Bow Design Figure 14 – Bow in tensile test machine Fig 17 – Typical stabiliser configuration


Figure 15 – Typical recurve loading and unloading curves


When assembling a bow it is normal to check the ‘bracing height’. This is the distance between the string and a point on the riser, usually the button position. Changing the bracing height changes the pre-load in the bow and string, altering the bow characteristics. Thus it is important for consistent shooting that the bracing height is constant. The length of the string is adjusted by changing the number of twists in the string. This changes the bracing height. While carrying out the research, we needed to know the tension in the string before it is drawn. We made up a dummy string with a load cell and an adjustable screw device to change the string length. {Fig 16] (A similar set-up can be assembled from a hand held luggage scale and a ‘bow press’, a screw device used when adjusting a compound bow.) With this we could change the string length and hence the bracing height significantly. We found that the string tension rose to a peak at a particular bracing height. This was shown to give the minimum bow vibration. (This doesn’t work for longbows.)


Fig 16 – Measuring string tension


Each bow maker includes small features which they believe makes their bows that bit better than the rest


22


Target bows are usually fitted with ‘stabiliser’ rods. [Fig 17] These have three purposes. They provide balance to the bow, moving the centre of mass to a point just in front of the handle, removing any tendency for the bow to tip. A bare bow has a significant rotational inertia in the roll and pitch directions but not in yaw. A long horizontal stabiliser rod, with a small weight on the end, gives the bow yaw inertia making it steadier during the shot. With appropriate construction, stabiliser rods can be made to absorb much of the bow vibration after the shot, making it feel better to shoot. Some are powder filled tubes; some have multiple rods (similar to carbon arrow shafts) with various spacers; some have rubber mounts or other damping materials. The usual configuration today is a long-rod


plus two short-rods angled to the sides near the riser. The short-rods can be adjusted, in position and weight, to counterbalance the mass offset caused by the bow window and bring the centre of mass to a point just in front of the handle. Other configurations abound.


Each bow maker includes small features which they believe makes their bows that bit better than the rest. There is a wide range of bows available costing from around £60 for a beginner’s bow, probably with a plastic riser and fibreglass limbs, to over £1200 for an all carbon-fibre high precision bow. Arrows can be from less than £2 for wooden longbow arrows to more than £25 for barrelled carbon shafts with tungsten points. Then you need sights, stabilisers, and a button. Archers wear various pieces of protective clothing and have a quiver for arrows, score pad, etc. (An old car sticker read, ‘Archers do it with a Quiver!’) A novice archer on the shooting line can be carrying around £150 of kit, while a top archer could have over £2000. The handicap scoring system used for many tournaments still makes it possible for the novice to come out on top.... occasionally.


If archery is something you have always wanted to try, find your local club via www. archerygb.org. Most clubs run ‘have-a-go’ days and beginner’s courses.


References For more on the history of longbow archery and its role in medieval warfare see: The Great Warbow, M Strickland and R Hardy, Sutton Publishing 2005. ISBN: 0-7509-3167-1.


For more technical details of both the archer and the modern recurve bow see: Archery Anatomy, Ray Axford, Souvenir Press 1995. ISBN: 0-285-63265-6


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