The Loose - Nock End

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When an arrow is released using fingers/tab the nock end of the arrow initially bends away from the bow. The nock end subsequently bends back towards the bow and then away again. Archer's paradox is the term used to describe this bending behaviour. The aim is to have the nock (fletching) end of the arrow bending away from the bow as it passes the body of the bow the purpose being to avoid any collision between the rear of the arrow and the bow. This section gives an overview of how this bending behaviour is generated. I will assume a right handed archer when describing directions of travel etc.

Points to bear in mind are that the force exerted by the string on the arrow varies as the arrow moves forward and also that the direction of the string force is always towards the string bracing height position.

The Loose - Nock End

The pressure button is set up for a recurve bow so that the arrow at full draw is pointing left away from the bow. The direction of the string force at full draw points towards the bracing height position and is therefore on the bow side of the arrow. If the loose was made using a mechanical release then the nock end of the arrow would bend towards the bow and the the pile end of the arrow would rotate away from the bow as illustrated. Ooops! The arrow would end up flying uncontrollably off to the left.

What initialises the the Archer's Paradox effect is the action of the fingers/tab on the bow string. At full draw the string force is balanced by an equal and opposite force on the tab. At the loose the string force causes the tab to rotate as the fingers are 'uncurled' (ref). At this point there are three forces acting, the string force towards the bracing height position, the tab reaction force at a right angle to the tab surface and a tab frictional force parallel to the tab surface. These three forces add together (ref) to produce a net force on the string forwards and to the left away from the bow. The sideways acceleration of the string causes the arrow shaft to bend away from the bow at the nock and as a consequence the string force ends up running across the arrow shaft. At the same time the forwards acceleration of the string transfers the load from the tab onto the nock end of the arrow shaft. The purpose in having a 'slippy' tab is that the lower the tab frictional force then the shorter is the time that the string is acting on the tab during the loose i.e. there is less time to mess it up by moving the string hand. The other effect the tab has in principle is on the effective arrow dynamic spine. The 'slippier' the tab then the higher the nock acceleration will be and the more the arrow will bend.

The result is that when the string force ends up acting on the arrow nock the string force runs across the arrow shaft and the reaction at the nock (which is at a right angle to its surface) is pointing backwards and away from the bow to the left. These two forces add together and the resultant force direction is forwards and to the left away from the bow i.e. the nock end of the arrow continues to bend away from the bow. This process is similar to the behaviour of a pole vaulter's pole after it is planted in the box.

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