Long Carbon Fiber Composite Lightens Wheelchair Caster Wheels 33%
People with physical disabilities who require wheelchairs for mobility literally feel every bump in the road as their chairs encounter obstacles as seemingly insignificant as cracks and uneven surfaces. A day filled with constant vibration and jolts from these hindrances causes fatigue and can produce spasms in riders that require treatment with medication.
While on a ski trip, a chance encounter with a quadriplegic rugby team and a late night discussion about their wheelchairs led Mark Chelgren to found Frog Legs and develop an alternative to the traditional rigid casters used for the front wheels on wheelchairs that allow them to make tight turns.
Instead of fixed forks that require the wheel to move up and over obstructions – bouncing the rider in the process – Chelgren designed his forks with a patented pivot point and wedge shaped shock absorber that allows them to move over impediments in an arc path, functioning much like airplane landing gear, smoothing out the ride.
“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.”
Testing performed by the Center for Industrial Research and Service at Iowa State University showed that 80% of the vibrations wheel chair riders experienced were created by the front caster wheels and that Chelgren’s design improvements reduced those vibrations from being transmitted to the wheelchair frame by 76%.
Twenty successful years later, Chelgren knew he and Frog Legs could do better. “We are always being pushed to lighten our product,” he said. “People with disabilities are greatly affected by any additional weight.”
“This time we looked at a different type of manufacturing process than machining aluminum,” said Chelgren. “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.”
“If the idea is to take an aluminum part and make it using carbon fiber, I think people are missing the boat,” he continued. “A carbon fiber part and an aluminum part shouldn’t look identical, instead you need to design for the material and process combined. It would be almost impossible to machine or forge the bottom part of our new fork design.”
Frog Legs’ second generation caster wheels make use of two different long carbon fiber reinforced composite materials supplied by thermoplastic composite technology firm PlastiComp of Winona, Minn. Machined aluminum has been replaced by a long carbon fiber reinforced nylon 6/6 composite in the wheel forks and a long carbon fiber reinforced thermoplastic polyurethane in the wheel hub.
PlastiComp custom developed the reinforced polyurethane composite to bond with the urethane used for the outer rolling surface of the wheel. “Using thermoplastic polyurethane for the hub allows us to obtain a chemical bond,” said Chelgren. “With an aluminum hub there are dissimilar materials that never fully bond and can slip.”
Changing from aluminum to long carbon fiber reinforced composites reduced the weight of a pair of Frog Legs caster wheels by one-third – over a half pound (280 grams) lighter – a reduction that will be noticeable and appreciated by riders.
The new design and materials passed RESNA (Rehabilitation Engineering Society of North America) industry standard testing that put them up against million cycle impact and drop forces. Frog Legs went on to test them to failure to have data to compare with traditional metal caster wheels.
“Then of course you have real world testing which effectively puts the product through the most extreme circumstances you can find to try and induce failure,” said Chelgren. “You are never 100% sure if something will work in the field differently than expected with regards to dynamic loading.”
“The difference with PlastiComp is they became our partner in making a better product.”
– Mark Chelgren, Frog Legs
Frog Legs took advantage of PlastiComp’s application design and performance analysis services throughout their product development cycle. “Overall the efficiency of our product is now much higher,” said Chelgren. “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.”
“In my opinion, the difference with PlastiComp is they became our partner in making a better product, it was more of a collaborative process – we weren’t going it alone,” Chelgren continued. “Someone should go to PlastiComp when they want more than just a sample and a ‘good luck.’ They have the technical engineering background and desire to work with people on a much higher level.”
Frog Legs’ shock absorbing caster wheels are not just a solution for wheelchairs, they can decrease vibration in all types of rolling equipment where there’s a weight versus performance equation, according to Chelgren.
Family-owned and operated, Frog Legs designs and manufactures its caster wheels in Ottumwa, Iowa. For more information, visit their website at froglegsinc.com.