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Crossbow parts

 

 

       
 

Riser

the metal piece that the limbs attach to.

The riser function is very basic as it should keep the limbs in position. Should be as light as possible as it mounted at the front of the crossbow and must allow free pass of the arrow including fletching. In order to be light and strong the riser is made of aluminum or magnesium, often machined with holes or pockets in order to reduce weight. The crossbow riser must be very rigid in order to keep the limbs perfectly aligned at full draw and during release. Every manufacturer has developed its own design that has become distinctive for its product line.

Flight rail/barrel

The flight rail/barrel is the component that supports the arrow and separates the riser/bow assembly from the trigger mechanism. This component, often made of extruded aluminum must be very rigid in order not to flex at full draw. Every manufactured developed its own solution: most use custom designed aluminum extruded beams, some use molded synthetic (special plastic loaded with filler such as glass fibers to add strength) component that integrates the stock. Some crossbow design named trackless do not have a flight rail (the arrow is not completely supported) but there still is a component that separates the riser from the trigger.

 

Power stroke

Power stroke is the distance from the rest position of the string to the fully cocked position into the trigger mechanism. As the string is released by the trigger the limb exert a force on the string that propels the arrow.

Power stroke is a fundamental parameter for a crossbow.

In physics the amount of work that a machine (crossbow) generates is equal to the product of a force by the distance:   W= F x d 

That explains clearly that all parameters being equal a crossbow with a longer power stroke generates more work. The work is used to accelerate the arrow thus longer power stroke = faster arrow.

 

Trigger mechanism

A mechanism, housed into the stock  at the end of the flight rail/barrel that keeps the arrow into the cocked position and releases it upon pressure on the trigger. The mechanism is fairly simple, being made of some levers but crossbow trigger face conflicting requirements: must be very strong to hold full force generated by the bow (sometimes over 200 lbs), should require low activation force (trigger pull is generally 3-5 lbs) should move enough to clear the arrow with a short activation distance. The trigger mechanism is complicated by the addition of a safety that prevents the trigger from being pulled (and the arrow released) without disengaging the safety first.

Stock

The stock is a shaped component that allows the shooter to shoulder the crossbow. Stocks are often made by molded synthetic plastic , a few manufacturer offer laminated wood stocks. The stock should allow the shooter to easily hold the crossbow and take aim, in order to do this a stock should allow the correct spacing from shoulder, allow the shooter to rest the cheek aligning the eye with the scope (or other aiming device). Some manufacturer offer adjustable stocks. As the stock integrates the rear grip so it must provide a comfortable grip and correct spacing from the thumb-index to the trigger.

Some stock designs integrate also the front grip: also in this instance the grip should be ergonomically correct both in positioning (for good balance) and dimensioning (for a good, positive grip).

Front grip design should provide some means to prevent the fingers from touching the string/cables

 

Limbs

Limbs are the flexible part of the bow. Limbs are made (very often) of laminated fiberglass. Fillers or exotic materials such as carbon fibers are sometimes used to tweak the characteristics of the bow.

Limbs are responsible for storing energy as they are flexed and releasing it as the crossbow is shot.

Limbs are very stressed components.

Limbs are composed of glass laminations glued together, for this reason should never be left in very hot places (in your car under the sun for example). Limbs may be damaged by dry firing.

 

Dry firing

The action ( either deliberate or accidental) of shooting a crossbow without an arrow.

Should never be done. Limbs may be permanently damaged as well as other components of the crossbow. The action is dangerous and may injury the shooter as well as other people standing close by. Some manufacturers equipped their crossbows with an anti dry firing mechanism that prevents the string to be released (or the trigger pulled) if an arrow is not present.

Semi dry fire

A rare event in which the string runs over the arrow causing a situation similar to dry fire

 

Yoke system

The connection of the cables to the cam axles. It is made by a loop connecting the cable end to the two tips of the cam axles (seen from behind seems a Y lying on the side). The two ends of the loops are (often but depends on crossbow design) twisted several times (usually the lower loop many times more than the upper) in order to create more tension on the lower limb , this helps the string remain in contact with the flight rail.

Reverse limbs crossbow

A crossbow design where the limbs instead of flexing toward the shooter flex toward the front of the crossbow. This design, originally conceived by Leonardo DaVinci offers advantages such as compactness and low vibration. Examples of commercial crossbow using this design are: Armcross, Scopryd, and some Horton models.

String

The piece of  rope that connects the limbs (in a recurve corossbow) , the often backmost rope in a compound crossbow,  the string does not touch either arrow nock or flight rail directly as a serving is interposed. Strings are made of high strength (more than steel) synthetic fibers such as dyneema fast flight. Strings should have low creep (stretch).

 

Serving

A spirally wounded filament that covers the center portion of the string. Serving prevents the string from being worn by friction onto the flight rail. Serving material is different from string material as main requirement is wear resistance. Should be replaced as needed. Lubricants are often applied to flight rail in order to reduce friction.

For a tutorial on how to apply serving see our technical corner here.

 

 

Cables

The rope that connects the cams in a compound crossbow. It used to keep the cams sincronized. High strength synthetic fibers are used. The part of the cable that contacts the cam is often covered with serving material.

 

Cam

A specially profiled (non circular) wheel used in the compound crossbow.

Cam profile is designed in order to maximize work that crossbow generates to launch the arrow. All parameters being equal crossbow with cam (vs wheel) is able to generate more work resulting in faster arrow.  

 

Cable Saver

A device used to reduce friction between the cables and the flight rail/barrel and the cable themselves. Sometime a specially designed piece of low friction material, can be as simple as a strip of Teflon.

 

Weaver rail

A military standard dovetail attachment used to attach optics and accessories (laser, flashlights, grips etc) to small arms. Weaver rail is a worldwide accepted standard therefore well suitable to allow interchangeability. The Weaver rail consist of a rail with locking slots, whose dimensions are defined in the standard,  that prevents the optics from moving. Sometimes confused with the Picatinny rail, that share same external dimensions, but differs in the slot dimensioning.

 

Cocking device

A device used to cock the crossbow while reducing the cocking effort.

Can be as simple as a pulley /rope mechanism that reduces the effort by 50% or a crank device (either removable or permanently integrated) that reduces the effort down to a few pounds. Cocking devices allow perfect cocking as the string is kept centered resulting in superior shooting consistency and precision. Each manufacturer offer its own solution sometimes customers are offered both choices, some manufacturers integrate the cocking device into the stock design.

   
             
         
             
       
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