Frog Legs wanted to switch the wheels of its wheelchairs from machined aluminum to carbon fiber composite. Mark Chelgren, the company’s founder, also wanted to create a wheelchair that had a smoother ride using a patented pivot point and wedge shaped shock absorber instead of having the wheels going up and over obstructions. The shock absorbers allow for the wheels to move over obstructions in an arc path like landing gear and make the ride smoother.
“An arc is a much more efficient movement,” said Chelgren in a press release. “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.”
The Center for Industrial Research and Service at Iowa State University tested the wheelchair vibrations that occurred from front caster wheels on wheelchairs. About 80% of vibrations felt in wheelchairs came from the front caster wheels and Chelgren’s new design reduced the vibrations by 76%.
“This time we looked at a different type of manufacturing process than machining aluminum,” Chelgren said. “Injection molding allowed us to have a much more complex shape, and carbon fiber composites really gave us advantages in what our design parameters could be.”
The wheels are made of two different long carbon fiber reinforced composite materials that were supplied by PlastiComp. The machined aluminum on the wheels was also replaced with long carbon fiber reinforced nylon 6/6 composite in the wheel forks and thermoplastic polyurethane in the wheel hub.
“Using thermoplastic polyurethane for the hub allows us to obtain a chemical bond. With an aluminum hub there are dissimilar materials that never fully bond and can slip,” said Chelgren.
The weight of the Frog Legs caster wheels was reduced by 33% and are over a half pound lighter. The design also passed Rehabilitation Engineering Society of North America (RESNA) standard testing. The company also suggests that the wheels can be used for a variety of moving objects, not just wheelchairs.
“Overall the efficiency of our product is now much higher,” Chelgren said. “We could see from the computer modeling what the benefits were going to be. I don’t think we could have developed a product nearly as good as the one we got without that assistance.”
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