assumed that reflex is good and deflex is bad but recently I have been pondering this, once the bow is braced, it doesn't care what its state was before that, the string will not allow the bow to return to its deflexed or reflexed shape so they have no real effect on anything once the string is released.  I think, that what deflex and reflex cause is a heavier or lighter build, a deflexed bow can be overbuilt - lets say you draw 25" and the bow was deflexed 3 inches, then it has little pre-stress so to get a 50lb draw weight you would build it with thicker limbs, it might have a slower cast though, due to the heavier build but it is only bending 22 inches.  A 4-inch reflexed bow has bags of pre-stressing so to get it to bend to 50lb at 25inches you would build it relatively light, so you get a faster cast as it is bending 29 inches.  How far off am I ?

[PatM]

 Quite a bit since the two are generally used in combination rather than separately.

[stuckinthemud]

I am fine with being a long way off, and d/r bows are something I have always wondered about since the one should cancel the other one out, shouldn't they? I know they can be very very fast, but I don't really understand why, maybe the deflex creates a very highly stressed mid/outer, the limb reflex section is still very thin and bendy, though. Maybe heavier built than they would be without the deflex? Also, they kind of cheat the system as the reflex is visible at brace, so the string is not preventing the bow going into reflex where in a straight limb the string does prevent the bow movement going into reflex.  Would all that mean I am not so far off, the deflex allows a heavier build, the reflex is lighter built to bend further???

Anyway, for my question, might be best if we assume a straight limb deflexed or reflexed at the handle. 

[Badger]

  When speaking of wood bows generally speaking you need more mass for higher stress designs than you do lower stress designs. The limbs might be thinner but they will also need to be wider. Reflex adds tension at brace and deflex is mainly to achieve better string angles for more favorable leverages that store more energy.

[Santanasaur]

Heres how i’m thinking about this, but I haven’t built many r/d bows to speak from experience.  Reflex and deflex aren’t exactly opposite so they don’t cancel out. If you reflex near the grip, that can make for a high stacking design while recurves at the tip actually improve string angle. You could totally cancel out your reflex with deflex and still benefit your force draw curve because of string angles.

When the string slams home and the tips stop moving, all that force used to brace the bow isn’t completely wasted. Pre stressing fattens the f/d curve representing more energy stored. This isn’t an efficient use of available stress, but it’s better than not using it at all.

Most horn sinew designs can afford plenty of reflex to fatten the f/d curve, but in a stressed wooden design, there is no leftover stress to throw away. Instead, stress is better applied where it can benefit the arrow more efficiently—during the power stroke and not during bracing, hence the r/d profile.

Contact recurves are a way for r/d bows to still have a fat initial f/d curve by starting off as a short stiff bow but without overstressing the design at full draw like reflex would cause.

[Halfbow]

Instinctively, I have always assumed that reflex is good and deflex is bad but recently I have been pondering this, once the bow is braced, it doesn't care what its state was before that, the string will not allow the bow to return to its deflexed or reflexed shape so they have no real effect on anything once the string is released.  I think, that what deflex and reflex cause is a heavier or lighter build, a deflexed bow can be overbuilt - lets say you draw 25" and the bow was deflexed 3 inches, then it has little pre-stress so to get a 50lb draw weight you would build it with thicker limbs, it might have a slower cast though, due to the heavier build but it is only bending 22 inches.  A 4-inch reflexed bow has bags of pre-stressing so to get it to bend to 50lb at 25inches you would build it relatively light, so you get a faster cast as it is bending 29 inches.  How far off am I ?

I think what you're saying is exactly right, with the exception of one thing.

Yes, a bow that starts with its tips in front of the handle will have to bend farther to get to full draw than a bow that starts with its tips behind the handle. If you make both of these bows the same draw weight, the bow with its tips in front of the handle will have lighter and easier to bend limbs.

The part where I disagree with you is when you say it has no real effect on anything once the string is released. It is true that it could potentially have no effect on the bows tiller or geometry throughout its power stroke, but the forces happening will be different. The more reflexed bow has lighter limbs and higher initial string tension. Lighter limbs are easier to move, and the string tension will fatten the f/d curve. Both of these things have an effect on how fast that bow springs back, and how fast the arrow goes.

These things are good, but if you try too hard to get these benefits you just quickly run in to the limits of wood. With highly reflexed designs, you get stuck between needing less wood to get to the draw weight you want, but needing more wood to avoid exploding/crushing.

[willie]

so putting the two together in a R/D design means the inners can be built heavier, and the outers built lighter, effectively changing the location in the limbs where working energy is stored?

[Halfbow]

so putting the two together in a R/D design means the inners can be built heavier, and the outers built lighter, effectively changing the location in the limbs where working energy is stored?

I understand why you asked that, but I don't think so. It's more about how far the limb can bend overall. Reflexed bows need to bend farther to get to full draw, deflexed bows less far, right? So if you have an R/D design where the tips are exactly in line with the handle, then those limbs just need to bend as much as a straight bow's limbs. Could be flexible where it's deflexed and stiff where its reflexed, or vice versa. Could be flexible near the handle, or could be whip tillered. Just so long as the limbs overall can bend as far as a straight bow's limbs.

[willie]

Could be flexible where it's deflexed and stiff where its reflexed, or vice versa. Could be flexible near the handle, or could be whip tillered. Just so long as the limbs overall can bend as far as a straight bow's limbs.

maybe, except energy used to move more limb would reduce efficiency....

as is the case with many things, it's most likely a little of this and a little of that all added up to give an advantage

[Halfbow]

Could be flexible where it's deflexed and stiff where its reflexed, or vice versa. Could be flexible near the handle, or could be whip tillered. Just so long as the limbs overall can bend as far as a straight bow's limbs.

maybe, except energy used to move more limb would reduce efficiency....

as is the case with many things, it's most likely a little of this and a little of that all added up to give an advantage

Totally agree. To be clear, I wasn't saying the things I listed were good ideas, or recommending any design over another. Just was talking about concepts and what's possible. If I started talking about what I think is good, that would be a whole other discussion.

[Stick Bender]

I think Steve is right on the money, I think the designs claim to fame is the superior string angle you could certainly build a reflex bow of the same length & draw that would have much greater early draw weight but would have less performance then the D/R of the same length/draw & mass !

[sleek]

  When speaking of wood bows generally speaking you need more mass for higher stress designs than you do lower stress designs. The limbs might be thinner but they will also need to be wider. Reflex adds tension at brace and deflex is mainly to achieve better string angles for more favorable leverages that store more energy.

I'd like to add to this if I may. Deflex also forces more thickness to be in a limb because it offers a leverage benefit over a straight or reflex limb. As a result, the limb has to be thicker to accomodate a reduced bending radius. That transferes to your reflexed portion so it doesnt end up overly thin and wide, but rather thicker and narrower which drops its mass allowing it to be faster.

[Selfbowman]

Guys I think the biggest advantage of a deflex in a wood bow reduces set and pushes the working part of the limb out on the bow limbs. That being said I like to load early and not have stack. The trick in wood bows is to prevent set with the reflex only design. I assume we are talking about incorporating both in a bow or just reflex in the bow. Arvin

[Halfbow]

I think Steve is right on the money, I think the designs claim to fame is the superior string angle you could certainly build a reflex bow of the same length & draw that would have much greater early draw weight but would have less performance then the D/R of the same length/draw & mass !

I agree with Steve too, I'm pretty sure. (Steve please correct me if I'm wrong about that and we disagree)

Common D/R bow designs definitely have great geometry for efficiency, and offer great string angles. And yeah I agree, it's easy to make a D/R bow that outperforms a reflexed bow of the same length and draw. Some of that is because of limitations of wood, but a lot of it is also those geometric benefits.

My one quibble with what you said is about mass. Assuming the same material, same length, and same draw, I think you'd have a hard time getting a D/R bow's limbs to be the same mass as a reflexed bow's limbs. Not without going with pretty unideal limb dimensions.

Deflex also forces more thickness to be in a limb because it offers a leverage benefit over a straight or reflex limb. As a result, the limb has to be thicker to accomodate a reduced bending radius.

Exactly. And the deflex further reduces the bending radius, requiring a yet more stout limb to get up to draw weight.

[SLIMBOB]

My opinion....Arvin nailed it!