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Leo Todeschini
Industry Professional
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 Posted: Tue 23 Apr, 2013 2:40 pm Post subject: |
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Jean-Henri Chandler wrote | Quote: | | Can we review why the replica bows are so much slower than modern crossbows? I have a cheap 150 lb hunting crossbow which makes 285 fps with a 20 -30 gram arrow. |
A few reasons.
the higher mass of steel limbs compared to carbon or glass fibre means more energy is used to move the limbs in steel bows rather than the bolt.
The traditional section profile of steel bows is say 4 or 5 :1 but you can make a much faster bow with a steel section of 2:1; but they didn't.
The string weight is far higher using traditional materials.
The bolt weight is higher using traditional materials.
But probably the most impact is due to draw length. Your bow probably draws 14" or maybe more, a medieval hunting bow will draw 5" so you have 3 times the distance in which to impart energy to the bolt.
In a nutshell your bow has light, computer designed limbs using a light string, accelerating a light bolt over a long distance.
Medieval bows have none of these speed advantages. They do however have soul........
Tod
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Bartek Strojek
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| Posted: Wed 24 Apr, 2013 3:44 am Post subject: |
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| Harri Kyllönen wrote: |
Is it a compound bow? The pulleys make quite a difference.
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Pulleys make huge difference in allowing to gather huge energy in the limb rather effortlessly.
As far as velocity goes though,, they are actually a problem, since they're heavy, clunky, and in pretty much most designs located at the very end of working limb.
But as mentioned, modern materials, engineering and designs are anyway perfectly sufficient to attain very fast bows.
I would suspect that the fact that most modern working parts of limbs are very short also makes the difference - they just cover all the distance from full span to rest very quickly.
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Jean Henri Chandler
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| Posted: Wed 24 Apr, 2013 9:58 pm Post subject: |
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| Leo Todeschini wrote: | Jean-Henri Chandler wrote | Quote: | | Can we review why the replica bows are so much slower than modern crossbows? I have a cheap 150 lb hunting crossbow which makes 285 fps with a 20 -30 gram arrow. |
But probably the most impact is due to draw length. Your bow probably draws 14" or maybe more, a medieval hunting bow will draw 5" so you have 3 times the distance in which to impart energy to the bolt.
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I agree this is the most important factor: but I do not buy this argument. They had bows, obviously, which drew back further than 14". And yet they chose to have this short powerstroke on the crossbows, particularly the crossbows made later in the era, when the technology was supposed to be at it's peak. Why? Crudity? Stupidity? I don't buy that.
Most period accounts credit these weapons with being at least as effective, and dangerous to their targets, as the most powerful self bows of the time (certainly, all the primary source accounts I'm reading from German, Czech and Polish sources), They overwhelmingly chose to use the crossbow for their militias, until it was very gradually (over the course of a century and a half) replaced by the firearm, in spite of it's high expense and difficulty of use.
Why would they make this design decision, to make a shorter powerstroke and a shorter bolt than they had to?
Why did they choose to have heavier bolts (since they obviously had light flight arrows available such as were so often used by the composite recurve bows of the steppe nomads)
Jean
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Leo Todeschini
Industry Professional
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| Posted: Thu 25 Apr, 2013 12:33 am Post subject: |
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If you make a war bow of say 1000lb, the bow will be around 16 x 50mm at the centre tapering to around 9 x 30 at the tips and it will draw to around 7". Beyond this the bow will bend or break; as a prospective owner you may decide that it is safer to limit this to 6.5"
if you want to increase the draw length you need to make the bow longer and this increases weight, causes efficiency issues and increases cumbersomeness.
Of course a draw of 14" is possible, but I would guess the bow would need to be around 1.2-1.3 meters long.
One of my many things to do is make a windlass bow on a stand with a large bow of around 1.4m length, and this will say what they draw to. Maybe one day
Tod
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Benjamin H. Abbott
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| Posted: Thu 25 Apr, 2013 6:34 am Post subject: |
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| Jean Henri Chandler wrote: | | I agree this is the most important factor: but I do not buy this argument. They had bows, obviously, which drew back further than 14". And yet they chose to have this short powerstroke on the crossbows, particularly the crossbows made later in the era, when the technology was supposed to be at it's peak. Why? Crudity? Stupidity? I don't buy that. |
How are you going to span a heavy crossbow with 14-inch powerstroke? Doing so with a windlass or cranequin would make the device larger and take longer. Doing so with a goat's-foots lever would require an extra long one, if you could do it at all. Modern crossbows use limb materials and span mechanism unavailable in medieval and Renaissance times. They also feature much lower draw weights than heavy crossbows from that era.
| Quote: | | Most period accounts credit these weapons with being at least as effective, and dangerous to their targets, as the most powerful self bows of the time |
The numbers from Ralph Payne-Gallwey's test - assuming they're reliable - stand consistent with the notion that heavy crossbows shot farther and hit harder than self bows. That was an extremely large crossbow, but it suggests few if any archers could match the power of 1000lb crossbow.
| Quote: | | Why did they choose to have heavier bolts (since they obviously had light flight arrows available such as were so often used by the composite recurve bows of the steppe nomads) |
You can't just any kind of arrow with any kind of bow. A heavy crossbow would destroy a light Turkish arrow; they're also too light to shoot profitably from English warbows. Furthermore, light arrows aren't necessarily optimal; Manchu bows are designed for and employed extremely heavy arrows. It depends what you're trying to accomplish.
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Jean Henri Chandler
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| Posted: Thu 25 Apr, 2013 8:31 am Post subject: |
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I think this is still missing the point. There are several logical inconsistencies.
If it was more efficient to have a lighter bow, because the steel is too heavy or can't be spanned at full length, then why wouldn't they just use a lighter bow that you can pull back 20" or 30" to get to that sweet spot? How much more efficient is the prod on my modern crossbow than say, a top quality of a Turkish or Mongol recurve bow ? Half again? Twice? Three times? Then I think they could emulate it and find the sweet spot for a 280 fps crossbow at 200 or 300 or 400 lbs.
For that matter, why not just lay down a recurve composite bow sideways and mount it on a tiller? You could put a very powerful recurve at the upper end of the spectrum for those weapons, say 160 lbs draw, spanned to the full length of the power stroke, and you could probably span that with a simple foot stirrup or at the worst, a belt hook. Then you would still have all the base properties of the crossbow: ability to hold it in readiness indefinitely, ability to lean it on a support before the shot and so on, and you could shoot a 30 gram arrow without damaging it in any way.
And yet instead they went for much heavier bows, heavier, shorter, thicker bolts, and a short power stroke. This is where they found the sweet spot after centuries of experimentation and practical use.
I think we must be missing something, unless I am still missing somethig logically.
J
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Leo Todeschini
Industry Professional
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| Posted: Thu 25 Apr, 2013 12:37 pm Post subject: |
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Jean Henri Chandler wrote | Quote: | think this is still missing the point. There are several logical inconsistencies.
If it was more efficient to have a lighter bow, because the steel is too heavy or can't be spanned at full length, then why wouldn't they just use a lighter bow that you can pull back 20" or 30" to get to that sweet spot? How much more efficient is the prod on my modern crossbow than say, a top quality of a Turkish or Mongol recurve bow ? Half again? Twice? Three times? Then I think they could emulate it and find the sweet spot for a 280 fps crossbow at 200 or 300 or 400 lbs.
For that matter, why not just lay down a recurve composite bow sideways and mount it on a tiller? You could put a very powerful recurve at the upper end of the spectrum for those weapons, say 160 lbs draw, spanned to the full length of the power stroke, and you could probably span that with a simple foot stirrup or at the worst, a belt hook. Then you would still have all the base properties of the crossbow: ability to hold it in readiness indefinitely, ability to lean it on a support before the shot and so on, and you could shoot a 30 gram arrow without damaging it in any way.
And yet instead they went for much heavier bows, heavier, shorter, thicker bolts, and a short power stroke. This is where they found the sweet spot after centuries of experimentation and practical use.
I think we must be missing something, unless I am still missing somethig logically.
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I am afraid that I think you are missing something. Horses for courses.
You are logically correct in that a bow as you describe would shoot very well, but that does not make it an effective weapon.
As you describe, the bow would be around 1.4 meters wide and with a .9m (30") draw plus the trigger mech and some distance to allow the trigger to work ergonimically for the hand, around 1.2m long. and would kill effectively at the same range as a comparable longbow, say 250 yds. A medieval style bow would be around 0.7m wide and 0.8 meters long and would also kill at 250yds.
This would make a standard medieval bow easier and more flexible to deploy with a greater concentration of firepower and far more manoeuvrable and with a similar killing range, but only at the expense of having higher draw weights. It may not be the most efficient but it worked well enough if you had high draw weights.
As a modern example, the US army has both 50cal Browning machine guns and M16 assault rifles. The 50 cal shoots further, hits harder and delivers more rounds per minute (I think) so why do they bother issuing troops with M16's? Simply put they fit the bill of what the average soldier needs far better. Sometimes the 50 cal is what is needed and so they are deployed; similarly there were windlass operated rampart bows and the massive German composite bows that I forget the name of when the situation required it, but that situation was not on the field.
I suspect it was not so much a sweet spot and an adequate spot.
Tod
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Benjamin H. Abbott
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| Posted: Thu 25 Apr, 2013 3:12 pm Post subject: |
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| Jean Henri Chandler wrote: | If it was more efficient to have a lighter bow, because the steel is too heavy or can't be spanned at full length, then why wouldn't they just use a lighter bow that you can pull back 20" or 30" to get to that sweet spot? How much more efficient is the prod on my modern crossbow than say, a top quality of a Turkish or Mongol recurve bow ? Half again? Twice? Three times? Then I think they could emulate it and find the sweet spot for a 280 fps crossbow at 200 or 300 or 400 lbs.
For that matter, why not just lay down a recurve composite bow sideways and mount it on a tiller? You could put a very powerful recurve at the upper end of the spectrum for those weapons, say 160 lbs draw, spanned to the full length of the power stroke, and you could probably span that with a simple foot stirrup or at the worst, a belt hook. Then you would still have all the base properties of the crossbow: ability to hold it in readiness indefinitely, ability to lean it on a support before the shot and so on, and you could shoot a 30 gram arrow without damaging it in any way. |
The Han Chinese arguably did exactly what you suggest. For example, check out this visual interpretation. It resembles a modern fiberglass crossbow. Calculations here suggest an initial velocity of 66 m/s and an initial kinetic energy of 152 J with a 70g bolt. The most powerful modern crossbow manages 207 J at 122 m/s with a 27.5g bolt.
Of course, the Han numbers are mostly speculation. Assuming a 350lb draw weight and 16-inch power stroke, I would expect rather higher performance. We need an in-depth study of the crossbow akin to The Great Warbow.
| Quote: | | And yet instead they went for much heavier bows, heavier, shorter, thicker bolts, and a short power stroke. This is where they found the sweet spot after centuries of experimentation and practical use. |
With a cranequin or windlass, this design has the advantage of requiring minimal physical effort to span the bow. Again, various spanning aids make longer draws difficult. Han soldiers spanned theirs by hand and had to be strong. I've additionally read that the relatively bulky European trigger mechanism interfered with long draw lengths.
As far as numbers go, a 300lb steel crossbow with an 8.5inch powerstroke here manages 88 J with an 81g bolt at 46.5 m/s and 51 J with a 26g bolt at 63 m/s. This stands roughly consistent with Payne-Gallwey's bow in terms of both energy and velocity. Remember, according to these numbers, heavy field crossbows drawing around 1000lbs would deliver 200+ J up close with heavy bolts and easily 160-180 J with lighter bolts. Few English warbow arrows would deliver even 150 J. I find these calculations in line with the historical evidence. Get much past 250 J and you'd have crossbows capable of piercing decent breastplates.
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Jean Henri Chandler
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| Posted: Fri 26 Apr, 2013 3:12 pm Post subject: |
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| Leo Todeschini wrote: |
I am afraid that I think you are missing something. Horses for courses.
You are logically correct in that a bow as you describe would shoot very well, but that does not make it an effective weapon.
As you describe, the bow would be around 1.4 meters wide |
I'm not an expert on recurves but I've definitely seen recurve composites a lot smaller than that, though it is somewhat beside the point.
| Quote: | | and with a .9m (30") draw plus the trigger mech and some distance to allow the trigger to work ergonimically for the hand, around 1.2m long. and would kill effectively at the same range as a comparable longbow, say 250 yds. A medieval style bow would be around 0.7m wide and 0.8 meters long and would also kill at 250yds. |
As you and I both know very well, medieval crossbows came in a wide range of sizes
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This would make a standard medieval bow easier and more flexible to deploy with a greater concentration of firepower and far more manoeuvrable and with a similar killing range, but only at the expense of having higher draw weights. It may not be the most efficient but it worked well enough if you had high draw weights. |
And yet far clumsier weapons were used in the field, for a long time right alongside crossbows
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As a modern example, the US army has both 50cal Browning machine guns and M16 assault rifles. The 50 cal shoots further, hits harder and delivers more rounds per minute (I think) so why do they bother issuing troops with M16's? |
Actually, 50 caliber M2 shoots at a much lower rate of fire than an M16 / M4 (et al) and was never intended for the same role. A more apt comparison perhaps would be the various .30 caliber (.308, .30-06) machine guns and rifles which the smaller M-16 replaced, but the .30 caliber weapons actually returned (both medium / light machine guns and assault / sniper rifle types) due to experiences in Afghanistan. Both are carried in the field.
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Simply put they fit the bill of what the average soldier needs far better. |
This doesn't make logical sense. These weapons were extremely expensive, tricky to use and bulky. Even an average crossbow in the 15th century cost the same as 3 tuns of beer or two swords. This is not even considering the price of a more powerful weapon or the spanners like a cranequin which were extremely expensive, often works of art in their own right. So the idea that it's a cheap weapon suitible for the rabble doesn't wash for me.
If a simple composite bow would do the same job, why not train with those instead? Why go the trouble and expense of making a steel prod and complicated machines? If the argument is that crossbows could be used by just anybody (as one often hears on The History Channel or BBC), that does not wash either - towns spent an enormous amount of money training crossbowmen for marksmanship. They built ranges and held contests for hunderds of shooters with huge amounts of prize money. I can cite examples if necessary. If they spent those kind of resources training people to use their crossbows, they could train people to use any self bow as well (as the English did with their longbow). But instead they overwhelmingly chose to use the crossbow.
| Quote: | | Sometimes the 50 cal is what is needed and so they are deployed; similarly there were windlass operated rampart bows and the massive German composite bows that I forget the name of when the situation required it, but that situation was not on the field. |
And yet these weapons were not light bows like the one I described (a composite recurve laid on it's side), they were also in the 1000 - 1200 range, and also had relatively short power strokes (certainly nothing like 30 inches).
And it's incorrect to suggest that these weapons had to be the way they were (with small, heavy steel or composite prods and short power strokes shooting small, thick bolts) because anything like the size of a real bow would be too bulky to use, because they had the Greek weapon called the Gastraphetes which was in effect, exactly that. This weapon was known to medieval people.
http://en.wikipedia.org/wiki/Gastraphetes
But for some reason they used the other.
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I suspect it was not so much a sweet spot and an adequate spot.
Tod |
I think we missed the spot here mate.
J
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Benjamin H. Abbott
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| Posted: Fri 26 Apr, 2013 5:44 pm Post subject: |
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I suspect towns chose crossbows because they're ideal weapons for defending walls. Because you don't have to worry about holding a string back while you aim, crossbows tend to be more accurate than bows. Han-style crossbows would be bulkier and exhausting comparing with European designs involving a cranequin or windlass. If my extrapolations are correct, such crossbows drawing about a half-ton would outperform a 150lb English warbow in range, armor penetration, and accuracy all while requiring minimal physical effort to span. They only disadvantage comes in speed of shooting, but that doesn't matter as much when you're defending a wall. Crossbows likely thrived on the castle-rich Continent for the same reasons. In addition to environmental factors, once a certain weapon acquires cultural importance and social infrastructure, it's not likely to be quickly or easily discarded. The weight of evidence indicates that steel crossbows, horn-&-sinew composite crossbows, yew or laminated bamboo longbows, and horn-&-sinew composite bow each served their purpose well and lacked the ability to displace one another.
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Jean Henri Chandler
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| Posted: Sat 27 Apr, 2013 11:27 pm Post subject: |
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I agree but the question really is why don't modern replicas show this level of performance?
J
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Randall Moffett
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| Posted: Sun 28 Apr, 2013 5:38 am Post subject: |
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Jean,
That is not 100% true. Jean Lebiel in his studies on crossbows and springalds got with his examples and models pretty similar results to what Ben is stating.
RPM
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Jean Henri Chandler
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| Posted: Sun 28 Apr, 2013 7:51 am Post subject: |
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Was this the fairly recent one with the composite prod weapons?
J
Books and games on Medieval Europe Codex Integrum
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Benjamin H. Abbott
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| Posted: Sun 28 Apr, 2013 1:29 pm Post subject: |
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When theorizing crossbows, it's worth noting that some evidence indicates that they proved effective even when they couldn't match - much less exceed - the performance of heavy bows. This paper details how Spanish crossbows designed for spanning with a goat's-foot level recovered from shipwrecks draw 250-500lbs with powerstrokes of 5-6 inches. I can't imagine that even the heaviest of these crossbows would manage much more than 100 J with heavy bolt; the lighter ones might have only delivered 40-50 J. Regardless, this would have been enough to kill. Interestingly, these energy numbers roughly parallel the energy figures for Turkish composite bows in the 80-110lb range common for cavalry use. Even with relatively low range, velocity, and penetrating power, the crossbow yet retains high accuracy, convenience, and ability to shoot from cover. In addition to colonial ventures in the Americas, such light goats-foot crossbows saw use aboard ships and by cavalry.
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Randall Moffett
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| Posted: Sun 28 Apr, 2013 2:29 pm Post subject: |
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Jean,
Let me double check my copy is at work. Iam fairly sure her uses steel prod for 1 foot crossbows.
Ben,
Good point. There are crossbows of many types and uses likely. That said some have thought the Padre Island crossbow for hunting.... which is another possible use and complication as many remaining xbows and bolts maybe intended for hunting and other non-military uses....
RPM
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Benjamin H. Abbott
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| Posted: Tue 30 Apr, 2013 2:05 pm Post subject: |
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I do have some trouble seeing the lighter Padre Island crossbows as serious weapons. Any projectile with initial kinetic energy below 60 J, though still dangerous, strikes me as unsuitable for the battlefield. Cavalry composite bow don't seem to have gotten much under 80lbs, and even an 80lb composite with a light arrow should manage 60-70 J. But without solid tests, we don't know exactly what these devices were capable of.
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Trevor Borden
Location: Phoenix az Joined: 05 Sep 2013
Posts: 4
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| Posted: Thu 05 Sep, 2013 8:20 pm Post subject: question on center serving |
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Im new the this form, the arbalest that I have built is like the one in the YouTube video that is built with leaf springs
mine is 415 lbs 20 in pull the prod is 3 springs one @ 47 in a backer @ 28 in and a third @ 14 in.
I know this is nowhere close to the quality or even remotely authenticpre but is just fun and being jy first build its a start. I do plan on building a more authentic one that should be around 56 in long and 1/2 in thick @ around 15 in draw it should be around 1600 lb draw but im not ready dfor that build.
now for my question
im having problems with the center serving on my string breaking, im using b50 string, with nylon serving (as suggested by string makers) I just made 2 new strings 1 made with leather thread and topped with nylon for the serving and 1 made with nylon topped with a drycon 100 they are being pre-stretched .
can anyone offer any suggestions on how to prevent the ripping off my serving
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Leo Todeschini
Industry Professional
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| Posted: Thu 05 Sep, 2013 11:53 pm Post subject: |
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Hi Trevor - Welcome.
You are right that it looks like fun and perhaps we can get a bit serious and forget much of this is for fun.
I suspect that a 56" bow will need to be much thicker than 1/2" if you want to hit 1600lb
I use 1/2' stock for around 600-800lb bows, albeit with a much shorter powerstroke, but also a much shorter length. I too want to build a bow of around the size you have given, but I am going to use a bow of around 35x90x 1500 mm. Not sure what the poundage will be but under 3000 I think.
I am not familiar with the nylons/dacrons/polyesters etc, but if the serving is wearing rapidly it is most likely to be because the string is rubbing too heavily on the stock. Try changing the angle of the bow to relive the downward pressure on the stock. This will also give a you few more percent power at exit. If this does not solve the problem, it is likely to be a rough trigger system, so give that a smooth and polish. Also try polishing the top of the stock and rounding off any sharp corners and giving it a wax.
Tod
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Trevor Borden
Location: Phoenix az Joined: 05 Sep 2013
Posts: 4
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| Posted: Fri 06 Sep, 2013 7:22 am Post subject: |
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Tod
thank you for the info.
I have rounded off the stock did a polishe, but the trigger is very crud, I know that I need to invest time in a better trigger system.
As for the down pressure on the stock it dose seem heavy, I have an other sit of springs that I can work with, I will be trying to tapper that set and set the ends of the prod high so i can get a better height with out a steep angle on the prod.
after that set is made I plan to shoot both sets to see if there is a loss in draw lbs or an increase of speed. I will post my findings to answer the question posted earlier about this design.
As far as the 1600 lb planned draw, I have bought a set of leaf springs for $40 usa pt#ssaa10 the maker of this leaf spring climes a minimum of 300 lb pre inch deflection so by that info if the prod moves 4 in with .09 stretch and a draw (in theory) of string of 14 to 18 in, I should be at 1200 to 1500 lbs but it does need to be made and tested
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Leo Todeschini
Industry Professional
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| Posted: Fri 06 Sep, 2013 10:34 am Post subject: |
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Trevor Borden wrote | Quote: | | As far as the 1600 lb planned draw, I have bought a set of leaf springs for $40 usa pt#ssaa10 the maker of this leaf spring climes a minimum of 300 lb pre inch deflection so by that info if the prod moves 4 in with .09 stretch and a draw (in theory) of string of 14 to 18 in, I should be at 1200 to 1500 lbs but it does need to be made and tested |
That may be so, but he has specified that for a standard leaf spring application, when the string is attached and you are pulling tips through vectors the sums all change........
Tod
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www.instagram.com/todsworkshop
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