Most people are familiar with bungee cords. They are a simple but useful tool that you probably have in your home or garage. They also have many applications in industrial, commercial, and even military environments. Bungee cord, also known as “shock cord,” is an incredibly versatile material. As a result, bungee cord manufacturing is more complex than most people expect.

The various sizes, materials, patterns, core compositions, sheathing, and ends allow for this singular cord to be useful in many different applications. Personal uses for bungee cords include car straps, truck tie-downs, camp tie-downs, storage retention, etc. In commercial and industrial settings, bungee cords find many additional uses: commercial tarping, securing signage, boating and ship storage, greenhouse agriculture, transportation, and more.

Bungee cords are composed of three primary parts: the core, the sheathing, and the end hardware.

The core of a bungee cord is what provides its elastic potential. Cores can be made of natural rubber and synthetic rubber and are often a combination of these two rubbers.. Natural rubber has a high strength and elasticity but is also quite vulnerable to degradation. Synthetic rubbers have a lower strength and elasticity than natural rubber but offer a greater resistance to degradation, making them more reliable over time.

The sheathing of a bungee cord protects it from the environment and from mechanical damage. Polypropylene and nylon are two of the most common types of sheathing. Polypropylene is affordable and has great chemical resistance, while nylon has a much greater resistance to mechanical abrasion and better elasticity. Polypropylene thread is coated for UV resistance making it great for general purpose indoor or outdoor applications. Sometimes polypropylene is called “fibertex”. Nylon is ideal for marine applications and places where maximum elasticity is required. Polyester sheaths perform well in marine applications too, and polyester is higher strength relative to nylon or polypropylene.

Additionally, sheathing gives shock cords their exterior color. MIL-C-5651 bungee is natural white cotton with color tracers for date traceability. The most common color for a commercial bungee cord is solid black, but they can also be white with a tracer. Other colors, with or without tracers, are also available.

End hardware comes in many shapes and sizes:

• Bungee balls installed on the end of a cord are used for tarps and retention in a grommet.
• Crimped wire ends, also known as “hog rings,” permanently attach a bungee cord to a surface, often for installing bungee hooks.
• PVC-coated and bare metal hook ends (plated steel or stainless steel)are used in many applications and are reusable as necessary.
• Barb tips allow a bungee to slide through a hole and then articulate to become anchored.
• Fiber rope sleeves are used to form loops on bungee cord or rope.

Stretch value refers to the amount of elongation for the shock cord. 50% stretch value means that 10” bungee cord length could stretch 5” to become 15” (10” relaxed and 15” fully stretched). 100% stretch value means that 10” bungee cord length could stretch by 10” up to become 20” (10” relaxed and 20” fully stretched). 150% stretch value means that 10” bungee cord length could stretch by 15” to become 25” (10” relaxed and 25” fully stretched). Keep in mind, you don’t want to use bungee cord at its maximum stretch. It’s suggested to only stretch within 80% of the available stretch. For example a 10” cord length with 100% stretch should only be safely stretched to 18” (even though it’s possible to stretch to 20”).

To understand the basics of shock cord/bungee cord science, it is important to understand Hooke’s Law and the principles of elastic force.

Hooke’s Law is an empirical law that states that the force needed to stretch or compress a spring scales linearly with the distance it is stretched. This means that if it requires “X” amount of force to stretch a spring “Y” distance, it requires “2X” amount of force to stretch that spring “2Y” distance.

Regarding bungee cords, their elasticity also depends upon their length and their cross-sectional area. The longer the cord, the more it stretches. Conversely, the thicker the cord, the less it stretches. A thick cord and a thin cord of the same length will break when stretched the same distance; it simply requires more force to stretch the thicker cord to that same length than the thinner one.

The point at which a given material begins to break is used to calculate its ultimate tensile strength. When something is pulled, it first stretches, but not so much that it will permanently deform. Then, past its “yield point,” it begins to deform permanently and won’t return to its original shape.

At some point, the stress in the material grows to a point where it begins to form localized stress points, called “necking.” It is at those stress points that the material will eventually break. The point where a material begins necking is its “ultimate tensile strength.”

The usefulness of bungee cords depends not only on their elasticity but also on their durability. A durable bungee cord can resist overloading, exposure, and general wear and tear.

Overloading a bungee cord is never a good idea, but mild overloading of a cord should not cause it to break. Repeatedly doing so might, however, reduce its performance and lead to breakage in the long term. Sun exposure and wear-and-tear from weathering or mechanical damage can also lead to degradation of the sheathing and eventual failure of the cord.

To preserve a bungee cord, it is important to inspect it regularly for damage, such as tears in the sheathing and lesser elasticity, where it doesn’t stretch as much as it did before. To prevent this damage, properly store your cords in a container not exposed to sunlight. When in use, prevent damage by avoiding rubbing them against cutting or abrasive materials and don’t overload them.

For safety, remember that bungee cords can carry a lot of potential energy. Thicker cords require more force to stretch, so they will snap back with more force if released or broken.

Be mindful of a principle called “safe stretch limit.” Safe stretch limit refers to the length to which the bungee cord can stretch, relative to its original length, without snapping. Generally, the safe stretch limit is 50% of the original length. For example, a 24 inch bungee cord can only be safely stretched to 36″. However, it is recommended to refer to your bungee cord manufacturer or vendor for precise stretch limit guidance.

It is important to understand your intended application for a bungee cord, a.k.a. shock cord. Smaller and shorter cords are useful for small applications around the home, but tying down tarps in a truck will require longer and thicker cords to resist stronger forces. Bungee cord can be supplied on spools, bulk boxes, pre-cut pieces or as finished assemblies.

Special hardware ends will be needed if the cords must turn or spin, while common hooks are necessary for cords that need to be removed and replaced frequently.

Marine applications will require a nylon sheathing rather than a polypropylene sheathing. MIL-C-5651 bungee cord is available for aerospace and defense. There are many more factors that must be considered when choosing the right bungee cord, so know your circumstances and consider your options carefully.

Bungee cords are versatile tools that provide a durable and cost-effective solution for retention and tie-down applications that need elasticity. When you understand the science of bungee cords and the options available, you are better equipped to determine exactly what kind of bungee cords you need.

Lexco® Cable is an industry leader in the manufacture of custom wire ropes and bungee cords. From different sheathings to a variety of end hardware and fittings, we have everything you need for the precise kind of bungee cord your project requires. For additional resources and to get a quote, check out our website. (Please be advised, Lexco does NOT supply bungee cord for supporting human weight.)