Bow Efficiency.

[greybeard]

I ran the numbers on two of my bows to find out how efficiently they utilised their stored energy.

The data required to arrive at a percentage value consisted of total stored energy in the bow [inch/pounds] obtained from the F/D curve, arrow velocity and arrow mass.

The chronograph recorded the 460 grain arrow at 166.9 fps from the 40# longbow.

If you look at the velocity values on the right hand side of the chart 166.9 fps shows an efficiency value just over 90%.

Longbow Arrow Mass And Bow Efficiency.jpg (55.8 KiB) Viewed 6363 times

The 500 grain arrow shot from the 43# takedown recurve registered a velocity of 172 fps showing an efficiency value just under 92%.

Takedown Recurve Arrow Mass And Bow Efficiency.jpg (55.46 KiB) Viewed 6363 times

It would be interesting to know what efficiency values the commercially made bows produce.

Daryl.

[longbow steve]

http://piratesofarchery.net/bb/viewtopi ... 9&start=20
Hi Darryl, your bows are up there with the best. Above is a link of FD charts for some speedy bows. Cheers Steve

[GrahameA]

Hi Steve and Daryl.

A few comments.

I have been lucky in that I have shot a fair few of Daryl's bows and spent a few hours talking bows in the workshop. Daryl's bow shoot nice and are pleasant to shot. One observation is that the bows shoot quiet and do not exhibit "shock". I attribute those two characteristics to the bows being Efficent in that most of the energy stored in the bow is transferred to the arrow. As distinct to causing the bow to vibrate and make lots of noise. The penalty of making bows well is that it takes a lot of time during the construction of the bow, espicially in getting the limbs to bend "smoothly".

A big side issue. I have always wonder why people add things like string silencers and various vibration absorbing devices to bows. If the bow is shooting "well" should it not be quiet and vibration free??

One good reason for fur string silencers is that they look "cool". How much arrow velocity do string silencers strip from a bow?

[Fanto]

Grahame,

Byron Ferguson suggests using the silencers to intentionally slow the string in order to "de-tune" the bow to match under-spined arrows.

One of my bows, the Martin Hunter 62", refuses to be quiet without wool puff silencers, the Mamba 58" on the other hand is extremely quiet??

another one of life's mysteries...

[GrahameA]

Morning.

Byron Ferguson suggests using the silencers to intentionally slow the string in order to "de-tune" the bow to match under-spined arrows.

Think about the logic here. You get a bow that shoots well and then you do things to it so that it can handle the wrong arrows and not be able to reach its full potential.

To my way of thinking rather than killing off the performance it would be better to get a set of arrows that matched the bow in the first place. Bit like Ferrari saying, "Hmmm,our latest race car is to fast. What we need to do is slow it down rather than putting a better driver in it".

I know where Ferguson is coming from however in isolation it just seems stupid.

One of my bows, the Martin Hunter 62", refuses to be quiet without wool puff silencers, the Mamba 58" on the other hand is extremely quiet??

another one of life's mysteries...

Try some heavier arrows. Try a thicker string. My Mamba shoots great with the standard arrows for it - it is not so good if Alloys are fed to it.

Wild comment for the Day.
Experience suggests that many people do not "tune" their bows. Many people would benefit by spending some time tuning their bows. Just adjusting the brace height can pay dividends - and working with a range of arrows, spine, mass, etc. ........ shock, horror.

[bigbob]

From the little I know Daryl your data does suggest a very efficient bow. I did read somewhere and I have no idea of its authenticity where for a straight laid long bow one 'took' a 'standard' of 115f.p.s. and add bow weight giving reasonable speed for that bow ie; 40# bow -115 + weight = 155. If there is any veracity in this equation then your bow is markedly higher. Of course I could be entirely erroneous with the 'data' which I hasten to add bears no reflection on the terrific performance that is visibly demonstrated in the graphs. Good work Daryl.

[longbowinfected]

Personally I like a slower string and arrow as it makes the gaps I use fit to the top of the target, then the middle then the bottom then the ground for 60, 50, 40, 30 metres for a FITA 60. My bow is almost centre shot and I put heavier 175 grain points on with 4 large 4 inch feathers. [under chin med release]. It also means these alloys are closer in mass to my timber arrows. Not perfect but very useful. So sometimes it is more about manipulation for convenience.
Then again not many longbowers want to shoot FITA 60.

Kevin

[GrahameA]

Hi Bob.

If there is any veracity in this equation then your bow is markedly higher.

That is a good question - and the sort of question I would ask. And since I wrote the spreadsheet it always possible to be wrong.

I did read somewhere and I have no idea of its authenticity where for a straight laid long bow one 'took' a 'standard' of 115f.p.s. and add bow weight giving reasonable speed for that bow ie; 40# bow -115 + weight = 155.

IMHO that sort of "Rule of Thumb" has the potential to show a wide variance between predicted values and actual values

[greybeard]

Hi Bob,

In the link below they use 100 plus the draw weight of the bow.

http://www.greenmanlongbows.co.uk/SPEED ... ometer.htm


This method is used as a base line to arrive at a number for comparing a bows performance but does not give a percentage value of the bows efficiency.

Check his site out, some nice looking bows on it.

Daryl.

[bigbob]

Using his method then your posted bows are at least equal and mostly superior in arrow speed. Given too that he had optimum conditions eg; light weight arrows etc.
Ps The veracity of equation I referred to, was for the figures I quoted re arrow speed. I wasn't casting aspersions on your work Grahame , as it seems you may have drawn that conclusion

[GrahameA]

Hi Bob.

Ps The veracity of equation I referred to, was for the figures I quoted re arrow speed. I wasn't casting aspersions on your work Grahame , as it seems you may have drawn that conclusion

You should always double-check my maths. I do not even trust it myself. As a general rule I rarely trust any quoted figures - I like to see some evidence.

Purely as an example.

I did read somewhere and I have no idea of its authenticity where for a straight laid long bow one 'took' a 'standard' of 115f.p.s. and add bow weight giving reasonable speed for that bow ie; 40# bow -115 + weight = 155.

I suggest the Holy Grail of Selfbows is 200 fps. Using that R.O.T. archers should be able to grab a 90lb longbow and expect that it should shoot 115+90=205. It is good to double check things and be a little cynical of things. Hmmm....... seems a tad fast ..... and a tad on the other side of unbeliverable.

[Fraser]

Hi guys, does the equation your using to calculate velocity/efficiency come from this artical?

http://www.currentmiddleages.org/artsci ... esting.pdf

I've only tested it on one bow but it gave a quite acurrate figure, it gave me a theoretical velocity of 185fps and the actual velocity is 183fps. Equation uses 90% as the efficiency for a longbow.

Cheers,

Fraser

[GrahameA]

Hi guys, does the equation your using to calculate velocity/efficiency come from this artical?

No.

[bigbob]

Hi guys, does the equation your using to calculate velocity/efficiency come from this artical?

No.

Grahame you will have to stop with such verbose answers. Some people may not have the internet time to read them!

[GrahameA]

Hi Bob.

Hi guys, does the equation your using to calculate velocity/efficiency come from this artical?

No.

Grahame you will have to stop with such verbose answers. Some people may not have the internet time to read them!

I will strive to be more circumlocutionary in my replies to questions.

[bigbob]

Hi Bob.



No.

Grahame you will have to stop with such verbose answers. Some people may not have the internet time to read them! [/quote]
I will strive to be more circumlocutionary in my replies to questions.[/quote]
Why are you circling diesel locomotives???

[dartonian]

I will strive to be more circumlocutionary in my replies to questions.

Please dont Grahame, I enjoy your posts and floccinaucinihilipilificate any circumlocution
Cheers,
Paul

[Dennis La Varenne]

The formula for arrow speed of 115fps plus draw weight that Bigbob refers to above is that proposed/guaranteed by Howard Hill Archery on the info sheet which accompanied HH bows.

I have not purchased one for some time now, so I do not know if that guarantee still applies. I still have at least one sheet somewhere here at home, but I cannot remember if that guaranteed speed was based upon a specific arrow mass - draw weight ratio which it would need to be.

Still, it is a big promise to make for any manufacturer and he would need to be very confident of his product to do so.

There is no way of telling either whether or not that figure is a minimum or a maximum guaranteed figure, and I cannot remember if the info sheet specified which. Anyway, I have acquired a few HH bows over some time and am well pleased with them, so I could do a bit of testing I suppose. My problem is that my arms are not long enough to allow a 28 inch draw at which draw length all my HH bows are rated. That puts a bit of a spanner in the works.

[Fraser]

Thanks Dennis, I'd wondered where that formula had come from. It came as a surprise to me how much of a difference a longer draw length makes. Makes it hard to compare bows unless the draw lengths are exactly the same.

Cheers,

Fraser

[greybeard]

Unfortunately arrow velocity used in isolation will not yield the required information to calculate bow efficiency.

Bow efficiency can be calculated by carrying out force/draw testing to obtain the stored energy data, chronograph information to obtain arrow velocity and grain scales to ascertain total arrow mass.

I did a simulated f/d curve for a 60# longbow and the stored energy was 717 inch/lbs.

Using the 115 plus 60# draw weight as suggested would yield 175 fps arrow velocity. By manipulating arrow mass I could change the bows efficiency rating between 80 and 100 percent.

Working on ten grains per pound of draw weight a 600 grain arrow showed the bows efficiency was 82.5%. At 12 grains per pound bows efficiency increased to 92%.

The above figures were generated by the data in the spread sheet using 175 fps as a base line. To obtain accurate results different weight arrows would need to be shot through a chronograph as actual arrow velocity is required.

Daryl.

[Dennis La Varenne]

Daryl is quite correct.

The Howard Hill Archery formula is only company's claim for the velocity output for their bows. High velocity is NOT the same as high efficiency, but commonly mistaken for it.

Efficiency relates to how much of the calculated stored energy in a bow is used to push an arrow of given mass. Each bow will have an optimum arrow mass which absorbs the greatest amount of stored energy for the greatest velocity for that specific arrow mass.

Above that point, arrows of lower mass may have greater velocity, but exhibit some vibration in the bow which is stored energy being wasted into the atmosphere because there isn't enough arrow mass to absorb it. Below that point, the mass of an arrow is so heavy that, as you would expect, velocity begins to fall off dramatically because, as arrow mass increases, the amount of stored energy in the bow is decreasingly enough to propel it - what Roger Ascham referred to in Toxophilus as a 'dead heavy shaft'.

[greybeard]

Efficiency relates to how much of the calculated stored energy in a bow is used to push an arrow of given mass. Each bow will have an optimum arrow mass which absorbs the greatest amount of stored energy for the greatest velocity for that specific arrow mass.

Above that point, arrows of lower mass may have greater velocity, but exhibit some vibration in the bow which is stored energy being wasted into the atmosphere because there isn't enough arrow mass to absorb it. Below that point, the mass of an arrow is so heavy that, as you would expect, velocity begins to fall off dramatically because, as arrow mass increases, the amount of stored energy in the bow is decreasingly enough to propel it - what Roger Ascham referred to in Toxophilus as a 'dead heavy shaft'.

For those who are interested I have added extra data to the original 40# longbow results.

The figures in the top row show the percentage of bow efficiency in relation to arrow velocity recorded by the chronograph shooting 460 grain arrows.

As you go down the subsequent rows of numbers you will notice the predicted arrow velocity reduces as arrow mass increases.

At 10 gns/lb. and 197.045 fps shows the bow at 100% efficiency. At 13gns/lb and 172.820 fps shows the bow at 100% efficiency.

There will always be trade offs such as sacrificing stability and how the bow feels in the hand if you are chasing maximum arrow velocity.

40 Lb Longbow.JPG (189.89 KiB) Viewed 6087 times

Daryl.

[looseplucker]

The knowledge you blokes have is scary. I can make a shooter. I can make arrows that the shooter will poke out with enough grunt to do the job.

But this technical stuff leaves me feeling a bit like a chef in a house of ill repute. I am perfectly competent in my own right, but have nothing to do with the business at hand. It really blows me away, the depth of this stuff.

[bigbob]

so pleased that Daryl and Dennis spend some of their valuable time posting information such as this . I am aware of the better part of the data but have a real problem these days with memory,or information retention and prolonged concentration. Being able to reference this data at will makes my sorry life just a little better [lol] I for one appreciate it.

[looseplucker]

What BigBob said. I take as much as I can out of this material too and turn wood into something. Works most of the time and all thanks to these blokes.

[Dennis La Varenne]

Daryl,

A few technical questions -

1. Why do you measure your draw length from the pivot point of your grip rather than the traditional method of from the back surface of the bow at the level of the arrow pass? All of us measure draw length from the back of the bow as has been standard practice for decades. I am interested in why you do it your way and how you consider that assists data accuracy.

2. What was the actual arrow speed of the bow used to project the data above. I understand that you use a spreadsheet to extrapolate for the different arrow weights, but I think some readers may think that the speeds you show at 100% efficiency are actual speeds from your bow rather than projections from an actual speed. It might help a bit if you explain this method a bit more.

3. Does your spreadsheet make an allowance for bow hysteresis? I have seen Norb Mullaney's method used to calculate bow efficiency which inputs bow hysteresis as a significant part of the input data. Without it, the efficiency numbers can appear much higher than they should. Mullaney's system would be an excellent candidate for a spreadsheet.

[Dennis La Varenne]

Daryl,

Cancel question 2. You did post the actual speed at the start of this thread - 460 grains @ 166.9fps - very impressive for such a light bow.

[greybeard]

Dennis,

I hope my original post did not mislead you as it refers to a 40# glass/bamboo laminated longbow and not a selfbow. Never the less I am happy with the bows performance and as a bonus it is gentle in the hand.

I prefer to use the true draw length [DLPP] as this remains constant whereas bow handles/riser can vary in depth. Brace height is measured from the pivot point of the riser so to my way of thinking it is logical to measure draw length in a similar method.

True Draw_DLPP.jpg (72.08 KiB) Viewed 6032 times

At the outset the spreadsheet was designed to enable me to carry out in shop testing on my own bows. I will need to check with Grahame Amy if hysteresis within the bow was factored into the spreadsheet. Although this will have some bearing on the percentage factor how do we factor in the values when we have selfbows, laminated wood bows and glass laminated bows utilizing various core materials and glue lines.

Arrow velocity is required to obtain the test results. Some factors that can skew the results is drag on the arrow i.e. fletchings that can be straight, angled, helical, length and profile height. Method of release such as fingers, release aid or shooting machine will yield differing results.

Perhaps one may cancel the other?

For in house testing I am dealing with a constant so if the values are plus or minus a little I can still evaluate my bows to see where there is room for improvement.


Daryl.

[bigbob]

I agree with you Daryl in relation to varying depths of riser material giving a non constant reading when measuring draw. For example even with a HH style riser and one having an overlay on the back of the bow and another not having, then there can be 5-6mm difference from that alone. Add to that one with a recurve style 'throat' versus HH style and there can be a big variance. there as well.That amounts to an obvious non conforming standard by which to measure accurate data. As long as you keep your data and utilize it for your calculations then it would be more accurate than accepting the 'norm'

[Dennis La Varenne]

Daryl,

All's fine. I did think the bow you used was one of your nifty little bamboo backed numbers and I was beginning to think that it was performing astonishingly well for a natural materials bow.

Actually, so far as your explanation about the DLPP (Draw Length Pivot Point) goes, it does make sense. However, it may be advisable in your posts to make sure that you are quite clear about that and what it actually is because it is not standard or traditional and may lead some readers to misinterpret what you are doing.

From a scientific perspective and endeavouring to create consistency in measuring draw-length from a point which does not vary from bow to bow because of the often considerable distances from back to belly in many bows, I do understand what you are doing.

As Bigbob alludes to, two bows which have an identical limb design, one of which has a 2 inch deep riser and the other having a 3 inch riser will yield two different draw weights at 28 inches as measured from the back of each bow because the bow with the 2 inch deep riser will require the limbs to be drawn one inch more to reach 28 inches from the back of that bow than another bow with a 3 inch riser, with a consequent difference in load on the limbs and subsequent significant differences in arrow speed.

However, the same problem can arise to some degree depending upon how deep the pivot point is cut into the riser from the belly side on a recurve for instance compared to that of a deep handled long bow.

The only way that I can see that consistency of draw weight can really be obtained is if the belly side of the handle of ALL BOWS is level with the belly surface of the limbs.

Even so, the system would still be fraught. With bow handles aligned with the belly surface of the limbs, there is the continuing problem of how far forward of this vertical alignment the limb tips lay. Furthermore, is it reasonable to compare the limb loading and consequent arrow speed relative to draw weight measured thus of a well-recurved recurve bow against one less well-recurved as well as with a conventionally reflexed, straight laid or string-follow long bow for instance?

It seems to me that even allowing for the DLPP argument, the relative positions of the bow tips relative to the DLPP is also a factor in the draw length/arrow speed relationship.

And so - the conundrum expands.