Perhaps the buzziest engineering material in the always-evolving ultralightweight manual wheelchair space is carbon fiber. Very lightweight, very strong and very attractive to look at, carbon fiber has often been associated with racing vehicles, where speed, light weight and sleek looks go hand in hand.
In complex rehab, where light weight can translate into such significant benefits as easier transfers and transportation, carbon fiber is being used not just for ultralight chair frames, but also in components, such as backrests.
Carbon Fiber Composites
Todd Hargroder of Accessible Designs Inc. (now part of Stealth Products) has worked with carbon fiber for years in wheelchair applications and is a long-time ultralightweight chair user.
“A buddy of mine was building a dragster, and he had made some panels out of carbon fiber,” Hargroder said. “That was my first exposure to it. I saw how pretty and how cool it was, and how lightweight it was. I made a backrest out of it, and just used the mounting hardware off of my old backrest that I had.
“The shell was great, but the mounting hardware was quite ugly, so my next step was to design mounting hardware. After putting the two together and going to a show, that pretty much launched a product line.”
As for working with it, Hargroder said, “Carbon fiber is a composite. It’s a flexible material until it’s mixed with resins or other materials, and it’s all about the mixture. You can make it flexible and bendable, or you can make it as stiff as can be. It’s a recipe that you have to work with depending on your application. It can be flat, it can be round. It all depends on how it’s molded and how it’s used. It can certainly be as strong [as aluminum or titanium] depending on how it’s laid up, how it’s used, how it’s engineered.”
He compared working with carbon fiber and finding the optimal blends to cooking. “It’s all about the recipe, how much of this, how much of that, and then how it’s cooked, the temperatures,” he said. “There’s certainly an art to composite manufacturing.”
Among the traits that Hargroder appreciates is carbon fiber’s literal flexibility.
“The great thing about carbon fiber is it is a composite, so you can have it flexible, or you can have it stiff,” he explained. “A backrest made out of aluminum is just made of aluminum. Aluminum is a dead, hard material, and it feels like that. A composite carbon fiber backrest will have some flexibility. We’ve designed flexibility into our backrest so it does absorb vibration, it does take away shock load, it moves with the body much better than a dead, metal back support.”
Hargroder also likes being able to adjust a carbon fiber “recipe” to attain the characteristics he wants.
“With the makeup of the composite back, you can add more material, you can add more layers of carbon, you can add different types of resin to make it stronger or more flexible — whatever your goal is, whatever you’re trying to achieve,” he said.
Still, while Hargroder has achieved much success from carbon fiber designs, he acknowledges the usefulness of different materials in complex rehab situations.
“Most of our mounting hardware is made of aluminum,” he pointed out. “We’ve done some composite mounting, but the majority of our mounting is made of aluminum.
“Aluminum is easier to work with, because you can just put it on a machine and mill it out. Carbon fiber certainly takes a lot more involvement. It all depends on your application. Some things are better made out of aluminum, and some things are better made out of carbon.”