draw length v`s arrow length

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weeniebeenie

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Hi everyone, my apologies if I`m retreading old ground here but i was wondering if anyone could shed some light here?
I was wondering if it is more of an advantage to have a long draw length(mine`s 31.5") or a shorter one? does the extra omph ( don`t want to get too technical here, ha ha) of a longer draw outweigh the extra drag + weight of a longer arrow. I know ther are a lot of variables here but just wondered what people think?
 

Grantmac

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A longer draw length is always an advantage. With the exception of some very specialized flightbows which perform on and past the ragged edge of the envelope.
If you've got the draw length then you can shoot a lighter bow and still get the same or better speed to someone with a shorter DL.

The only disadvantage can be finding arrows, but that is really only an issue past 32".

-Grant
 

tinkerer

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I'm not so sure, but I doubt if it's so. As DL increases, so does energy stored, but so also does the mass of bow and arrow. At lower draw weights it's hard for the longer arrow to maintain enough spine without becoming too heavy. At the warbow end of the scale longer arrows seem to have the advantage.

But for an average strength elb there's probably not a great deal of difference between 31.5 and 28ins.

David
 

Bender

New member
Sorry tinkerer, but at any draw weight that extra 3.5 inches of draw means HUGE gains in additional stored energy.
Take a bow, any bow. Plot its force/draw curve to 28 inches of draw. Then plot it again to 31.5" of draw, and you'll see.
 

tinkerer

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Bender. Yes, of course the extra energy is there. But, if the mass of the bow and arrow inevitably rise in proportion to the energy stored, then where is the advantage? I don't think you have thought through this other side of the equation.

Why are the gent's elb flight records for 50 and 35lbs held by bows shooting 28 and 25inch arrows respectively, not 31.5ins?

David
 

Grantmac

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Flight arrows for relatively light draw-weights cannot be built light enough past that length. Very specific situation. You really would not want to use a flight-weight arrow in anything other than a flight bow, it wouldn't last very long!

-Grant
 

tinkerer

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I use the flight example because it illustrates the same physical laws that apply to all bows, just taken to the extreme in this case.

I don't think we'll agree on the theory but as to target shooting I don't get the impression that the most successful archers need to pull 31.5ins in order to put the pile on the gold at 100yds.

As I said earlier there may not be a big advantage either way if you shoot 50lbs.

David
 

Bender

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Using a longbow as an example, since it acts as asimple linear rate spring, a nice long 70" so stack isn't a problem
50# 6" brace drawn to 28" stores 45.8 ft-lbs KE. Its not a barn burner so at 80% efficiency 36.6 ft-lbs is delivered to the arrow. Lets start with a common 10 grains per lb of draw arrow for a 500 grain arrow. Doing the math that arrow leaves the bow at 181.5 fps.
Now same bow drawn an additional 3.5". So the arrows you make aren't so hot and gain 20 grains per inch. That's now a 570 grain arrow being shot from a bow that stores 60.5 ft-lbs KE. Again at 80% efficiency thats 48.8 ft-lbs delivered to the arrow. (actually efficiency goes UP slightly with a heavier arrow, but to give tinkerer the advantage we'll ignore that) Again doing the math that arrow leaves the bow at 195.5 fps. That's 14 fps faster!!!!!!! Even a crappy shooter can take advantage of that large of a difference.
Rather than me not thinking it all the way through, it seems that actually others have failed to to do the testing, lack the experience, haven't done the math, and failed in due diligence. Mere opinions just don't cut it sometimes. One shouldn't be presenting opinions and misinformation as actual technical fact.
 

Yew Selfbow

Active member
As DL increases, so does energy stored, but so also does the mass of bow and arrow.
David
David,
I'm a little confused as to how the mass of the bow increases. I understood that mass (the amount of material within a given space) was a constant as apposed to weight which is determined by gravity.
Could you explain ?
Cheers
 

fanio

Active member
presumably he means that the bow needs more material to allow for a longer draw length (makes sense if you want to be keeping all other things - e.g. string angle - exactly equal between the draw lengths). The mass of the arrow does NOT increase in proportion to the increase in energy stored, though - there is a huge increase in stored energy (under force draw curve) and a much smaller increase in required arrow weight (can't be R-sed to do the maths now but you're welcome to). Also, the heavier arrow means slight increase in bow efficienccy (i.e. stored energy -> kinetic energy; effectively you get a bit more fps per grain of arrow weight with a heavier arrow)
 

CraigMBeckett

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Most of the answers given are based on the supposition that the longer draw bow is of a higher weight, why cannot the longer draw be from a bow that has the same ultimate weight as the bow used for the shorter draw length, say both pull 65 lbs, one at 28 inches the other at 32 inches and to bring David's concern about the additional mass in the longer draw bow, the 28 inch draw bow is 65 inches long while the one for a 32 inch draw is say 74 inches long, (maintaining the draw length x 2.3 = bow length relationship, I know it should be 64.4 inches and 73.6).

I would suggest that in accordance with the principle that shorter bows stack more quickly that longer bows then the longer bow would store more energy having a better FD curve and as more energy is stored and applied over a longer power stroke then the longer bow would shoot with more power. despite the fact there is more mass in the limbs (even if optimised using the mass principal, and probably more in the arrows.

Of course this would only occur for two bows of equal quality of build.

Craig.
 

tinkerer

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Bender Obviously we should be comparing bows of the same final draw weight but varying in draw length here. I assume that was what wb had in mind. He wasn't asking if drawing 57# at 31.5ins will be better than 50# at 28ins??

CB has stated the problem. But that 73in bow will be plenty more massive than the shorter one. It's not only a lot longer but must also be a lot thicker to maintain stiffness, as will the arrow. Now it seems to me that if energy storage, bow mass and arrow mass all rise by 12% say, not much changes.

Selfbow, I should have made it clearer, as, hopefully, in the above.

David
 

tinkerer

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Fanio-A huge increase in stored energy? The energy increase will be roughly 31.5/28 or 11.25% plus a certain amount due to reduced stacking (??)

The arrow shaft will be around 11% longer but also thicker to maintain spine, so likely to go up in proportion to the energy increase, at least.

And look at the bow, say 11% longer and 8% thicker. The energy storage is looking more modest.

David
 

fanio

Active member
no it's a bit more than 11%, as JUST the power stroke is longer (assuming equal brace height, which we have to do for apples vs apples).

if brace height is say 8", then it's (31.5-8)/(28-8)=(23.5/20)=17.5% (assuming same draw weight etc as above).

that's equivalent to comparing a 50# bow to a 58.75# bow.

but I take the point that you will lose some (but not all) of that due to heavier limbs, heavier arrow, etc.

but we all know that the variance between 2 traditional bows / ELBs - even with identical draw weight and drawn to the same length - is possibly more than this difference anyway (due to design, wood, build execution, etc).

So while it's an interesting theoretical discussions, the practical bottom line is: get a good quality bow that fits you and feels good, and don't try to "massage the figures" by shooting a bow that's too long.
 

tinkerer

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Fanio, Sloppy of me to ignore powerstroke; fair point. But it still seems evenly poised. (Especially if brace height drops a bit). It's hard to compare practical examples without bringing in a host of other variables but an interesting question.

David
 

Bender

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Take a 43# @ 28" bow. It will gain about 2# for each additional inch drawn. Draw it to 31.5" and now you have your 50# @ 31.5" I guess the limbs weigh more due to the additional energy stored and since E=mc^ its a relativistic effect? Yes it requires a long nock to nock bow to avoid stacking. But the limbs do not need to weigh more.
Sure I granted a real poor arrow that gained 20 grains for each additional inch in length. But that was unrealistic. It does take some skill, time, effort, and lots of sorting and culling but its perfectly possible to build a 50# spine arrow at sub 400 grains. Its totally possible to build the arrow for the 50# @ 28" bow to weigh 500 grains then build the arrow for the 50# @ 31.5" bow and have it weigh LESS while still being the proper spine and well tuned.
The assertion that the gain is neglible is misinformation based on false assumptions.
 

tinkerer

New member
I put this question on paleoplanet and had a few replies, all supporting the longer is better view; not the response I was hoping for lol. I'm still in a minority of 1.

At any rate I have a fast elb 67.5ins ntn that pulls 46lb at 25ins and with it I was able to reach 100yards with the point of the arrow on or just above the top of the boss. Ordinary target arrows. Keep in mind that this bow is overbuilt for this drawlength, a bow 62ins or so ntn and with the same DL would be faster again. There was a light breeze from the side and slightly behind the flight which probably helped a little.

So, whether your natural drawlength is 25 or 32ins the best thing is to tailor the bow to suit your drawlength, not the other way around.

Bender If you use all your skill to make the 31.5in DL bow and arrow as light as possible whilst still being robust and performing well, and use just the same skill in making the shorter combination, then there will be a significant differemce in mass. The 25in arrows I shot weigh just over 300grains but are sufficiently stiff, strong (not being barrelled or tapered) and reasonably consistent in accuracy. And very easy to make.

David
 
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