Torsion Bundles

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Terminology's Torsion page

How Torsion Bundles Work

A torsion bundle uses energy stored in the stretched rope. Once the bundle is loaded by twisting both ends in the forward direction (The direction that the arm rotates when throwing) a torque is applied to the arm. The increased tension in the bundle from the twisting also holds the arm in place. Then the arm is released, after being rotated backwards, the bundle will turn is forwards providing the power for the throw.



Onagers have one torsion bundle parallel to the ground which swings the arm in a vertical plane.


Ballista have two torsion bundles which turn the arms to function like a cross bow.


The type ans size of rope used is very important. Please see the rope page for details.


Torque and Rotation Speed

Torque generates rotation speed, however a higher static torque does not always produce a higher rotation speed. This is because there are limits on how fast the bundle can rotate. This is caused by the properties of the rope such as the recovery rate. The torque drops as the rotation accelerates, which means that in some cases longer arms can increase efficiency.

Tensile Strength

The tensile strength of rope is how much load it can take before breaking. It is important to note that this is different from working load which is calculated from the tensile strength. The working load is usually around 1/4 to 1/5 of the tensile strength but it varies. Sometimes it is under 1/10 of the tensile strength. It is not uncommon to see the same rope for sale labeled with different working loads, sometimes different by over a factor five. The load in the bundle relative to its tensile strength is important because it effects the type of stretch.

Types of Stretch


Structural is when the rope is lengthened by a tightening of the weave, or other movement of the strands in the rope. Structural elongation involves a lot of friction and the recovery rate is very low. Torsion bundles should avoid structural elongation by operating at tensions high enough to achieve mainly material elongation. This can be done my removing rope, or by increasing tension.

Material Elongation

The point at which the stretch is due to material elongation is not easily determined. Material elongation stretch has fast and efficient recovery rates.


For the direct effects of varying specific characteristics see Tuning




Load Calculations





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