Post by Icarus on Feb 13, 2008 19:37:35 GMT -5
How Crossbows Work
by Tracy V. Wilson
The crossbow, a weapon popular with Wookiees, vampire slayers and some modern hunters, looks like a cross between a bow and a rifle. Like a bow, it uses a fast-moving string to launch projectiles, but it also has a trigger and a stock, like a rifle. Based on these similarities, some people may think that the inventor of the crossbow took pieces from a rifle and attached them to a bow.
But crossbows have been around much longer than guns have. Archeologists have found crossbows in 2,500-year-old Chinese graves, and some historians believe that they existed in China as early as 2000 B.C. Crossbows also appeared in the Mediterranean region by the fourth century A.D. Roman military writer Flavius Vegetius Renatus mentions them in a text written in 385 AD, and they were prevalent in Europe throughout the Middle Ages.
In both Chinese and Mediterranean societies, early crossbows had the same basic parts. They were essentially bows mounted horizontally to wooden tillers, or stocks. When shot, an arrow, or bolt, traveled down a groove or through a notch in the tiller. Both styles also incorporated devices to make drawing the string easier. One common device was a stirrup at the front of the crossbow. A person could brace the crossbow with his foot while drawing the string with both hands or with a hook attached to his belt.
Both types of crossbows employed a trigger to release the cocked string. Roman crossbows used a rotating nut held in place by a lever, and Chinese crossbows used a system of hooks and levers. Since the two types of crossbows have such different firing mechanisms, historians believe that the two cultures developed them independently of each other.
It may seem hard to believe that two different cultures could invent roughly the same mechanical device. But the crossbow is really a logical improvement of the bow, which has existed since prehistory. Early bows were simply pieces of wood with their ends connected by a bowstring. Improvements to the basic bow, such as more effective bow shapes and better materials, made them more powerful and efficient. But firing even the best bow quickly and accurately still requires strength, agility, speed and a fair amount of skill. Becoming an expert archer with a longbow can take years.
Crossbows, on the other hand, don't require the same physical strength or training that ordinary bows do:
• When using a traditional bow, an archer must draw, aim and shoot in fairly quick succession. The longer it takes the archer to aim, the more fatigued his arm will become, and the less accurate his shot will be. Being able to aim and shoot quickly requires lots of practice. But if the archer uses a crossbow, he can draw the string -- or cock the crossbow -- and leave it that way as long as he needs to.
• An archer who isn't very tall can't use a very long bow. If he isn't very strong, he also can't draw the string on a powerful bow. In other words, a person's size and upper body strength limit the size and strength of the bow he can use. With a crossbow, though, an archer can use his strongest muscle groups -- the ones found in his thighs and buttocks -- to draw the string. A crossbowman can even use tools like levers or cranks to supplement his strength. This means that a crossbowman can use a more powerful weapon than a traditional archer with the same amount of strength.
Early crossbows did have a few disadvantages, though. An archer could load, aim and shoot a bow in around six seconds, but a crossbowman required nearly a minute to do the same task. Crossbows also had far more moving parts than plain longbows. But crossbows gave armies the option of arming recruits with ranged weapons regardless of their level of skill at archery.
In general, military crossbows could be very fast or very powerful, but not both. For example, in the 14th century, European crossbow makers began making weapons from steel and incorporating crannequins in their design. A crannequin was a toothed wheel attached to a crank. When a soldier turned the crank, the wheel moved a toothed rod, which pulled the bowstring and cocked the crossbow. With a crannequin, a soldier could cock crossbows that he would not have the strength to cock on his own. However, the soldier had to remove the crannequin before each shot, and re-cocking and reloading a crossbow with a crannequin could take several minutes.
At the other end of the spectrum, some Chinese crossbows were built for speed. Designers added cocking levers and magazines full of bolts to the basic crossbow. Bolts would automatically fall from the magazine when the string was cocked. Often, the crossbow then automatically released the bolt. With this type of crossbow, a soldier could fire several shots per second, although the bolts did not travel as fast or do as much damage as a bolt shot from a steel crossbow with a crannequin.
But regardless of whether it's built for strength or speed, a crossbow's basic function is basically the same as a bow's. We'll look at the factors that affect a bow's power and speed in the next section.
science.howstuffworks.com/crossbow.htm
by Tracy V. Wilson
The crossbow, a weapon popular with Wookiees, vampire slayers and some modern hunters, looks like a cross between a bow and a rifle. Like a bow, it uses a fast-moving string to launch projectiles, but it also has a trigger and a stock, like a rifle. Based on these similarities, some people may think that the inventor of the crossbow took pieces from a rifle and attached them to a bow.
But crossbows have been around much longer than guns have. Archeologists have found crossbows in 2,500-year-old Chinese graves, and some historians believe that they existed in China as early as 2000 B.C. Crossbows also appeared in the Mediterranean region by the fourth century A.D. Roman military writer Flavius Vegetius Renatus mentions them in a text written in 385 AD, and they were prevalent in Europe throughout the Middle Ages.
In both Chinese and Mediterranean societies, early crossbows had the same basic parts. They were essentially bows mounted horizontally to wooden tillers, or stocks. When shot, an arrow, or bolt, traveled down a groove or through a notch in the tiller. Both styles also incorporated devices to make drawing the string easier. One common device was a stirrup at the front of the crossbow. A person could brace the crossbow with his foot while drawing the string with both hands or with a hook attached to his belt.
Both types of crossbows employed a trigger to release the cocked string. Roman crossbows used a rotating nut held in place by a lever, and Chinese crossbows used a system of hooks and levers. Since the two types of crossbows have such different firing mechanisms, historians believe that the two cultures developed them independently of each other.
It may seem hard to believe that two different cultures could invent roughly the same mechanical device. But the crossbow is really a logical improvement of the bow, which has existed since prehistory. Early bows were simply pieces of wood with their ends connected by a bowstring. Improvements to the basic bow, such as more effective bow shapes and better materials, made them more powerful and efficient. But firing even the best bow quickly and accurately still requires strength, agility, speed and a fair amount of skill. Becoming an expert archer with a longbow can take years.
Crossbows, on the other hand, don't require the same physical strength or training that ordinary bows do:
• When using a traditional bow, an archer must draw, aim and shoot in fairly quick succession. The longer it takes the archer to aim, the more fatigued his arm will become, and the less accurate his shot will be. Being able to aim and shoot quickly requires lots of practice. But if the archer uses a crossbow, he can draw the string -- or cock the crossbow -- and leave it that way as long as he needs to.
• An archer who isn't very tall can't use a very long bow. If he isn't very strong, he also can't draw the string on a powerful bow. In other words, a person's size and upper body strength limit the size and strength of the bow he can use. With a crossbow, though, an archer can use his strongest muscle groups -- the ones found in his thighs and buttocks -- to draw the string. A crossbowman can even use tools like levers or cranks to supplement his strength. This means that a crossbowman can use a more powerful weapon than a traditional archer with the same amount of strength.
Early crossbows did have a few disadvantages, though. An archer could load, aim and shoot a bow in around six seconds, but a crossbowman required nearly a minute to do the same task. Crossbows also had far more moving parts than plain longbows. But crossbows gave armies the option of arming recruits with ranged weapons regardless of their level of skill at archery.
In general, military crossbows could be very fast or very powerful, but not both. For example, in the 14th century, European crossbow makers began making weapons from steel and incorporating crannequins in their design. A crannequin was a toothed wheel attached to a crank. When a soldier turned the crank, the wheel moved a toothed rod, which pulled the bowstring and cocked the crossbow. With a crannequin, a soldier could cock crossbows that he would not have the strength to cock on his own. However, the soldier had to remove the crannequin before each shot, and re-cocking and reloading a crossbow with a crannequin could take several minutes.
At the other end of the spectrum, some Chinese crossbows were built for speed. Designers added cocking levers and magazines full of bolts to the basic crossbow. Bolts would automatically fall from the magazine when the string was cocked. Often, the crossbow then automatically released the bolt. With this type of crossbow, a soldier could fire several shots per second, although the bolts did not travel as fast or do as much damage as a bolt shot from a steel crossbow with a crannequin.
But regardless of whether it's built for strength or speed, a crossbow's basic function is basically the same as a bow's. We'll look at the factors that affect a bow's power and speed in the next section.
science.howstuffworks.com/crossbow.htm