The Modern Crossbow

by Terry Stewart

  Practically all crossbows are constructed with the pivot pins passing transversely through the stock from side to side, the ends being flush with the stock and retained in position by cover plates of hardwood or metal screwed to the stock. If it is required to examine the trigger group, these plates are unscrewed and the pivot pins pushed out. An alternative and neater method is to have the trigger group and pivots contained in a light metal four-sided box to slip into the latch recess. This shows no pins or cover plates and is very neat and strong, keeping the latch recess without pivot holes through the body of the stock.

  The draw weight of your bow has only slight effect on trigger pull off if the correct angles are obtained between trigger and latch contact faces and the pivots are correctly placed to give proper leverage. It should be as easy to discharge a 100 lb. draw weight as it would be a 50 lb. draw weight.

  A number of ingenious release mechanisms have been devised over many centuries, though the original 'nut' type, or free revolving rolling block latch was not improved on for 400 years, and then only by a rather more complex mechanism of a double trigger, three springs and two moving parts. The free revolving or rolling block latch was held in a fitted metal casing, this casing was a close fit on both sides and bottom of the latch. A retaining pin was inserted through the centre of the latch to prevent the latch falling out of its casing. This pin was purely a retaining pin and not a pivot. The latch pivoted in its metal casing and no strain whatsoever was placed on the pin. This type of release was used during centuries of crossbow construction, and was capable of holding with safety and discharging with a smooth accurate release, powerful battle crossbows drawing up to half a ton.

  For simplicity, accuracy and safety, the free revolving latch, as described, is probably the best for any crossbow in use today whether for hunting or target use. The most common cause of a dry release on an otherwise well designed and constructed latch, particularly among the heavy draw weight hunting crossbows, is due to the bending of the latch pivot. No such occurrence is possible utillslng a free revolving latch as no pressure whatsoever is placed on the pin.

  The majority of modern crossbows use some variation or adaptation of the latch described. The basic design is retained regarding the actual method of release, etc., and the following variations are common to simplify construction and to create a self-cocking latch. The latch pivots on a steel pin of suitable strength, and a cocking projection is added to the rear of the latch. The long medieval 'trigger' is replaced with a conventional trigger.

  The improved release mechanism devised during the 16th century, incorporating a double trigger, was most advantageous from the safety point of view, and for its exceptionally fast release, it was impossible for this latch to release the bowstring accidentally, as it would not discharge until the safety trigger was set, this being done immediately prior to release. Upon pressing the release trigger the locking device struck the latch lever a sharp blow knocking it free of the latch and thus allowed the latch to drop, releasing the bowstring. This latch was cocked in the conventional manner by the bowstring, and once cocked would not release until the rear trigger was set and the front trigger pressed.

  The simplest, safest and most accurate releases for the amateur to construct are the two following types; both adaptations of medieval releases, and both, for ease in making, of fabricated construction. For the purpose of readily identifying the mechanisms I propose to refer to one as the stonebow release and the other as the nut release.

  The stonebow release is a modern adaptation of the type used in medieval stonebows. This was a type of catapult crossbow with double string and pouch, designed to discharge small round pebbles at birds and other small game.

  The nut release is a modern adaptation of the basic medieval nut release used in crossbows discharging bolts.

  Both types of release may be cut from solid steel if desired, though if the amateur feels this is a little beyond his capabilities, my method of fabricated latch construction is as efficient and strong as one of solid construction. Both the stonebow and nut release are suitable for any modern crossbow up to 100 lb. draw weight. Over 100 lb. draw weight I recommend the use of the rolling block release as used in medieval crossbows with the retention of a modern trigger in place of the trip lever device.

  The mechanism shown on the opposite page (p.12.jpg) is a modern adaptation of the release used in medieval stonebows. This type of mechanism has an extremely high safety factor due to the 'hook' construction of the trigger contact point. This 'hook' makes it virtually impossible for the latch to drop accidentally, as any forward pull on the latch engages the hook more firmly into the sear pin. This can only be released by pressing the trigger; and so pivoting the hook forward and off the pin. The cocked and released positions, as shown in the diagram, illustrates this admirably.

  A helical spring attached to the lower front sectIon of Part B and the centre front section of the trigger, Part C, does the work of trigger spring and also acts as a foolproof non-return spring, preventing the latch bouncing up again to the cocked position after release; a fault found in some mechanisms. An efficient latch of this type may be easily constructed using the fabricated method. Three pieces of light alloy make up the mechanism. Two pieces shaped as Part A and one only piece shaped as Part B. All three parts drilled through and riveted together complete the latch. The sear pin on which the trigger hook holds must be of steel and, when in position through the rear of the latch, perfectly horizontal. The sear pin should be cut long enough to allow riveting into position.

  The Trigger, Part C, is made entirely of steel with the hook surface, where it will bear on the latch sear pin, case hardened. It is a simple matter to make this latch with a 'second pressure' on the trigger, as is found on many first class rifles. To do so all that is required is to pass the edge of a file across the underside of the trigger hook at one eighth of an inch in from the hook tip. Use enough pressure only to cause a narrow indentatIon. This slight indentation is quite distinctly felt through the trigger as a 'second pressure', indicating that the trigger has but a further eighth of an inch to travel before release. To return the trigger to first pressure, and safe, is accomplished by pushing the trigger forward. The indentation across the trigger hook face must not be too deep, or wide, or it will defeat its own purpose. Use only the edge of the file, going no deeper than one sixteenth of an inch. Do remember that if you decide to have a second pressure on your trigger, the trigger hook must be case hardened after the indentation has been filed.

  We come now to the nut release as used in one variation or another in most modern crossbows in use today.

  This mechanism may also be easily and efficiently constructed using the fabricated method. A latch of this type is quite simply made by the following method. Three pieces of flat bar metal strip are used, two pieces of light alloy (A) and one of mild steel (B). After cutting to shape and drilling through, the three pieces are laminated together with two small steel rivets. The centre laminate of steel (B), later case hardened, forms the contact point of the latch at the trigger (C). The two alloy pieces, riveted each side of the steel laminate, act as latch forks to allow the butt end of the bolt through to contact the bowstring. The spring for this latch is made from a piece of clock or old lock spring screwed to the stock inside the back of the trigger recess. When bent over the spring presses against the rear of the trigger to provide resistance when the trigger is pressed. The general principles of good latch and trigger construction as outlined earlier in this chapter are of paramount importance and should be carefully observed. When this type of latch is used it is most important to ensure that the correct angles are obtained between the latch and trigger faces and that the pivots are precisely positioned. If the trigger is heavy and jerky to discharge, particularly during the last part of the pull, it is invariably due to the latch being forced up and back when the trigger is pressed. The trigger face pushing the latch up throws the entire draw weight of the bow on the trigger causing
a heavy pull off.

  A fault of this nature in the release mechanism is due to incorrect angles between latch and trigger faces, which may be caused by badly placed pivots or incorrect contact angles between latch and trigger.

  To locate the pivot positions and contact angles accurately it is advisable to cut a stiff cardboard template of the stock over where the mechanism should fit. Mark the pivot holes on the template and try the latch and trigger against it, in both cocked and released positions, so as to determine accurate positioning. When satisfied, mark the pivot positions, using the template on the stock when drilling the pivot holes.

  The latch cover is in the form of a hardwood or metal bridge passing directly over the latch and screwed firmly into the stock behind the latch recess. This cover allows the string to pass under, protecting the trigger and latch from dust and rain. This cover is also used to attach a bolt clip to grip the rear end of the bolt prior to release, enabling the crossbow to be pointed vertically up or down without the bolt falling out.

  Bows

  We come now to the source of power that shoots the bolt from the stock, the bow. Provided attention has been paid to the bow's fitting, angle to the stock, and height in stock, the power and speed with which a crossbow bow will throw a particular bolt depends on how fast the limbs return from the bent to normal positions. The speed at which they return depends on bow design, stiffness of the limbs and how far the limbs are bent. The choice of bows is numerous as regards material. You may have self wood, composite, solid fibre glass, spring steel or alloy. The choice is yours. Bear in mind that you will seldom have the required cast and power in a self wood bow for hunting purposes. A self wood bow will generally shoot sweetly but not fast or with a very flat trajectory, and will also tend to 'tire' over a day's shooting, and may not be carried at full draw for long periods. A composite bow is probably the best, even though a little expensive. Steel bows are good if they have no tendency to kick, but are much heavier in the stock than other types.

  Solid fibre glass is used in the U.S.A. with some success, but apparently is not readily obtainable in Britain. I find solid fibre glass bows to be robust, reliable, showing good cast, and are of moderate price. These bows must inevitably be purchased from commercial sources as their construction is beyond the capabilities of the amateur craftsman.

  Alloy bows are light in weight, easy to construct or cheap to purchase, and have good cast for their weight. This is contrary to an opinion once passed by an archery authority as being poor in cast.

  One of my hunting crossbows with an alloy bow drawing 85 lb. threw an ordinary target bolt 390 yards. I am in no doubt that with a flight bolt I could have reached the 400 yard mark with ease.

  A noticeable advantage of an alloy bow is its ability to be carried at full draw for long periods without loss of cast. Non-ferrous alloys will not last indefinitely as bows, and should be discarded and replaced after some 2,000 to 3,000 shots. Alloy bows are so cheap to construct that replacement is not as financially disastrous as may be thought.

  Although this alloy shows good cast, some controversy exists concerning its safety factor, as sudden breakages have been experienced with this material. Any flexed bow that breaks is dangerous both to the shooter and bystander, and every precaution should be observed to avoid such an occurrence. Alloy bows must be of such a finish that no scratches or unevenness is apparent at any point whatsoever. Over a period of eight years I have experienced only two breakages in bows of this material, both occurring on cocking the crossbow, the bow limb breaking off close against the stock at the bow plate. These breakages I put down to the bow design allowing too much flexion at this point. After adjusting the design to stiffen the bow centre at the stock and allowing more flexion along the bow limb to the bow tips, no further breakages were experienced.

  There is no such thing as an unbreakable bow whether it be self alloy, steel, solid fibre glass or composite. Flex a bow enough times and it will eventually break. It may take many thousands of flexes over a period of some years to do so, but no bow will last for ever. Some handbow manufacturers deliberately break bows in a machine that repeatedly flexes the bow limbs many thousands of times, (this is to ascertain the average life of the bow), and so are able to guarantee their bow for a period of one to three years at the most.

  A fine piece of tough rubber glued on the back and belly of an alloy bow at the centre section, where the bow is in contact with the stock and bowplates, helps to absorb recoil and provides a resilient surface for the bow limb to pull against. This is a considerable help in reducing the possibility of a premature breakage of the bow limb close to the stock. As an alternative material, fine leather makes a good substitute if rubber is not available.

  Should an alloy bow snap off at the bowplate close to the stock it could be dangerous, as a flying bow limb could cause a nasty cut. To prevent an accident of this nature, safety stops fitted to the stock will effectively prevent the bow limb flying back at you, and are simply fitted by the following method.

  The bow plates on my crossbows are 1/8" thick mild steel with a 1/4" diameter hole either side of the bow slot. These plates are clamped firmly to the stock by 1/4" diameter screwed rods passed through the bowplates, and the stock itself, and tightened up with four hexagonal nuts. Should a safety stop be required, simply leave the screwed rod behind the bow limb long enough to protrude 2-1/2" to 3" out from the side of the stock. Firmly clamped against the bowplates, these protrusions will effectively deflect to the side any bow limb breaking off at the stock. Leave the other hole and screwed rod in front of the bow limb clamped tight against the stock in the usual way as no protruding rod is required in front of the bow.

  A composite bow is by far the best for either target or hunting crossbows, having usually excellent cast. You may expect to pay considerably more for a good composite bow than for any other type, though the extra cost is well worth the advantages you will have over the other types.

  Velocity or speed of cast depends mainly on bow design and draw length. A flat trajectory is dependent on velocity; the higher the velocity the flatter the trajectory. A high or low trajectory on a target crossbow when shooting at regulation ranges is of no real consequence, whereas on a hunting crossbow as flat a trajectory as possible is essential. A bow with recurve tips will throw a bolt with faster cast than a straight-ended bow because when drawn the recurve unbends and upon release acts as a spring giving extra 'snap' to the limbs.

  The shorter the bow the greater will be the cast for a given draw length. Although a longer bow will shoot more steadily and smoothly, it lacks the cast of the shorter bow. A bow of three feet, drawing sixteen inches, will shoot faster than a bow of three feet six inches, drawing the same weight at sixteen inches, but the shorter bow will not shoot as steadily.

  Crossbow bows are practically identical to flight bows and when drawn form almost a true ellipse between bow and string. The farther you draw your string back to form an ellipse the faster cast your bow will have. The danger is in trying to force the bow to have too long a draw for its length causing it to break. As a long bow shoots steadily it is best for target work, while the shorter bow throwing its bolt flatter will be more useful for hunting purposes. A flat trajectory reduces your difficulty in range estimation, increases hitting velocity at the longer ranges; all this adding up to increased effective hunting ranges.

  Irrespective of draw weight, the recoil or 'kick' on a crossbow when shot should be practically negligible or entirely non-existent. The absence of recoil will depend a great deal on stock construction and weight in relation to bow design and the manner in which the bow is fitted to the stock. The principle of good bow fitting is to have the bow immovably fixed in the stock. This must at least efficiently prevent any lateral or vertical movement of the bow. A number of crossbow bows are fitted into rubber mountings in such a manner as to prevent any movement other than the natural forward and backward movement when shot. This rubber mounting will effectively absorb recoil. />
  All bows should be mounted at 5� to vertical in the stock as this angle gives the fairest pull on the limbs. Over 5� gives a faster loose but may tend to twist the bow limbs. Your bow should be fitted into the stock as close to the bolt groove as safety will allow, as this will assist the string in a fast loose. The bolt groove, being 1/2" deep, will allow the bow to be fitted with its top edge 1/8" below the groove, in all a total of 5/8" below barrel level. This, together with the bow fitted at 5� to vertical, reduces the pressure of the string bearing down along the barrel. The higher in the stock your bow is fitted the better the cast your bow will have.

  Many self wood and composite bows have a 1/4" deep cut out at centre to barrel thickness, this further reducing friction by raising the bow limbs a further quarter of an inch when fitted to the crossbow stock.

  The best Sixteenth Century crossbows, and some of later date, were fitted with a canted limb steel bow, fitted in the stock at right angles. String movement along the barrel was absolutely friction-free, the canted limbs allowing the string to skim fractionally above the barrel delivering every ounce of draw weight to the bolt.

  Canted bow limbs do not, to date, appear to have been utilised in modern bow construction, though I consider solid fibre glass a likely possibility for this.