Carbon-fiber composite wheelchair casters function like airplane landing gear

Great ideas come to people in unsuspecting ways. In 1997, Mark Chelgren had a chance encounter with a quadriplegic rugby team. A late night discussion about their wheelchairs led him to develop an alternative to the traditional rigid casters used for the front wheels, and Frog Legs Inc. was born.

The Ottumwa, Iowa–based company specializes in precision built, impact-absorbing products for wheelchair suspensions. Recently, Chelgren decided he wanted to switch from machined aluminum to a carbon-fiber composite for the new generation of wheelchair caster wheels. PlastiComp Inc. of Winona, MN, was just the right partner, not only for the carbon-fiber-composite materials but also for assistance in ensuring the material change would be successful. PlastiComp specializes in long-fiber-reinforced thermoplastic materials and technologies.

Frog LegsInstead of fixed forks that require the wheel to move up and over obstructions, bouncing the rider in the process, Chelgren designed forks with a patented pivot point and wedge-shaped shock absorber that allow the wheels to move over impediments in an arc path, functioning much like airplane landing gear.

“An arc is a much more efficient movement,” said Chelgren. “But its compression zones are not linear, so instead of a traditional cylinder-shaped shock absorber, Frog Legs uses a wedge shape to better handle the differential loadings from front to rear.”

People with physical disabilities who require wheelchairs for mobility literally feel every bump in the road as they navigate obstacles as seemingly insignificant as cracks and uneven surfaces. A day filled with constant vibration and jolts causes fatigue and can produce spasms in riders, who may require treatment with medication.

Testing performed by the Center for Industrial Research and Service at Iowa State University showed that 80% of the vibrations experienced by wheelchair riders came from the front caster wheels. Chelgren’s design improvements reduced those vibrations in the wheelchair frame by 76%.

Knowing that Frog Legs was searching for an alternative to aluminum, one of the researchers at ISU attended a webinar presented by PlastiComp to learn more about long-carbon-fiber-reinforced thermoplastics and the injection molding process, explained PlastiComp Media Marketing Manager Kirk Fratzke to PlasticsToday.

“One of the services we offer is helping companies throughout the design and product development cycle, including computer modeling, to achieve solutions to their requirements,” said Fratzke.

Chelgren knew he and Frog Legs could do better than the current method of machining aluminum. “We are always being pushed to lighten our product. People with disabilities are greatly affected by any additional weight,” he said. “Injection molding allowed us to have a much more complex shape, and carbon-fiber composites really gave us advantages in [terms of] our design parameters.” 

Fratzke noted that long-fiber materials, whether glass- or carbon-fiber reinforced, have long been best suited for metal replacement because of their performance characteristics. “If someone is currently using metal and wants to move to thermoplastics, long fiber is a good place to start,” he said. “It’s as close as you can get to the mechanical properties in metals.”

PlastiComp custom developed the reinforced polyurethane composite to bond with the urethane used for the outer rolling surface of

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