THE SAFE BOUNCE
Chasing superior functionality
By Michael Herman
Over one million trampolines are sold each year in North America alone but the growing incidence of backyard trampoline injuries points to a serious design flaw in the traditional trampoline, emphasizing that while they might be fun they’re certainly not safe.
With its origins in the ancient civilizations of China, Egypt and Persia, the universally popular manufactured trampoline was an invention of the 1930s. A taut, resilient fabric stretched over a steel frame with a multitude of coiled springs to provide a rebounding force, the traditional trampoline remained largely unchanged until Kiwi engineer and Canterbury University senior lecturer Dr Keith Alexander undertook a detailed analysis of worldwide trampoline injury data and came up with what parents had long been asking for – the safe trampoline.
“The old design is pretty stupid. You’ve got this really exciting bouncy surface kids can play on and do stupid stuff – and they love to – and right beside it you’ve got a steel bar and these springs with holes all around them. And you put the whole thing high up off the ground so if they misjudge anything they break their teeth, fall through the holes or fall some distance to the ground. It just didn’t seem like a good design from the functionality point of view given what kids are like and what kids want to do,” Alexander says.
His injury analysis confirmed trampolines did a good job of propelling jumpers into the air but it also proved another point: they fared much worse at ensuring users returned to the ground uninjured.
With the preliminary research complete, he set out to design a trampoline that addressed the safety component failings of the original design. Alexander says the safe trampoline concept required the device to have the characteristics of a rubber room, providing a secure environment with no impact zones.
“There must be nothing there for [kids] to bang into and hurt themselves because kids are going to be kids and do their craziest.”
While identifying the safety flaws of the original design was easy, engineering a device that was appropriate for getting the job done – propelling jumpers into the air and returning them to the ground safely every time – proved to be a lot harder, taking eleven years of design, materials innovation, prototyping, and a significant chunk of international venture capital before his SET Technology (SoftEdge Trampoline) was ready to be commercialised.
“It ended up being very tricky and I quite soon found out why no-one had done it before – it was not at all easy to get rid of that frame around the edge and still have a taut mat that was nicely stretched and had nothing hard or sharp anywhere near it.”
Alexander and his team did not just jump from the original trampoline design to the safe trampoline. The final design was the result of refining several different concepts and matching the most effective of these to cost considerations and perceived market needs.
One of his original prototypes, the inflatable trampoline is an inflated ring or octagonal shape with a trampoline mat across the top and elevated off the ground to the same height as a standard chair. Suitable for toddlers, who are content to bounce “just a little way”, the inflatable trampoline was acceptable to mothers with its soft surfaces, low height aspect and constrained 'bouncability', but presented a “hopeless” business model.
“The materials were far too expensive and the inflatable tubes always had a leak somewhere,” Alexander explains. Moreover, these expensive devices were suitable only for a small although rapidly growing slice of the market – toddlers – making them a hard sell despite unanimous approval of its safety features by mothers.
Following the market disappointment of the inflatable trampoline Alexander started working on spring designs and later began using pultruded fiberglass rods to replace the springs.
Interest in the pultruded fiberglass rod prototype from a manufacturer resulted in several new requirements being added to the list of challenges, including easy assembly “by a solo mother with no tools in half an hour on Christmas eve” and maintaining the safety factors while increasing the height – and therefore drop – of the trampoline to satisfy the risk expectation of older children.
“He said it was too low and that there was no sense of danger: ‘You’re not going to get a teenager bouncing unless it’s higher.’ We were looking at safety and at least some sort of jumping surface; he was looking at shipping, manufacture, sales outlets and assembling.”
Armed with more insight into what was required to take a safe trampoline to market, Alexander worked through the engineering and financial challenges of producing prototypes for each new design before attracting the interest of a Canadian investor with yet more design requirements: a larger mat higher off the ground surrounded by a net and overall weighing less than the prototype.
Until then trampoline net systems used nets mounted onto rigid big steel poles set against the mat edge, a technique not available to Alexander as the emergent SpringFree trampoline had a flexible edge. Also, the poles merely added additional impact zones, rendering them unsuitable and unwelcome conceptually.
A unique net system using pultruded fiberglass rods was built, providing enough flexibility and resilience to support the combined mass of two teenagers and for the net to automatically spring back to shape as soon as they move off the net’s surface, completing the initial product development and fulfilling the functionality requirements of the manufacturing and marketing functions, the channel and the customers.
Alexander says about 30,000 SpringFree trampolines have been sold so far, with most sales occurring in North America and new sizes and shapes in the pipeline.
Michael Herman is the principal of Knowledge Resources Limited, a Christchurch company working in the areas of custom research, technical writing and knowledge management. A former technology editor at The Press, Michael specialises in turn-key documentation consulting to assist organisations improve productivity and to maximise future sales value. He can be contacted at email@example.com
© designindustry 2006