Posts: 2 Location: Croatia
Sun 11 Dec, 2011 1:56 pm
When comparing bows it is not enough to look just at arrow weights and energy storage curves. Material of which it is made has a huge influence, when you shoot a 70 gram arrow out of a warbow that warbow also has to move itself, which is at most somewhere under two pounds, most commonly between 25-30 ounces, while steel prod is considerably heavier, provided it stores the same amount of energy which is an entirely different topic in itself.
This poundage to limb mass ratio is critical for performance, Adam Karpowicz's book on turkish bows has a great info on this. According to him, bows made of wood have this ratio more favourable than composites up to about 100 pounds of weight, after that composites become bettter while selfbows suffer, that's the reason for the mentioned longbow's sweet spot. After you reach a certain weight you need disproportionaly more limb mass, whoever has made a classic backyard practice selfbow knows that difference between 20 and 40 pounder is few hairbreadths of wood while heavier ones are much more forgiving during tillering. You also need a longer bow because limbs are now thicker and cannot stand the same amount of deformation but then lever effect comes into play and you need more thickness again and so forth and so forth. That's why dense growing yew was favoured, it can pack more weight in the same space. Note difference between weight (poundage) and physical mass.
It is much repeated statement that composites are lighter than wood bows, actually 6 foot composite would weight more than a 6 foot wooden bow, since composites are made of tissues that sink in water for example. The thing is they can be made shorter and put under greater strain to offset this mass disadvantage. Now imagine by how much iron is denser than horn, it would need proportionately higher draw weight to become truly effective in this context. So, sweet spot for steel bows should be determined and that would answer whether they were made for energy effectiveness or knocking out, as Leo said.
To be honest I'm not sure that this same kind of wood-composite swapping is possible with composite-steel, because composites, though denser, are more elastic than wood, while steel has to be even less strained than wood.
Crossbows are by definition made to be held at draw with no effort, but there is a problem to this. Materials like wood, horn and sinew creep when under stress, and longer they are under it the weaker they become. If steel can stand it for much longer it would be a good explanation for it's use, it would be great for mounted soldiers closing in on their targets and picking spots, or for sniping over built defences or any kind of stalking as in hunting. Of course crossbows were used for that before invention of steel but it's introduction could have raised their use to another level, comparable to that of first wheellocks. Or they did it not to increase range but to launch heavier bolts to already common distances. But that opens a new problem when considering sporting hunting bows with steel prods. You dont need armor piercing projectiles when hunting deer. Relative ease of construction and maintenance probably held a big role in it's development.
So there it is, I'm sorry if this is all confusing but it's complicated subject and it's late here, I'm not that good on historical sources but I've made a handful of wooden bows and read a lot on their construction theory so wanted to say something about this part of the story which to me seemed ignored except for this
Quote: |
Crossbow arms are heavier, but the longbow arms travel a longer distance. |
again I will quote Adam Karpowicz who I believe knocked this whole topic on the head in his book. Bow is a spring, and springs oscillate in a frequency which is always the same and depends on spring length and thickness. Just like a tuning fork. If you clamp a bow by its grip in a vice and pull-release a limb it will go back and forth. It will go back and forth same amount in a a given time and that's it's frequency. The further you pull it away from the rest position actually the faster the recovery so it can meet that frequency. When designing a bow you want to pull it as far as possible without breaking or suffering permanent deformation. With that said if-ever-achieved 300 fps mark set above will not be a consequence of poundage, as Jean Thibodeau said, but of frequency to which a certain prod is "tuned" and how far it is stressed
Sime Ivic