Recurve, compound and draw weights: load cells in archery bow testing

The World Archery Championships last weekend in Mexico City drew huge crowds to the city’s main square. Thousands watched the world’s best archers compete for the World Championship title shooting at a target centre as little as 8cm in diameter from 70 metres away!

 

Take aim

While the aim of target archery has remained the same for centuries (hit the middle of the target as often as possible!), the bows used by modern archers are incredibly high tech. The testing and setting up of these bows is of crucial importance, and load cells can play a vital role in establishing the draw weight. This is how much weight an archer pulls when they draw the bow back, and is always measured in imperial pounds.

It’s important to establish the right draw weight for the archer. Too heavy a draw weight will make it very hard work for the archer, and they won’t pull back the string to its optimum position. It could also cause muscle strain or injury. Of course, measuring the draw weight of a bow is a simple task when you have a load cell!

 

Draw weights and bow types

The Championships feature two types of bow, recurve and compound.

The recurve bow has a traditional bow shape, with a handle section (riser) in the middle and two curved sections (limbs) top and bottom. The limbs are pre-calibrated to a particular draw weight, such as 28lbs at 68 inches. (That’s the total height of limbs plus riser, if a 25” riser is used.) Once the bow is strung, the limbs bend to form the familiar bow shape. Limb weights are calculated at a nominal 28” draw length – that’s how far back the archer can draw the string back when holding it at arm’s length.

Compound bows look more like an explosion of cogs and strings, and work differently. The bow configuration enables an archer to pull a heavier weight, say 40lbs, to their draw length. The compound bow ‘holds’ that position while the archer takes aim and then releases. Again, too heavy a draw weight and the archer won’t be able to get to the ‘hold’ position safely.

 

Measuring draw weight with load cells

In his paper “Applications of Physics to Archery”, H O Meyer of Indiana University describes a simple but effective rig to measure the poundage of a compound bow.

 

“The bow is tied down at the grip and pulled up by the force Fd, applied at the nocking point in the direction indicated at the top of the figure. This ‘draw force’ is a function of the distance s between the grip and the nocking point. For a relaxed bow this distance equals s0, called brace height. While the bow is drawn, the nocking point moves by a distance sD, and at full draw s = s0 + sD, the true draw length.

The bow is supported just at the grip and at the nocking point and can rotate around a straight line through these two points. The wooden bar on the left is there to prevent this. The force is applied by a rope attached to the nocking point. A block and tackle (not shown) is used to help pulling the rope. The force is measured by a piezo-electric load cell and is displayed on a read-out at the bottom of the picture.”

 

Unlike a luggage scale that gives a single reading, this rig enables any archer to accurately measure the poundage at every stage of the draw, giving much more accurate information.

 

Manufacturer’s bow testing

In Britain, we may not win a lot of archery medals and titles, but we do have a manufacturer of world-class bows in Mybo. In a fascinating (if slightly long) video, they show the whole process of making their Origin compound bow; we’ve skipped forward to 15.45” to show the testing of the bow, both strung and unstrung.

 

Strings and arrows

Load cells can also be used to test the other two crucial components of a bow, the string and the arrows.

Strings are made from a single strand of high-performance polyester string material wound into a continuous loop, such that the string ends up as a bound up bundle of multiple strands. The elasticity of the string is important: too much stretch and the string will change length as it is used, which in turn of course affects the height of the bow and the draw weight. The best recurve strings, for example, are pretensioned to a massive 300lbs.

Arrows are made from aluminium, carbon fibre, or a mixture of both, and come in a range of stiffnesses that define how much the arrow can bend. Again, Meyer describes the physics of how an arrow is constructed, and has a simple way of testing the stiffness using a luggage scale. And again, a load cell would be way more accurate, and enable archers to text arrows over time, especially if any remedial straightening is required.

 

Put it all together…

When a bow is properly set up for an archer, the accuracy they can achieve is breathtaking. Look out for those wiggling arrows!

 

Testing sports equipment with load cells

If you want to test any sports equipment using load cells, either as a sportsperson or a manufacturer, call us. We can help create highly accurate testing solutions using our years of experience in the design and manufacture of load cells for specific testing requirements, including high-end commercial testing machines. Call us with your requirements.