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Gary Teuscher
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 Posted: Wed 07 Jan, 2009 9:04 am Post subject: Crossbows |
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Does anyone know of any finds of crossbows from the 14th century or earlier? And any info for their dimensions and pound pulls?
I'm more concerned with true finds over any replicas of course. I try to stay away from the later periods because from what I know many of the later finds were crossbows that were likley used as hunting or sport weapons.
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Randall Moffett
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| Posted: Wed 07 Jan, 2009 11:52 am Post subject: |
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Gary,
The Glasgow Museum has a selfbow crossbow of yew but I do not have any real info on it. It is in the real fighting stuff by T. Capwell.
RPM
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Martin Jahn
Location: Germany Joined: 20 Mar 2008
Posts: 1
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Richard Hare
Location: Alberta, canada Joined: 15 Mar 2008
Posts: 135
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| Posted: Fri 09 Jan, 2009 7:40 am Post subject: |
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Hi Gary,
Martin's book reccomendation should be an interesting read,...even if the text takes a bit of getting through.
I know not of what I speak, other than earlier bows had a foot stirrup, and could be spanned with only the hands and foot, and later larger stirrups were used for both feet.
This doesn't tell us much, other than the poundage was at the time limited to what a man could draw in this manner.
I stand to be corrected, but think that mechanical aids to spanning, ie 'goatsfoot' etc, only came in with the laminated bows.
Though the self-bows were weaker, they were at the time still a terrifying weapon, judging by accounts.
I find this a very interesting subject, and am sorry I have nothing worthwhile to contribute.
R
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Gary Teuscher
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| Posted: Fri 09 Jan, 2009 7:52 am Post subject: |
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Thanks for the info so far everyone!
| Quote: | This doesn't tell us much, other than the poundage was at the time limited to what a man could draw in this manner.
I stand to be corrected, but think that mechanical aids to spanning, ie 'goatsfoot' etc, only came in with the laminated bows.
Though the self-bows were weaker, they were at the time still a terrifying weapon, judging by accounts. |
From what I have gathered, collaborated by a few sources, is that a 200-250 lb pull range was probably what the hand and foot spanned ones ran, 300-350 pounds or so with a very simple belt claw.
This is not based on specific findings though, but some knowledgable peoples estimates, some of these crossbow makers.
There are many accounts of the crossbows power and range, based on the ballisitcs the draw eights would have to be somewhere it the above range it would seem to have the power they are given by contemporary cources, the higher pulls making up for the shorter draws.
One interesting thing though - It seems while a roughly 72 inch longbow may have a 30" draw, with crossbows the ratio seems different, draw length being closer to 50% of prod length with many "accurate" replicas, or in the 67% range with modern crossbows, though these are a different bird.
I've heard that earlier crossbows lacking the pound pull needed a longer draw to be effective - and that to long of a draw makes a cumbersome weapon. But it seems a 15" draw could be achieved with a 30" prod - making it not as cumbersome I think as some would suggest.
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Richard Hare
Location: Alberta, canada Joined: 15 Mar 2008
Posts: 135
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| Posted: Fri 09 Jan, 2009 11:10 pm Post subject: |
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Hi Gary,
I think you're probably right about the 200 to 250 lb pull for hand and foot spanning.
I read somewhere, maybe Sir RPG's book, that a self-bow needed a longer prod so it wasn't overworked, and about 36" was average.
This would enable it to pick up some extra speed from more arrow thrust, rather than from sheer poundage.
as an example, and I might be remembering this wrong!;
I00 lb longbow, drawn 24" from brace,== 2400 lbs/inches of thrust
200 lb crossbow drawing 8" from brace, ==1600 lbs/inches of thrust.
So if we had a self-bow with a bit longer prod, and could draw it 12 inches from brace, we could have;
200 lb crossbow, drawn 12" from brace, producing 2400 lbs/inches of thrust.
If I have screwed this up, I stand to be corrected....but I Still find this interesting!!
R.
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Leo Todeschini
Industry Professional
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| Posted: Sat 10 Jan, 2009 2:22 pm Post subject: |
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| Quote: | Gary Teuscher wrote
One interesting thing though - It seems while a roughly 72 inch longbow may have a 30" draw, with crossbows the ratio seems different, draw length being closer to 50% of prod length with many "accurate" replicas, or in the 67% range with modern crossbows, though these are a different bird.
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A steel bow of 25" will have a draw of between 4.5" to 5" in modern times, and this allows for all those lovely margins of error that engineers, spring makers and law courts love so much, in medieval times when you didn't get sued this could be a bit more, maybe to 6" but beyond this and you will get deformation or breakage, wooden bows tended to be longer so you could get higher thrust distances as the poundages of the bows tended to be lower. Short bows and short draws were still used though on wooden prodds and as a rough guide a wooden longbow, rather than flat bows, needs to be 2.5 times its draw length. A heavy wooden prodd with an even thicker section to length ratio would have a smaller draw to length ratio than longbows, so that it seems they were similar to steel bows in ratio at about 5:1 or 20%.
Medieval crossbows tended to have between 4 and 6.5" of draw so the thrust distances are quite low which is why even an 800lb cross bow is not that much better at distance than a 140lb long bow due to this disparity in thrust distances.
The thrust load is also not constant, as at 1" draw the bow may be taking 100lb, at 2" 200lb, at 3" 350lb at 4" 650lb and at 4.5" 820lb (for example) and at brace the draw load may be 50lb so the thrust/distance relationship is a curve rather than a constant and that curve starts at a value rather than 0 and in the case of a steel bow stacks heavily and in the case of a longbow, I believe that it stacks heavily and then starts to drop off at high draw distances. As a seperate point the start of the thrust line at a value is why a bow with a brace height and so a minimum thrust, performs far better than a bow with no brace height and so zero minimum thrust.
So I would guess that a 100lb long bow starting at 7" brace, drawing to 32" = 25" draw going from 10lb at brace to 100lb at full, assuming no stacking this has an average thrust of 55lb so 55 x 25 = 1375lbs of total thrust
A 200lb crossbow with a 5" draw, starting at 30lb and assuming some stacking (I have done this to stop my brain melting trying to work it out and because a steel bow stacks far more than a wooden one) goes to 200lb so the average draw is 115lb, but up this a bit because of the stacking so lets say 135lb = 135 x 5 = 675lbs total thrust
A 800lb cross bow with a 5" draw starting at 80lb and again assuming some bias for stacking so the average draw is 440lb, but up this a bit due to the stacking, so lets say 520lb = 520 x 5 = 2600lbs total thrust (approx 350-400yds range)
For interest if we do a 140lb long bow at 32" we get 10lb at brace, 140 at full, over 25" so and average of 75 x 25" = 1875lbs total thrust. (approx 250yds range)
For interest: a 100lb bow with a 32" draw and no brace = 32" x 50lb average (says logic), though (no sums here) instinct tells you that there will be virtually no load for the first 10" say and a low load for next 6", at which it will probably be flexed at or around the brace height flex so only at say 16" will it start to do any real work, so assuming that the load is 10lbs at 16" this leaves 16" of a decent load upto full draw, but of course because you started with no brace height, you may pull back 32", but the limb flex will equate to a draw length 7" less than this and so a draw load of say 70lb rather than the 100lb expected. So 16 x average of 40lbs = 640lbs. By the way if you try this as a real experiment you will kill your bow. So all guess work, but if you didn't brace your longbow and shot it with a slack string it would deliver half the thrust than if you brace it.
This is the sort of stuff that always goes through my head when I am making something, but in all honesty this is the first time I have put any kind of framework or numbers to it and I feel that this actually reflects pretty well what we know about these weapons.
DISCLAIMER - I am neither a mathmetician nor have I spent a great deal of time working this through and if it is right well thats great, if it is wrong, well A. I look like a dunce and B. it should encourage some more debate.
Tod
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Randall Moffett
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| Posted: Sun 11 Jan, 2009 10:59 am Post subject: |
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I think I must have gotten lost somewhere. 4-6" draw as in where the crossbow string is when fully extended from at rest right? Maybe it is the headache I have and I missed it. This is not the bow's back to cocked distance right?
Most medieval bolts I have seen range from 10-14 or a bit more inches so if you have 4-7 inches draw this should work from the string at rest to cocked placement. Anything shorter would leave much of the bolt before the bow and this would just waste more energy of the crossbow.
The crossbows to bows power is rather complicated. If a crossbow is 400 or 600 pounds at the ready it is 400-600 pounds draw. The poundage is well more than an average warbow's and than it can achieve. The draw length being 1/3-2/3 that of a bow is important as it limits the potential energy.Since the bolts can be 2 times plus the weight of a warbow arrow another factor involved which I think is missing from the equation as thrust is important but projectile mass is as well. In the end I have never seen an adaquate study done on the subject so it is all rather in the air. Liebel has done some interesting work on crossbows and their energy but I do not know if anyone has done the same for bows really.
I'd highly recommend this book by Liebel.
http://www.royalarmouriesshop.org/cgi-bin/sh0...98#aR00598
RPM
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Gary Teuscher
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| Posted: Sun 11 Jan, 2009 12:52 pm Post subject: |
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| Quote: | | A steel bow of 25" will have a draw of between 4.5" to 5" in modern times, |
I think here you mean late middle ages - One crossbow I've run some numbers with is the Excalibur - It has less than a 30" prod, and 22" draw length. Most other modern crossbows have similar ratios.
Leo Todeschini wrote:
| Quote: | | Short bows and short draws were still used though on wooden prodds and as a rough guide a wooden longbow, rather than flat bows, needs to be 2.5 times its draw length. A heavy wooden prodd with an even thicker section to length ratio would have a smaller draw to length ratio than longbows, so that it seems they were similar to steel bows in ratio at about 5:1 or 20%. |
I have no idea upon what bows you are basing this. The "Longbow" numbers are pretty well on, but there is very litttle information to base a wood or composite crossbow from say the 11-13th century on. Many seem to apply the numbers from steel crossbows, however I would not think this is correct, as the lack of poundage in the draw compared to a steel bow would make a woooden crossbow of steel crossbow dimenions useless for the most part.
| Quote: | | A steel bow of 25" will have a draw of between 4.5" to 5" in modern times, and this allows for all those lovely margins of error that engineers, spring makers and law courts love so much, in medieval times when you didn't get sued this could be a bit more, maybe to 6" but beyond this and you will get deformation or breakage, wooden bows tended to be longer so you could get higher thrust distances as the poundages of the bows tended to be lower. |
You are using the "powerstroke" numbers here, not the draw length. Power stroke draw length minus brace length. And once again, these are numbers for a renaissance era steel crossbow.
QAs far as the calcualtion for stored energy, I have spoken with a fair amount of replica manufacturers as well as some bow "experts" (OK< I guess I mean emailed, not spoke). The formula that seems to work the best for stored energy is to take the draw length of the bow, say with a 30" 100lb draw bow it starts at 3000 foot inches, or with a 1000lb 9 inch draw steel crossbow 9000 foot inches. For the Brace, you take the brace length times the pound pull, subtract this from the draw weight. Assuming an 8" brace for the longbow, it's at about 2787, the Crossbow with a 4" (has a very short 5" powerstroke) brace is at 7222 foot inches. As these are the calcualtions for a spring, you then halve them and of course divide by 12 for foot pounds, giving 116 foot pounds for the bow, 301 for the crossbow. Then there is a somewhat arbitrary "efficiency factor" to throw in, probably in the range of .70 for steel, .75 or so for wood. Composites might be in the .80 or so range, modern .90 or so. I have seen others use lower efficiency numbers, but they do not halve the base foot pounds. Steel while it can handle higher draw lengths better than the others, seems to be a bit less efficient.
So we have a longbow with 87 foot pounds or so. The crossbow would be at about 211 foot pounds. Now throw a 1000 grain or so projectile on the longbow, a 2000-2500 grain projectile on the crossbow, and do the ballistic numbers. They should work out pretty well.
One thing this does not factor is the maximum velocity of the limbs (a bow is a spring, and there is a limit to how fast the limbs can move, even propelling no mass), and with this is the ssue that very long limbs lower this maximum velocity as they have a further distance to move. I've also heard that overly thick limbs (like that of a crossbow) can couse this same problem. But as long as you are using appropriate arrow/bolt weights, this should not be a huge factor.
But say you try to loose a 500 grain bolt with the heavy crossbow - my guess the numbers may show a terminal velocity of over 400 fps, but in actuality it would likley at the most be 250 fps.
Modern bow materials make a better spring and are "quicker", but they still have the same issues, maybe more so due to the fact that the emphasis on modern bows is velocity, not launching a large grain projectile. For instance that Excalibur I mentioned is a 22" draw, 5.5" brace, 225 pound draw weapon. It uses a 360 grain projectile! While the ballisitcs show the bolt should travel at 450 or so FPS, it's real velocity is in the 350 fps range. Even modern bow material don't move fast enough to impart their full stored kinetic energy to the projectile.
My guess it's not a magical "terminal velocity" that the bows have, but more of a bleed off - loosing a 2500 grain projectile might be ideal for the above 1000 lb draw crossbow, but with a 1500 grain projectile you will gain some velocity, but not that indicated directly by ballistics. A 500 grain projectile, if it does not become damaged in the launch, won't probaby gain much velocity over the 1500 grain projectile, and certainly won't be close to numbers indicated on the foot pounds vs. grains ballisitcs.
| Quote: | | In the end I have never seen an adaquate study done on the subject so it is all rather in the air. |
Absolutely agree here, Randall. I'm pretty comfortable with the base ballistic numbers for the most part, while I think they are accurate to a point the "efficiency factors" of bows is where I feel the most potential error would be. But as to the terminal velocity and velocity bleed off numbers, I really don't have a real good idea.
I've read that estimates put exit velocities on middle ages bows and crossbows in the 180-200 fps range, no higher than 250 fps. My guess is these numbers come from a combination of purported ranges of weapons, and tests on either found bow and/or as accurate as possible replicas from the same.
Due to this IMO the bleed off would occur in most bows starting at that 180-200 fps range or so, of course more "springy" materials would have a higher number before bleed off.
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Leo Todeschini
Industry Professional
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| Posted: Sun 11 Jan, 2009 2:21 pm Post subject: |
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| Quote: | Gary Teuscher wrote
I think here you mean late middle ages - One crossbow I've run some numbers with is the Excalibur - It has less than a 30" prod, and 22" draw length. Most other modern crossbows have similar ratios.
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No I really meant modern as in today. If you make a steel bow to roughly medieval proportions, it will be pretty much the same, in medieval times you could stress a bow a bot more because nobody was going to sue yuo if it broke, nowadays you have to have a factor of safety and litigation and so you have to reduce the draw a bit to be safe so a 25" prod made to medieval specs can be drawn between 4.5 and 5" approx in 2009. I don't know excalibur, but I would bet it has either an aluminium prodd or has spent a long time getting acquinted with Finite Element Analysis as nothing like these numbers were ever seen in medieval times.
| Quote: | Gary Teuscher wrote
I have no idea upon what bows you are basing this. The "Longbow" numbers are pretty well on, but there is very litttle information to base a wood or composite crossbow from say the 11-13th century on. Many seem to apply the numbers from steel crossbows, however I would not think this is correct, as the lack of poundage in the draw compared to a steel bow would make a woooden crossbow of steel crossbow dimenions useless for the most part.
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Look at any manuscript or painting or artefact in a museum of a normal hand bow size ie prodd at 60-70cm and stock at 70-85 cm and estimate the draw length from brace position to nut you will struggle to find anything over 6"
| Quote: | Randall Moffett wrote
I think I must have gotten lost somewhere. 4-6" draw as in where the crossbow string is when fully extended from at rest right? Maybe it is the headache I have and I missed it. This is not the bow's back to cocked distance right?
Most medieval bolts I have seen range from 10-14 or a bit more inches so if you have 4-7 inches draw this should work from the string at rest to cocked placement. Anything shorter would leave much of the bolt before the bow and this would just waste more energy of the crossbow.
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Roughly a medieval steel bow will have a height at brace of about 3.5" and from there a draw of about 5" so from the bow about 8.5", the bow will be set back from the front of the stock by say 1/2", so a total groove length of about 9" Off the top of my head a composite one is about the same, but with possibly a bit more of both.
10" for a medieval bolt would be really short for a full sized bow and much more likely to be 14" and so yes a deal of the bolt is sticking out the front and again this is so often shown in manuscripts and also accounts for why spring clips were a great invention. As regards wasting energy, I guess because you feel that you are propelling more bolt than you need to. Well perhaps that is so, but I can tell you making a bolt stable in launch and flight is no easy task and with a reasonable weight pile and a 1/2" shaft or so 12-14" is a good area to work in, below this and the bolts tend to buck horribly on launch and lose loads of speed before straightening up.
As regards fps of projectiles? - I just make the things and I got as far as I can with the maths in my last post so this is my reply as an English reencactor, when a visitor asks how fast they shoot, my stock line is 'Faster than a Frenchman can run' apologies to any French readers out there.
Tod
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Gary Teuscher
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| Posted: Sun 11 Jan, 2009 4:35 pm Post subject: |
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| Quote: | | Look at any manuscript or painting or artefact in a museum of a normal hand bow size ie prodd at 60-70cm and stock at 70-85 cm and estimate the draw length from brace position to nut you will struggle to find anything over 6" |
Are there any of these drawings for the earlier period crossbows? From what I have seen, most are illustrations of bows from the late 14th century on, not much on earlier period bows. and the earlier period drawings I have seen I'd hate to base any type of scaling upon.
| Quote: | | but I would bet it has either an aluminium prodd or has spent a long time getting acquinted with Finite Element Analysis as nothing like these numbers were ever seen in medieval times. |
Modern Composite Construction.
http://excaliburcrossbow.com/demo/listings.php?category_id=41
IIRX it is the exocet version.
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Randall Moffett
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| Posted: Sun 11 Jan, 2009 10:54 pm Post subject: |
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Todd,
The lengths are ones I have measured so I am fairly sure about them. They were all from English museums as well. The thing about having extra mass in front of the bow is that it will be dead weight. It does not add to the potential energy and in fact only detracts from it therefore is mechanically weaker than if it were behind it, at least this is how it was explained to me by someone that understands physics. I am not sure why having it before the bow would add or decrease stability over behind it but you are right that some pictures show some of the bolt before the bow but typically from what I have seem it is just the head which is more acceptable than 2-3 inches of bolt. Since there are a great number not showing this I am not sure how much trust I could put either way in artwork and artistic convention for bolt length and hang over. At least during the mid 14th we know often how long the bolts were as we have a great deal of purchase records for the tower and other royal castles. Seems 1/3-2/3 a yard is fairly common bolt length. That said the only thing we know of the crossbows is they were one foot and two foot one which does not really tell you the distance from the crossbow nut to the bow or front of the crossbow.
RPM
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Leo Todeschini
Industry Professional
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| Posted: Mon 12 Jan, 2009 1:12 am Post subject: |
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| Quote: | Randall Moffett wrote
but you are right that some pictures show some of the bolt before the bow but typically from what I have seem it is just the head which is more acceptable than 2-3 inches of bolt. Since there are a great number not showing this I am not sure how much trust I could put either way in artwork and artistic convention for bolt length and hang over. At least during the mid 14th we know often how long the bolts were as we have a great deal of purchase records for the tower and other royal castles. Seems 1/3-2/3 a yard is fairly common bolt length. That said the only thing we know of the crossbows is they were one foot and two foot one which does not really tell you the distance from the crossbow nut to the bow or front of the crossbow.
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Artistic convention is always tricky and to take one picture and read everyting into that is wrong, but when a great many show something then I think that there is a story there.
Bolts at 1/3 yd is 12" and 2/3" is 24" and 24" is for sure a big bow so I would guess some sort of rampart/siege/pedastal bow, but 12" is a reasonable bolt size.
If we get Joseph Alms book out and I am estimating where the brace is and a few other factors.
Page 34 large windlass, stock 90cm picture shows stock at 110mm and brace to nut at 22mm, so 8.2 = factor so draw = 180mm ie 7"
page 35 composite hunting bow stock length 75, picture shows stock at 70mm, factor = 10.7, brace to nut = 13mm so 139mm or 5.5"
page 36 composite hunting bow stock length 80cm, picture shows stock at 135mm, factor = 5.9, brace to nut = 22mm so draw = 130mm or 51/4" bolt length = 354mm or 14"
page 30 composite stock 85cm, picture shows stock at 76mm so factor is 11.2, brace to nut is 14mm so draw is 156mm or 6.25"
page 58 steel hunting bow, stock length 75cm, picture shows stock at 72 so factor is 10.4, brace to nut is 12mm so draw is 125mm or 5"
page 66 windlass, stock length 90cm, picture shows stock (from butt to bow) at 10cm, so factor is 9, brace to nut is 17mm so draw is 153mm or 6"
The real fighting stuff, page 15, 14th C yew prodd bow, can't measure it, but clearly a short draw.
Wallace collection (I have some pictures I can't post for copyright reasons)
A1032 steel bavarian hunting bow, draw is 132mm or 5 1/4"
A1033, steel 15thC? war bow, draw is 99mm or 4"
A1034 composite, draw 140 or 5.5"
A1035 steel, draw 135 or 5.5"
A1037 steel, draw 140mm or 5.5"
There are only two windlass bows in this rough survey and they both come out at longer draws but this is to be expected as the bows are generally longer nock to nock.
Going back, these show that normal goats foot or cranequin bows seem to have a draw between about 5-6" on the whole and with a bolt length of 12-14" this will leave a few inches sticking out the front, though a couple of the Wallace bows have a very deep belly and so long brace distance and this would help the bolts. That aside a brace of say 3.5" and a draw of say 5" and bow set in of 1/2" still leaves at 3-5" of bolt sticking out the front. Windlass bows have a draw 1 to 1.5" longer.
| Quote: | | Randall Moffett wrote I am not sure why having it before the bow would add or decrease stability |
Sorry I just meant that a short bolt is not very stable so that you need 12-14" to make a decent bolt and so whether they liked it or not that is what they had to deal with.
| Quote: | Leo Todeschini wrote
Look at any manuscript or painting or artefact in a museum of a normal hand bow size ie prodd at 60-70cm and stock at 70-85 cm and estimate the draw length from brace position to nut you will struggle to find anything over 6" |
Ok I take it back, windlass bows seem to be heading to 7" and I am not so familiar with those so apologies, cranequin and goats foot are between 5-6" it appears or a touch more - apologies.
Regards
Tod
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Leo Todeschini
Industry Professional
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| Posted: Mon 12 Jan, 2009 1:30 am Post subject: |
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While we are involved in this rather interesting talk that is taking up far too much of my time............
| Quote: | Randall Moffett wrote
Since the bolts can be 2 times plus the weight of a warbow arrow |
Have you got any stats, as I have not, but when I look at cross bow bolts with quarel piles for example and wooden fletches, as I handled at the VandA early last year (but didn't weigh them) I was not struck by weight.
They were about 14", ash shaft and ash fletches, steel quarrel head and about 1/2" barrelled, so basically pretty much the stats of a longbow war arrow (as I understand them), but 18" of ash shaft less, the pile was pretty comparable so how double the weight? Equally the bolts shown in the Alm book look pretty much, size wise, as I have described the VandA ones.
Thoughts?
Tod
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Randall Moffett
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| Posted: Mon 12 Jan, 2009 2:35 am Post subject: |
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Tod,
I have some weights somewhere but would need to look in my old notes which are not on computer but on paper. Several of the bolts at their thickest were around 3/4-7/8 inch though which would greatly increase their weight. Let me see if I can find them. Some might have been sent back stateside as I likely will be moving and have already started moving some of the stuff there. I think just like warbow arrows there will be a great deal of weight difference between crossbows, especially with the massive variety of crossbow types so my statement would be a bit over simplistic, especially comparing some heavy arrows to lighter bolts. I honestly think there were a wide variety of bolt types just like that of arrows. I do think the 24 inch bolts were indeed from a siege crossbow device. The biggest problem is likely the fact so little has been researched in sufficient numbers to get a decent idea. Same goes with armour thickness and various other arms and armour topics though so I appreciate the hands on and research you have put into this. More curious than anything regarding crossbows. I think we talked about this earlier that bolts likely were made for specific crossbows and that might be the explanation for various length and weight bolts.
RPM
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Richard Hare
Location: Alberta, canada Joined: 15 Mar 2008
Posts: 135
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| Posted: Mon 12 Jan, 2009 7:31 am Post subject: |
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Re. the weight of bolts;
Sir RPG in his book "The Crossbow" states that in his experience bolts of 2-2 1/2 ounces flew the best, and barreled down from 5/8" shaft.
This appears to be the middel ground by what I can gather....and of course depends on the poundage of the bow.
Still, it gives a good indication of bolt weight, as he had vast experience with original pieces.
This weight is on a par with warbow arrows, as far as I know. ...but I don't know very far!
R.
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Gary Teuscher
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| Posted: Mon 12 Jan, 2009 7:52 am Post subject: |
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Hey Tod, I think we are talking about two different ways of measuring "draw length", whcih is probably some of the confusion and why I thought your draw lengths were ridiculously low 
Lets take a selfbow for instance. maybe it's 72" long, a 30" draw, a 7-9" brace would be normal. While the draw is ststed at 30", this is basically nock of the arrow when fully drawn to center of stave. So the brace length is included in total draw length.
For example, that longbow above if measured as you are meansuring the draw length of a crossbow has a 38" draw (assuming 8" average for brace). This would require a 40" arrow or so if using this way of measuring.
As Mary Rose arrows are more in the 30" or so range, add an arrow head and it's 32" - comfortable for a 30" draw as I am calculating (there were many arrows of 27-29" as well - which would imply either there were two types of arrows used on the same bows, or the draw lenghs of the bows varied, which might make a 27-30" draw length more appropriate for a longbow.
Using this way of measuring, a crossbow with a 4" brace and a 6" what you call draw length and I refer to as powerstroke, we have a 10" draw, and those numbers make sense to me for steel bows.
Wood or composites have more flex, and with lighter poundages compared to the top end 1000# draws could have had higher draw lengths and longer prods.
Someone explained to me basically a bow of any type is a spring. It tries to go back to straight when drawn. Now by being strung, it is already under tension.
When draw to full draw length, it is x pounds at x length of draw. The last few inches of brace to not help propel it of course, as it cannot go back to straight. But of course the draw becomes progressively harder as it is drawn back further, so the real force you lose from brace inches is calculated as total draw (brace + powerstroke) x pounds, then divide the brace by the total draw (on the crossbow above total draw is 10", brace is 4, so 40% or 400 pounds if 1000 pound draw), then multiply by brace length. This "brace factor" is subtracted from total poundage.
So for that 1000 pound bow, 10" total draw, 6" powerstroke, 4" brace, we have a base of 10,000 foot inches, -1600 foot inches from the brace factor. This is 8400 foot inches, divide by 2 as a spring only transmits 50% of stored energy, we are at 4200 foot inches.
The part I have the greatest misgivings about is the factor based on construction material, in this case .7 for a steel bow. While this is a definite factor, I'm thinking the numbers are partially guesswork. And of course even the same material can have a different degree of efficiency, but it's the best I have and probably fairly accurate.
But that puts us at 2940 foot inches, or 245 foot pounds. Put a 2000 grain or so projectile in, and the ballisitcs say about 235 fps exit velocity. That might be a tad bit light for projectile weight, maybe a bit of the "bleed off" I mentioned occurring.
The bleed off is something even modern bows, self or cross have. I looked at the numbers on a 100 pound 30" draw longbow with an 8" brace made of modern materials. It used a 600 grain arrow, light IMO when you look at that it stores energy better than a english longbow due to construction, and has similar draw weight/length. Ran the ballistics, and it should have been loosing an arrow about 50 fps faster.
How this bleed off works I have no idea. Since the common thought by many bow experts have a 180-200 fps average for middle ages bows and a max of 250 fps, my guess it occurs at these velocities where the "spring" of the stave is unable to move fast enough to loose the arrow at the velocity it's stored energy suggests. Modern bows it would make sense have a higher bleed off number, as they are designed to loose light projectiles at hgh velocites, but even they have their limits as shown by the numbers on the modern longbow I mentioned and the excalibur crossbow.
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Gary Teuscher
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| Posted: Mon 12 Jan, 2009 9:23 am Post subject: |
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| Quote: | Sir RPG in his book "The Crossbow" states that in his experience bolts of 2-2 1/2 ounces flew the best, and barreled down from 5/8" shaft.
This appears to be the middel ground by what I can gather....and of course depends on the poundage of the bow.
Still, it gives a good indication of bolt weight, as he had vast experience with original pieces.
This weight is on a par with warbow arrows, as far as I know. ...but I don't know very far! |
Thats in the 900-1100 or so grain range. Seems a bit light to me, but it depends upon bow KE of course. Seem light for a 1000# steel bow, but seems a lot closer for something more in the 300-500 or so range.
Any idea of what draw weight bows he was using for this?
And the other thing, a higher FPS will make the arrow "fly better", though it may not maximize the bows KE potential.
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Randall Moffett
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| Posted: Mon 12 Jan, 2009 10:19 am Post subject: |
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I do not have the book on hand but my guess is he is talking about averages for some type of flight bolts. I agree as an average it seems very low if considering all types of bolts. I cannot find my notes so I might see if I cannot find someone to measure them again.
RPM
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Leo Todeschini
Industry Professional
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| Posted: Mon 12 Jan, 2009 12:23 pm Post subject: |
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well I for one am certainly out technicalled here, so I will dip out of any FPS etc discussions etc, as much as anything because much as topic interests me I have a backlog to deal with and however much I tell my wife that this is research, she will only accept it so far.
That aside, Gary you are right, I have muddied the waters. For some reason I always refer to a prodd having a draw length from the positon of the string at brace to the final position whereas I refer to a longbow having a draw from the bow to the final string position. Not sure if that is just me or that it is a common convention either way it does explain some confusion and well spotted.
I am going to look into this bolt weight business as I have a 800lb on the way and am curious.
Tod
PS When are you off Randall, before March?
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