Compound bow
From Free net encyclopedia
Image:Compound Bow full.jpg A compound bow is a modern bow that has pulleys or cams at the end of each limb through which the bow string passes. As the bow is pulled back (drawn) the pulleys or cams turn which, in turn, reduce the amount of force needed to completely draw the bow. They are little affected by changes of temperature and humidity and give superior accuracy, velocity, and distance in comparison to the traditional longbow. Unlike traditional bows that are usually made of wood or wood laminated with other materials, compound bows are usually made of aluminium and composite materials. They were first developed and patented by Holless Wilbur Allen in the USA in the 1960s and have become increasingly popular.
A composite bow is made from different materials laminated together, usually applied under tension. Modern composite bows use laminated wood, plastic, fiberglass and carbon foam. The use of these materials means that the bow is not affected much by changes in temperature and humidity and this gives the bow superior accuracy, velocity, and distance in comparison to bows made out of natural materials such as the classic longbow. With recurve bows, the shape curves back on itself. It is this design that gives the bows tremendous power compared to their size.
With a traditional single string bow as the string is pulled back the tension increases, so the bow must be aimed and released quickly (else the archer's arm will tire, reducing accuracy). On release the string rapidly accelerates to its fastest and then decelerates for the rest of the string's return to stationary.
The compound bow uses very stiff limbs that can operate with greater energy efficiency than less stiff limbs. An arrangement of pulleys and cables gives the archer the necessary leverage to bend the limbs.
The pulley system usually includes some rubber-covered blocks that act as draw-stops. This provides a solid "wall" that the archer can draw against. These draw stops can be adjusted to suit the archer's natural draw-length.
There are mechanical advantages to pulleys:
- As the bow is drawn, the draw weight increases to a peak and then "lets off". The let-off is usually between 40% and 60% of the peak weight, but some concept bows have a let-off of 99% and even 100%. This enables the archer to hold the bow fully drawn and take more time to aim.
- The pulleys enable the archer to draw a bow with a much higher draw weight (due to the reduction of the draw weight at break) than they could manage with a conventional single stringed bow (there are very few people alive today who could shoot accurately with a single string using the draw weights of the longbows found on the Mary Rose). (note: Two separate tests of the longbows recovered from the Mary Rose - one by duplication and one by mathematics indicate the average draw of the Mary Rose longbows was 105 lbs.)
- The string continues to accelerate from the release to rest so imparting more power (and hence speed) to the arrow.
- The "break" of the draw weight helps the archer achieve a consistent anchor point (the amount of draw the arrow is held at prior to release), further increasing accuracy.
Archers in modern archery competitions usually use a release aid to hold the string steady. This attaches to the bowstring at a point and permits the archer to release the string with a pull of a trigger. Compound archery (unlike recurve and traditional archery) usually permits the use of a magnifying sight.
Due to the greater forces the compound bow places on the arrow, only arrows designed for a compound bow should be used with a compound bow. Wooden arrows intended only for a recurve bow may break when fired from a compound bow, possibly driving the broken shaft into the archer's arm. However, most modern arrows are made from materials capable of withstanding the forces of a compound bow. Image:Compound Bow Pulley.jpg
