Bow Design
longbows, with some slightly recurved, still made all in one piece.
Fig 5 – Ladies at leisure Fig 6 – Olympics London 1908
Recent developments After World War II, the availability of aircraft grade aluminium opened up new options for bow design. Tubular aluminium arrow shafts, with steel ‘piles’ (the pointy bit) were lighter than the older wooden ones and plastic fletchings replaced feathers. More recently, tubular carbon-fibre arrows, which are lighter still for the same stiffness, have become the arrows of choice. All aluminium bows were produced in the 1950s, which could be taken apart in two pieces with a socket joint in the handle. However, with little inherent damping, metal bows relied on the archer to absorb the vibration after the shot, which wasn’t popular. Archery has become an Olympic sport again and the coverage on television has helped make people more aware of the sport.
Fig 7 – Patent for first compound 1967
The idea of a bow which could be broken down into pieces for transport and storage led to the most prevalent modern version of the recurve bow. The bow separates into a stiff centre section, known as a ‘riser’, and two flexible limbs. Most bows now use a standard joint, which allows limbs and risers sold by different makers to be used together. Limbs of different lengths and stiffness can be combined with a few different lengths of riser to produce bows to fit any archer. Limbs are usually made of fibreglass or carbon-fibre laminate, tapering in width and thickness towards the tip. Risers are usually made of laminated wood, machined aluminium or carbon-fibre laminate and are shaped to provide a handle and standard attachment points for the limbs as well as an arrow rest, sights, stabilisers and buttons (see later). They also sweep to one side just above the handle (a ‘bow window’) to allow an arrow to lie closer to the central plane of the bow and give the archer a clearer sight of the target.
Longbows, shooting wooden arrows, are still popular, but now there is a third type of bow first patented in the USA in 1967. [Fig 7] The compound bow is a very stiff bow
drawn using a set of pulleys and cams with strings which pass several times across the bow. There are several configurations and variations of cam shapes and pulley sizes. [Fig 8] The compound advantage can best be understood by comparing the behaviour of all three bow types. [Fig 9] A simple draw force against draw length graph shows that a longbow gets stiffer with the curve getting steeper as it is drawn. The energy put into the bow is given by the area under the graph. The same graph for a recurve bow shows a straight line; the bow has the same stiffness throughout the draw. The curve shows a significantly higher energy input. In addition, the use of modern laminates instead of wood gives a more efficient bow putting a higher proportion of that energy into the arrow. The compound bow is a very different device. The draw force rises rapidly to a peak, then falls away to as little as 15% of the peak when fully drawn. This puts a lot of energy into the bow, but makes it much easier to hold and aim at full draw. Since the stiff bow only deflects a little under load, dynamic energy losses and vibration are reduced increasing its efficiency.
Typical speeds of heavier wooden longbow arrows are in the 80-100 mph range with accelerations up to a few hundred g. Lighter carbon-fibre arrows leave recurve bows at 120-150 mph with accelerations of 600- 1000g. Typical compound bows shoot at 180-220 mph with peak accelerations of 1400-1800g. Special compound configurations have produced speeds approaching 300 mph, but tend to be less consistent from shot to shot as they are very sensitive to archer variation. Most target compounds are shot using a release aid, a device held in the draw hand which releases the string when a trigger is lightly pressed. Compound bows are not currently allowed for Olympic competitions, but are popular overseas for both hunting and target shooting, particularly in the USA.
The recurve bow If a bow was symmetrical, when the string was loosed, the ‘nocking point’, where the
Fig 8 – Typical compound bow – Hoyt Turbohawk
The last major battle against the French, where archery was decisive, was at Agincourt on 25 October 1415
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