I am wondering what the difference in speed is for a two identical design bows with one bow that is 55 lbs @ 28 inches and another that shoots 55 lbs @ 30 inch draw.
Does anyone know if there is a general number for increase in arrow speed per extra inch of draw length? I did a search and could not find anything covering this one. I know I have read in posts somewhere that the longer draw length makes the 55 lb @ 30 inch bow the equivalent of a higher poundage bow drawn to 28 inches.
For speed tests they use a 30 inch draw usually. I always thought it was to make the bow 'look' faster.
In addition does anyone know if limb length changes the speed of a bow?
I know with wheelie bows it runs about 10 fps per inch of draw length.
With trad bows I think it runs about half that or about 5 fps per inch of draw.
For the same grain per pound arrow, speed will increase about 3 fps for each inch of draw over 28". Thus from 28" to 30" gain is about 6 FPS.
While Dynamic efficiency remains almost identical, the store energy per per pound of draw force will increase about 3 to 4% for each inch over 28" , that's how longer draw length will increase arrow speed.
Longer limbs shorter risers equil faster bows.
Stu's calc doesn't reflect effects of power stroke length on velocity while all other factors remain constant. He is aware of it and would be a good source to address your question.
If you do follow-up with Stu, then please post you feedback.
With precisely the same grains per pound of arrow weight the difference in two inches of draw is very close to 10 fps for almost every bow I have ever tested (5 fps per inch of draw).
ok with that being said how much draw weight is need to add the same 10fps
Take a look at the difference between 26-27" to 28 "
guk, it depends on the bow and how efficiently it stores energy and how efficiently it delivers that stored energy.
swampthing, for a while I tessted all bows at 26", 28", and 30" AMO (all with 9 grain per pound arrows at their respective draw lengths and corresponding draw weights). The difference from 26" to 27" and from 27" to 28" is the same - right at 5 fps per inch of draw.
Draw weight is but one measure of a bow. As John says, bow design matters more. A 45 lb self bow is not a 45 lb ACS. Over generalizations are misleading and unproductive. There may be some patterns, but specifics will vary greatly by design, materials, etc. etc...
QuoteOriginally posted by Ground Hunter:
Draw weight is but one measure of a bow. As John says, bow design matters more. A 45 lb self bow is not a 45 lb ACS. Over generalizations are misleading and unproductive. There may be some patterns, but specifics will vary greatly by design, materials, etc. etc...
Correct.
But this discussion is more about draw length and it's relation to arrow speed gained or lost as draw length is increased or decreased, not as much about draw weight comparing from bow to bow.
QuoteOriginally posted by guk:
[qb] ok with that being said how much draw weight is need to add the same 10fps [/b]
Seems to me to be about draw weight - and to that I added my comment. I hope that's ok with you?
OK, I am a little confused why we are referring to gpp when the bows are both supposed to be 55#. Is this what you are saying, John , that say a 55# acs @ 28" will be 10fps slower than an acs that is 55# @ 30"?
And what, roughly, would the increase in speed be for an acs that is 55#@28" drawn at 28" versus 30", assuming the same total arrow weight, but adjusted for.proper tuning.
So how would the 2 inches of arrow(for the 30" draw)compare for weight and drag to the 28" arrow draw.
QuoteOriginally posted by Ground Hunter:
QuoteOriginally posted by guk:
[qb] ok with that being said how much draw weight is need to add the same 10fps [/b]
Seems to me to be about draw weight - and to that I added my comment. I hope that's ok with you? [/b]
Sorry dude, I didn't notice the comment about the draw weight. Nothing personal intended. My bad. :)
I was referring to the original post, which is what this topic was originally intended to be about and I didn't think anybody was overgeneralizing that.
John, I think there is an apples / oranges thing going on here.
By using the same bow, you are also increasing horsepower with each inch of draw.
I believe the question compares different bows, one 50 @ 28 one 50 @ 29 one 50 @ 30, all else being equal.
There is all sorts of talk of power stroke length, but I am questioning all that myself. I think it all depends on a bows design and quickness to recover. Heck, build a 95 foot tall bow and using that logic with a 90 foot power stroke that arrow might hit light speed !
Anyone that is actually smart (knows about this stuff, instead of my feeble guesses) got any answers based on testing and not theory ?
ChuckC
QuoteOriginally posted by John Havard:
With precisely the same grains per pound of arrow weight the difference in two inches of draw is very close to 10 fps for almost every bow I have ever tested (5 fps per inch of draw).
In reference to Clay's original question, is this roughly applicable to 2 bows, one 55@28 and the other 55@30, all else being equal?
I think someone mentioned power stroke earlier on and this term sounds like it fits the question at hand.
Thanks for the input guys!
Chuck,
My answers are based on testing the same bows at 26", 28", and 30" - all at 9 grains per pound of draw at each draw length. There's NO way to compare different bows without having to test each bow individually. I'll refer you to this website. Of particular applicability is example #4 in the attached link (toward the very end). It gives a real-world example of what can be accomplished with the same arrow and with draw weight changes.
http://dryadbows.com/Defining%20Bow%20Performance%20Dryad.pdf
A lot of the responses deal with the increased speed/performance attained by the same bow as it's drawn farther. It gains speed/performance because it gains draw weight as the draw is increased from 28 to 30 inches. But that's not the question asked.
The question asked is, is a bow drawn to 50#@30 inches faster than a bow drawn to 50#@28 inches, all other things being equal, and by how much? Most assume that the longer power stroke of the 30-inch draw would yield a faster arrow, but so far, no one has provided proof, an example of that being so. I don't have proof of it either, but that's the way to bet. :dunno:
The examples I have offered are based on ceteris paribus - all things being equal. If a bow is drawn to 28" and a 9 grain per pound arrow is shot from it, and the same bow is drawn to 30" and a 9 grain per pound arrow is shot from it (ALL things being equal including brace height and string weight), the 30" 9gpp arrow will be 10 fps (within 1-2 fps) faster than the 9 gpp arrow shot with a 28" draw length. Several examples are offered with explanations in the Dryad website that explain how/why that is.
John - Other information posted from the wheelie world relayed the same ~10 fps gain, though these tests were completed maintaining the same arrow wt, draw wt and merely altering the power stroke.
The original post was about differing draw lengths for the same exact bow design at 55lbs of draw weight.
So what I am getting from the discussion and based on tests that have been run is that there is approximately 5 fps gained for every inch of increased draw length in this situation.
This brings on the next question. What difference in speed can you expect to gain or will any speed be gained for two bows of identical design shooting a 10 gpp arrow at 50 and 60 lbs. Theoretically if the energy curves are the same and the arrows gpp at that draw weight are the same, I would think you would be shooting the arrow at the same speed out of the two bows.
My recurves gain a bit more than 2 lb per inch. Longbows a bit more. 'With the numbers John gives that would be about 4.8 fps gain per lb. That would be one efficient bow.
God bless you all, Steve
Lowell,
A 50# @ 28" bow shooting a 500 grain arrow (10 gpp) will shoot "X" fps. The same exact bow design that's 60# @ 28" will shoot a 600 grain arrow (same 10 gpp) at the same "X" fps. There will be NO difference in arrow speed at the same draw length and at the same grains per pound.
Another way to say the same thing is that any bow of a particular design will shoot the same gpp at any draw length the same speed.
Oops, I didn't see that you were keeping gpp constant in your first post, John. As you point out, another way of measuring the same thing. :knothead:
John,
I want to say thanks for posting the answers you have based on testing. I am overly curious about technical details and would have eventually let it drive me to testing all these things myself. Your experience and results are greatly appreciated.
While I have your attention I would like to ask one more question. Have you done the testing to know what equivalent draw weight of the same design bow you would need to match a 55 lb bow drawn to 30 inches? I have read and understand that adding 10 fps due to 2 extra inches of draw is giving you the equivalent fps of a heavier bow at a shorter draw length. I suppose you could calculate it if you knew a set increase in fps per lb of draw weight.
OOPs, Sorry there, I misread. John's right on. Should have known that. Its a gain of about 1 fps per lb. That jives. LOL lots of oops ssss there
God bless you all, Steve
John,
I want to say thanks for posting the answers you have that are based in actual testing. I am overly curious about technical details and would have eventually let it drive me to testing all these things myself. Your experience and results are greatly appreciated.
While I have your attention I would like to ask a couple more questions. Have you done the testing to know what equivalent draw weight at 28 inches in the same design bow you would need to match a 55 lb bow drawn to 30 inches?
Is there a rough constant of increased fps at a set gpp of arrow weight per lb of draw weight?
Easy one.... approx 5 fps per Inch of draw, sooooo.
55# @ 30 and a 65#@28, should, be about the same speed with the same weight tuned arrow.
But. and this is a big one......
One major oversight by a lot of peeps is the diference will be offset by the additional 2" of arrow length. Take a wood arrow of 55-60# and then one for 65-70# you would need for the 30" draw with a 125g point, if one used a heavier 160g point they would need probably 70-75#. These 2 different spine shafts will weigh "a bit different" you could say. Or for tech comparo, about 50-75g more. With carbon you would find that the difference would be a bit less, to the tune of 25g or so.
I find that for my Hill style bows, they typically and linear- ally drop 1fps for every 10g arrow weight. So in this instance the 30" draw would loose 5fps and 2.5fps, anyway, over what the math says the difference would be between the 2 different draw lengths.
Using the average of:
A 30" draw is 10fps faster than a 28" draw with same bow, that can handle a 30" draw. Take away 5 fps for the heavier arrow needed for the longer draw, so we are now at 5fps faster with the 30" draw and longer arrow. Pretty close already, just remember though upping draw weight will require stiffer/possibly heavier arrows so we will need to take that into consideration. So about 5#+/- a pound or 2 more for a 28" draw should get you right in there with the speed of the 30" draw.
Please remember that my model was a Hill Style bow, not affected "as much" to arrow weight through my chronograph. It consistently shows that for every 10g arrow weight will affect speed by 1fps. Some of the newer bows, D/R bows and recurves, do not exhibit the same change in speed in regards to the same difference in arrow weight, usually the are bogged down a little more per 10g of arrow weight then a Hill.
Hi Lowell,
I haven't done that exact testing but the results are fairly easy to calculate given enough background data.
First of all, you need to know SE/PDF28 and SE/PDF30 for the bow in question. Let's say that SE/PDF 28 is 0.900 and SE/PDF30 is 1.000. That means that the 55# @ 30" bow stores 55 ft-lbs of energy.
Second, we need to know what the DE (dynamic efficiency) of the bow design being tested is. My testing results suggest that there's very little (so small it's hard to measure) difference in DE at varying draw lengths (within reason GIVEN CONSTANT GRAINS PER POUND ARROW WEIGHT).
But, given that DE varies with grains per pound, let's assume in this example that the 55# @ 30" bow is shooting a 10 gpp arrow, or 550 grains. Further, let's assume that in this example we want to know what draw weight in the same design bow is needed to shoot THE SAME 550-GRAIN ARROW THE SAME SPEED.
Now we calculate: the 55# @ 30" bow stores 55 ft-lbs of energy and the arrow shot from that bow (because of the DE of 85% - a very high number FWIW) contains 55 ft-lbs*0.85=46.75 ft-lbs of energy. Knowing the equation for kinetic energy, we can calculate the speed of the arrow being shot from this particular 55# @ 30" bow. An arrow weighing 550 grains (10 gpp) with 46.75 ft-lbs of energy is traveling at 195.6 fps.
To quote from one of my college differential equations books "therefore it should be obvious to the most casual observer that....." the same bow design being drawn to 28" and shooting the same 550-grain arrow the same speed will also have to deliver 46.75 ft-lbs of energy to the arrow. The DE for the 28" example will be lower because in order to shoot the same arrow the same speed (and therefore have the same total ft-lbs of energy), the bow being drawn to 28" will have to have a higher draw weight at 28" than 55#. Iteration here is necessary to fine-tune the answer but I'll cut to the chase with a simplifying assumption: With the higher draw weight at 28" let's assume DE drops from .85 to .835 when shooting the same 550 grain arrow at the same speed of 195.6 fps.
Now the solution is easy and simple. The same bow design being drawn to 28" delivers 46.75 ft-lbs to the arrow. Its DE is .835, so that means it must store 46.75/.835=56 ft-lbs of energy. And given the fact that SE/PDF28 = 0.900 the bow being drawn to 28" must have a draw weight of 56/.900=62.2#.
The answer from this example based on reasonable numbers for a better-than-average recurve is: Bow of "X" design and manufacture being drawn to 55# @ 30" would have to be 62.2# @ 28" in order to shoot the same 550-grain arrow the same speed.
Now, if we wanted to keep the gpp constant the DE for the 28" bow would be higher, and through iteration we could hone in on the answer. Suffice to say that somewhere in the 60-62# range at 28" would shoot the same arrow the same speed as a bow of precisely the same design being drawn to 55# @ 30".
There are several more examples in our website that provide more background and explanation if you're interested:
http://www.dryadbows.com/Defining%20Bow%20Performance%20Dryad.pdf
Nerd ALERT!!!
Now you know why John is such a good guy to have on board when you are designing a new limb.
LOL
Mike
I resemble that remark Mike.
QuoteOriginally posted by John Havard:
Chuck,
My answers are based on testing the same bows at 26", 28", and 30" - all at 9 grains per pound of draw at each draw length. There's NO way to compare different bows without having to test each bow individually. I'll refer you to this website. Of particular applicability is example #4 in the attached link (toward the very end). It gives a real-world example of what can be accomplished with the same arrow and with draw weight changes.
http://dryadbows.com/Defining%20Bow%20Performance%20Dryad.pdf
John i went on your link and found (you wrote it and i also measured 3 to 4% variation) that for 1" of draw SE/PDF is increasing with 3,5% and DE remains the same and of course arrow mass (m) is identical and V is arrow speed
here is what you wrote on Dryad link about Store energy variation according to draw length:
(http://i758.photobucket.com/albums/xx225/hybridbowhunter/SE.jpg)
We can do the math:
Dynamic Efficiency X Store Energy = arrow Kinetic Energy= 1/2 X m X V2
So V2= SE X 2DE/m and V= Square Root(SE X 2DE/m)
If you increase SE by 7 % then for 30" then speed becomes
SqR(1,07SE X 2DE/m)= SqR(1,07) X SqR(SE X 2DE/m)= SqR(1,07) X V so the factor of speed gain is SqR(1,07)=1,0344
So Arrow Speed@30" = Arrow Speed@28" X 1,0344
If You test a Recurve bow giving 180 FPS # @ 28" then Arrow speed @ 30" is 180 X 1,0344= 186,19 fps
If you test an ACS CX @ 10 gpp and 28" you get 190 fps.
At 30" speed will be 190 X 1,0344=196,53 fps
If you test a Howard Hill bow giving 160 fps @ 10 gpp/28" speed when drawn to 30" will be 165,5 fps
From 28" to 30", speed gain is about 6 fps...
Laurent
QuoteOriginally posted by wingnut:
Nerd ALERT!!!
Now you know why John is such a good guy to have on board when you are designing a new limb.
LOL
Mike
:biglaugh: :biglaugh:
It's good to know that you guys really understand what you're doing. :cool:
Speaking from the perspective of a dummy who never even took physics in high school, I think I'll just take John's word for it and be happy that I have long arms. From John's explaination I'm pretty sure that's a good thing....
I'll try to remember this thread the next time someone asks what the minimum bow weight is to hunt elk. It all depends .... ;) :biglaugh:
Merci Laurent!
John,
I understood the equations, and all our remarks. Thanks for clearning that one up nicely.
Clay
I would say based on design it will vary but a good rule of thumb in my expierence is 5-8 fps aross all draw lengths on average. If all things are equal it is in my opinion the variable that affects performance
i've been following the thread since the beginning. suddenly, i am very tired...
bill
Thanks for all the answers guys!
Now how 'bout another question: are longer limbs faster then shorter limbs? :D
Cheers,
Chris Mallett
John,
I read your article about bow performance, thank you very much that clears a lot up for me. Excellent piece thank you. One question though, on page 6 the table about DE has the ACS longbow more efficient then the new ACS recurve. Is this a mistake or if it isn't any chance you could explain why?
My apologies if this is wandering off topic.
Chris M
Chris,
I'd be glad to explain. The ACS recurve has very high DE for a recurve because we're able to make the limb tips very thin through the use of ACS cross-sectional geometry (don't have to make portions of the limb wider or thicker in order to make them stiffer). However, any recurve limb tip is heavier than the limb tip of a longbow - especially the limb tip of an ACS longbow. Therefore more of the stored energy is spent in the acceleration of the limb tip forward = lower DE.
Where the ACS recurve makes up for its lower DE (an unavoidable problem with a recurve) is through increased energy storage. Our ACS recurve stores 7% more energy than our longbow, and is therefore a tad faster in spite of having a lower DE. In fact (while we're going to wait and let Blacky's third-party impartial test numbers do the talking) I can honestly say that the AMO speeds I have gotten on our ACS recurve tests are at a minimum 2 fps faster than the next fastest bow I have ever tested.
Thanks for all the work John. Now I have to read thru it and TRY to understand it,
ChuckC
So what would the speed be with a, 66" ACS design, 62# @26.5" draw, shooting a 510g arrow?
OK, I'm hanging in there (sort of)but if someone can answer this question for me I think it will make it clear for my little pea brain.
Bowyer Bob builds me a recurve that it is 50lbs at my 28" draw.
Bowyer Bob also builds my buddy the exact same recurve, except it is 50lbs at his 30" draw.
Both bows are strapped in shooting machines and drawn to their respective 28" and 30" draws. They are both loaded with a perfectly tuned 500 grain arrow. Both shooting machines are triggered at the exact same time. Which arrow reaches the target first?
Burnsie,
The 30" draw gets there first by about 6 fps.
Mike
With Bear takedowns way back, when we had a truck load of limbs to play with, we found a couple of cases where certain limbs were faster than average and shot about the same and other cases where even a one inch longer draw made 6 fps. I have always been jealous of those long armed guys, but with the Bear takedowns we found that it was about 4 fps per inch either way an inch from 28" on average. Mathematically speaking, I wonder if with any limb length or style there is a point where the increase changes with additional inches of draw, does the fps per inch increase or is there an outside chance that the mechanics of a particular bow can get less efficient with extra long draws? For draws under 28" it does not seem to be an even drop in fps when decreasing the draw multiple inches.
Guys read John's performance paper (linked earlier in this thread) it really explains it all. Even if the math is over your head a little, just scroll through to the examples at the end and smile at the awesome :)
John, thanks for the response! Sounds like it's time to get saving for a new ACS recurve...
Cheers,
Chris
John,
I just read thru the link in your earlier post. I am a die hard Wes Wallace Royal fan. I am going to have to talk with you guys to see if I can have bows made with matching grips or if I can buy bow blanks and form the handles myself.
I am looking into African and Australian hunting trips for the wife and I. She shoots 40 lbs, and I shoot 55. I have been plotting the purchase of two R/D longbows with increased performance for these hunts. My intent is to get bows that will shoot heavier arrows at speeds matching our existing bows. I will run the calcs to figure out what weight bow I need to have to shoot a 650 grain arrow at the same speed I am shooting my 560 grain arrows at now. I will check out your website and see what is available.
Thanks for posting up your results and links. I love the technical side and enjoyed learning about your testing process. You just moved dryad to the top of my next bow purchase list.