Jump cords are bundles of rubber strands, usually natural (latex) rubber, inside a fabric or rubber cover. Natural rubber stretches a long way, springs back to its original length and has high tensile strength, which makes it a good material for bungee cords [1]. You don’t need to know an operator’s exact recipe, but you should be able to ask how it inspects its cords, counts jumps and retires them.
How a bungee cord is made
Many jump operators design and make their own cords, and the process is usually kept secret. A typical commercial or military-spec cord is made like this [1]:
- Extruding: rubber is heated and pushed through a die to make long, thin ribbons, up to about 100 feet (30 m) long.
- Bundling: the number of strands sets the cord’s diameter and strength. The strands are dusted with talc so they don’t stick together.
- Covering: the bundle is stretched slightly and fed through a braiding machine, which weaves a tight cover around it.
Jump cords are usually about 1.6 cm (0.62 in) thick, and each company designs its own to suit its sites [1]. Ohio’s rules describe two types in use in the US: military-spec cords with a fabric sheath, and “New Zealand” style cords [2]. Operators match the cord, or the number of cords, to each jumper’s weight, which is why you’re weighed at check-in [3][4].
How cords are tested
US state rules give a clear picture of what good testing looks like. Ohio’s rule says [2]:
- Safety factor of five: the most force a jumper can put on a cord can be no more than 20% of the cord’s minimum breaking strength.
- G-force limits: no more than 4.5 g with a waist and chest harness, and 3.5 g with an ankle harness.
- Break testing: manufacturers must break-test samples of every batch, made with real end loops, with the tests done or supervised by an independent certified testing authority or engineer.
- Stretch data: the load-versus-stretch results are used to check the g-force and safety factor.
Pennsylvania publishes similar cord specifications [5]. For how stretch and g-force feel during the fall, see the physics of bungee jumping.
How long a bungee cord lasts
A cord is rated for a set number of jumps, not a set time. In Ohio, manufacturers must state the maximum number of jumps each cord can take, and operators must keep a record of jumps on every cord that can be audited. Cords must be retired when they show wear or damage, stop behaving as specified, or reach their jump limit. Retired cords are then cut into short lengths so they can’t be reused [2].
Illinois has similar rules. Each cord must carry an ID that’s recorded in a log and color-coded as set out in the operations manual. The jump master must inspect the whole cord before jumping starts and at least four times during the day, and record each check. If a binding breaks, the cord comes out of use until it’s repaired. Withdrawn cords are destroyed by cutting them into lengths of 5 feet or less [4].
What you can check on the day
You won’t see the factory tests, but you will see some of the routine. Staff weigh you and match you to a cord or setup [3]. They check connections and the cord itself, and swap out any gear that looks wrong. It’s reasonable to ask three simple questions:
- How do you record the number of jumps on each cord?
- When do you retire a cord?
- Who double-checks my connections before I jump?
A good operator will answer these easily. If the staff can’t, or tell you not to worry about it, think about jumping somewhere else. See how to choose a reputable bungee operator.
New technology in bungee jumping
Some newer systems are already in use. Macau Tower’s 233 m jump uses what Skypark calls the world’s only guide cable bungy system, which keeps jumpers away from the tower [6]. Booking, photo and video systems have moved online, and serious operators log every jump on every cord, as the rules above require [2][4].
VR headsets can give you a preview of the height and the platform, which may help with nerves. They can’t show you what the stretch, the upside-down bounce or being lowered or raised at the end feel like, so treat VR as practice, not a replacement for the real thing.
Whatever changes, the basics stay the same: an honest weight check, weather limits, and staff who will stop a jump if something isn’t right.
FAQ
What are bungee cords made of?
Usually strands of natural (latex) rubber inside a braided cover [1].
How strong is a bungee cord?
In Ohio, a cord’s minimum breaking strength must be at least five times the maximum force a jumper can put on it [2].
How often are bungee cords replaced?
Cords are retired after a set number of jumps specified by the manufacturer, or sooner if they show wear or don’t perform as expected [2].
How often are bungee cords inspected?
In Illinois, the jump master must inspect the whole cord before operations and at least four times during the day, and record each check [4].
Why do they weigh you before a bungee jump?
So staff can match you to the right cord or setup for your weight [3][4].
Sources
- How Products Are Made: Bungee cord (materials and manufacturing)
- Ohio Admin. Code 901:9-1-29: Bungee cord specifications
- AJ Hackett: how to prepare for a bungy (check-in and weighing)
- Illinois Administrative Code tit. 56, § 6000.340: Bungee jumping (cord identification, daily inspections, destruction)
- 7 Pa. Code § 139a.27: Bungee cord specifications
- Skypark Macau: Macau Tower bungy (233 m, guide cable system)
Image credits
- Photo: MirandaUrbina / Wikimedia Commons (CC BY-SA 3.0)
- Photo: Andrzej19 / Wikimedia Commons (CC BY 3.0)