As mentioned in other sections, the crossbow's earliest widespread use was probably in China, during the 3rd century B.C. or earlier. On single-shot crossbows, one type of latch/trigger mechanism was a very clever precision bronze casting with three moving parts and no springs. Surviving wooden stocks end in a type of pistol grip. Their laths were either of composite construction or made from multiple bamboo slats bound like an automobile leaf spring.
Another type of crossbow used by the Chinese since at least 210 B.C. was a repeating design with a gravity-fed box magazine! The magazine was situated above the bolt track. When the lever at the rear of the crossbow was first raised and then lowered, the box moved forward, caught the string in a wooden recess and drew it to full cock, dropped a bolt into the track and released the string. These crossbows were neither powerful nor accurate, but they could launch a bolt every second or two until the magazine emptied. Poison was usually smeared on the points to increase their lethality.
In the manner of handbows of the same period, early Western crossbows featured wood laths and long power strokes (compared to later examples.) The most common latch mechanism was a rotating nut of bone, ivory or antler. To achieve greater power, massive "composite" laths made from sinew, horn or baleen, and wood came into use; these were shorter and much stiffer than earlier wood laths. As draw weights increased, new methods and devices for spanning had to be employed, which included the cord and pulley, belt claw, "goat's foot", bending lever, cranequin and windlass. Steel laths later provided even greater power. Spanning devices made reloading a slow process compared with hand bows. Crossbows were more useful for hunting and siegecraft than in open battle, where their slow rate of fire was a serious handicap.
Features usually found on military and hunting crossbows of the 14th to 16th centuries include a fairly plain, straight stock, a sinew bridle binding the lath to the stock, a cylindrical latch nut and a long iron trigger. It would have either a simple rest or a grooved track to guide the bolt; a stirrup, cocking ring, or cocking lugs would be present depending on which cocking device was to be used. The stock could be held in the same manner as a firearm, or rested on top of the shoulder and the trigger manipulated with the thumb. The bolt's point usually served as the front sight when aiming.
Sporting crossbows of the 17th to 19th centuries were used for formal target competitions and hunting. Aperture sights and set triggers were usually present on target crossbows. Bow irons and similar fittings for securing the lath replaced the sinew bridle. Bullet crossbows became popular for small game hunting and informal target shooting, using a double bowstring with a leather pouch to launch a lead, clay or stone balls. The barreled crossbow or slurbow also shot round balls, using a conventional bowstring and a tubular barrel. The range of features found on sporting crossbows of this period is better seen than described; the books by Payne-Gallwey, Stevens, Bilson, Heath and Paterson listed earlier include illustrations.
The crossbow was (and in some cases still is) a popular hunting weapon in Southern Asia and parts of Africa. The construction used in both areas is similar in that a relatively weak wood lath is mounted to a straight stock with a bolt track. The latch is simply a notch in the stock; the trigger is a peg that is pushes the string out of the notch from below. On some examples, the stock is horizontally split for part of its length, so that pressing the two halves together pushes the trigger peg upward. Since bolts from these crossbows have little kinetic energy, they are invariably poisoned. Bolts are slivers of hardwood or bamboo, usually with simple leaf fletchings.
Crossbows of medieval and renaissance design were very inefficient devices. Modern tests indicate that armor-piercing bolts, while heavier than war arrows, achieved about the same velocity (130-40 fps) from a 700 lb. draw crossbow as an arrow did from a 80 lb. draw longbow. The initial velocity imparted to a crossbow bolt is governed by the velocity of the bow tips as the bolt and string part company. Despite their heavy draw weights, medieval laths were too massive to accelerate rapidly. This was made worse by short draw lengths, which reduced the time available for the tips to accelerate. In addition, the massive bowstrings required for such heavy draw weights robbed energy from the bolt. Balanced against these faults is the higher ballistic coefficient of the short, heavy crossbow bolt, as compared with an arrow. This meant that crossbows often could shoot further and hit harder than hand bows.
Modern hunting crossbows are engineered to launch 400+ grain bolts at initial velocities in excess of 200 fps, with draw weights of about 150 lbs. This provides ample kinetic energy for big game hunting with a far lower draw weight than would be the case with a medieval crossbow of similar power. A longer power stroke coupled with a less massive fiberglass lath makes the difference.
Modern target competition with the crossbow falls into two quite different classes. In international 10 meter competition, shooters use a crossbow that marries the elaborate stock and sights of a smallbore target rifle with a short-draw steel lath. The draw weight is well over 100 lbs., so cocking is performed using a long steel bending lever. Bolts are about 6" long and made of unfletched wood; their metal points are threaded like a coarse woodscrew to facilitate removal from the lead plates used as backstops. Field crossbow competition takes place at 30, 40 and 50 yards, with bolts similar to those used in hunting. Because lighter-drawing field target crossbows are shot over greater distances than in international 10 meter, their stocks and sights must be suited to a broader range of adjustments. Field target crossbows are usually hand-made, often home-made by their users.
When making bolts for a factory-made crossbow, try to obtain at least one factory bolt to measure its length and weight. Bolt length does not seem to be very critical on tracked crossbows, but you would do well to make your bolts be the same weight or slightly heavier. Lighter bolts will fly faster, but may shorten the working life of the lath. Bolt weight is a careful compromise, usually determined at the factory through destructive testing, and it would be most cost-effective to accept their recommendation.
Bowstrings for crossbows, though shorter, are also made in the same manner as for handbows. Obtain the length and number of strands by inspecting a factory string and make yours to the same specifications. If you don't wish to make your own strings, you should still obtain a serving jig and a spool of serving thread for re-serving the centers whenever necessary. Abrasion from track and latch contact wears through crossbow center servings very quickly. Depending on the design of your weapon and the waxes and lubricants used to reduce friction, the center serving may begin to fray after only a few dozen shots; by re-serving the center as needed, a crossbow bowstring should last thousands of shots.
For those wishing to make their own crossbows, the National Crossbowmen of the USA offers plans and a partly fabricated lath for sale at a nominal cost. See the Archery Organizations FAQ for their address.
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I made a crossbow about six years ago with reasonable success and learnt a bit in the process. I'll discuss what I know of these parts of the crossbow.
Prod:- I made mine from annealed spring steel that was 6mm thick. Following a plan that I had I tapered it linearly using an angle grinder. This required a lot of metal to be removed and took a long time. It was bent to shape approximately like a recurve and then hardened and tempered at the place I originally bought the steel from. It was fairly large, being 36 inches across, but it turned out to be about the weight that I desired.
Since then, I've put some more thought into it and probably the easiest way of making a bow prod is to get an old leaf spring from a car wrecker. A leaf spring consists of a number of leaves, all but one of which should have their taper already formed. They are already hardened and tempered, so all that would be required would be to angle grind the grooves in the ends for attaching the string.
Bolts:- I bought these from a shop. Obviously there is no knock. The other big difference was that they had large vanes with a higher twist rate than normal arrows.
General construction:- Make it out of a good solid seasoned hardwood. I used high density softwood but in the end it cracked behind the prod. The slot for the bolt vane should be wide enough to accomodate the high twist of the vane. I glued laminex to the body to provide a smooth (low-friction was the idea) surface for the string to move over. The most important thing to consider is the design of the release mechanism. This should have as low a moment of inertia as possible. My own bow had a relatively high moment of inertia, so what tended to happen was, when fired, the string would jump over the top of the release mechanism and also over the top of the bolt. It would take me a moment to realise that I hadn't lost sight of the bolt in flight, but that it was still sitting in the crossbow. The one thing that saved the bow from being a total disaster was that the release mechanism had two prongs to hold the string (rather than just a single post), and the bolt fit between them so that it could rest directly against the string. Whenever I pushed the bolt back properly against the string it didn't misfire.
That crossbow was quite powerful and accurate. It was fun to make and also fun to use. Since its demise I bought an old compound bow and enjoy that probably more because it requires a bit more skill in aiming and releasing.
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Mal Goris
mgoris@rp.csiro.au
Last-modified: 24 November 2004
FAQ - Crossbows
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