How Does Carbon Fiber Measure Up Among Ultralightweight Choices?
- By Laurie Watanabe
- Jan 01, 2018
CARBON FIBER: ISTOCKPHOTO.COM/TIERO
Each August, Beloit College in Beloit, Wisc., distributes a new Mindset List, a collection of facts describing the perspectives and experiences of the incoming freshman class. Initially, the list was meant to help professors make more relevant references when communicating with their young students — for instance, the Mindset List notes that for students starting college this year, The Daily Show has always existed; nicotine has always been known as an addictive drug regulated by the U.S. Food & Drug Administration; and Hong Kong has always been part of China. But the List has become popular even among those far removed from their college days, because the factoids serve as a way to measure time and marvel at how things have changed.
Veterans of the complex rehab technology (CRT) industry — including those who remember when it wasn’t called CRT — can mark mobility milestones in a similar manner. For instance, CRT professionals currently serve clients with Post-polio syndrome, and those end users might have pushed steel-framed manual wheelchairs once upon a time.
Today’s self-propelled manual wheelchairs include ultralightweight (K0005) versions that seem only distantly related to those heavy steel behemoths. The ultralightweight segment has benefited from several space-age material advances, from high-quality aluminum used in aircraft and spacecraft to titanium and increasingly, to carbon fiber.
Which always raises the question: For ultralightweight chairs, which one is best?
As is so often the answer in CRT: It depends. All three materials offer wheelchair engineers (and, eventually, consumers) advantages and relative disadvantages.
For example, Jesus Ibarra, product manager for manual wheelchairs, Sunrise Medical, said of carbon fiber, titanium and high-end aluminum, “The main advantage of all of them is that they’re lightweight materials. Each of them has pros and cons: e.g., carbon fiber will get you the lightest weight, but overall, all of them are considered light weight.”
All three materials are also very strong.
“Each material has different strength values,” Ibarra said, “therefore giving you the ability to use different amounts of each material in order to achieve the desired strength.”
As an example, he compared a quantity of titanium with a quantity of steel that weighs the same amount.
“You have a titanium block that can weigh the same as a block of steel, but if you reduce the amount of material in that titanium block, you will not compromise the strength by going to less material. So for each material (carbon fiber, titanium or aluminum), its strength-to-weight ratio is very high. This allows you to use less of each material and get the same equivalent strength.”
The three materials also differ from an engineering perspective, said Eric Simoneau, CEO of Motion Composites.
“Not all materials are equal when comparing their density, mechanical strength and cost,” Simoneau pointed out. “A material might be highly efficient for certain designs, and could be just the opposite if used for another purpose.”
He described aluminum, for example, as a commonly available material, “offering good mechanical strength and easy to machine. It has one of the lowest densities of the metal alloys family. It needs less knowledge and experience to design and for fabrication than other compared materials.”
By comparison, Simoneau said titanium “is considered an exotic alloy and is a high-capacity material. It has a lower density than steel, and also offers extremely high mechanical strength. Its fatigue strength makes it a good material for wheelchair applications.”
And what about carbon fiber, the material that Motion Composites is perhaps best known for?
“Carbon fiber is also considered exotic due to its special manufacturing process and high material cost,” Simoneau said. “However, its density is lower than both titanium and aluminum, and its strength-to-density ratio makes it a top performer. Its characteristics can also lead to good energy absorption, making this material a good candidate for a comfortable wheelchair. Carbon fiber composites are highly resistant to fatigue and absorb high energy, which means it resists impacts. Moreover, carbon fiber composites are appreciated for wheelchair designing because the manufacturing process allows complex and optimized shapes.”
The costs — not only the initial costs of buying the materials, but also how much time and money needs to be invested to work with the materials — is another important factor that manufacturers consider when making their ultralight frames.
“Aluminum is strong, it’s easy to work with, it’s easy to manufacture — to bend, to form, to weld — so it’s the lowest cost out of all three,” Ibarra said. “Titanium is high strength, but is difficult to work with — bending, cutting, welding — so all of the aspects of manufacturing get more expensive, and [titanium] is more expensive than aluminum.”
Carbon fiber is lightweight, Ibarra acknowledged, “but it’s difficult to work with in tooling and manufacturing. You can’t make a carbon fiber tube and then bend it into shape. The tooling and material costs are higher than titanium.”
Finding Material Balances
Those practical issues, whether it’s the cost of a certain material or the material’s inherent strengths and weaknesses, currently mean that ultralightweight wheelchair manufacturers typically work with multiple materials, even if they have a reputation for preferring one over the others.
Consider, for instance, Permobil. Its ultralightweight manual chair brand is TiLite, named for and famous for its work and success with titanium. But Permobil still works with additional materials in creating its ultralight chairs.
Prashant Srinivasan is Permobil’s Global Product Manager for Business Unit Manual.
“Titanium is a very durable wheelchair frame material that has proven its value over time,” he said. “It has a non-corrosive, natural finish that doesn’t require paint or powder coat at all. It’s a finish that can look great for years and not show scratches from daily wear.”
A frame made of titanium, Srinivasan said, “retains shape and tension despite wear and tear over time due to better fatigue strength, and titanium’s dampening properties can result in a smoother ride for the client.”
Still, the manufacturer continues to work with and explore other materials.
“TiLite has also built chairs using magnesium alloys, and we are constantly testing and evaluating materials like carbon fiber and composites for frame components,” Srinivasan acknowledged. “While carbon fiber does have advantages in terms of weight savings, the way that it is applied in the market today means you lose much of the customization opportunity that we take advantage of using aluminum and titanium.
“We use carbon fiber on components today like backrests, sideguards, etc., to shave weight, but use titanium as the frame material that allows a custom-fitting wheelchair that provides an advantage in terms of functionality. An ideal marriage would be a blend of every technology that best suits the application of the frame or component. That said, one of the challenges that users face today is that reimbursement for chairs made with newer materials is limited, perhaps due to a fundamental lack of understanding by insurance as to how using chairs with these materials provides meaningful and comprehensive benefits to the users in terms of health and well-being, ability to perform tasks at work or just be better able to do things with family.”
The Importance of Weight
Weight differences can be significant and even extreme when comparing chairs in different segments — for instance, a lightweight chair vs. an ultralightweight chair. But by definition, every ultralightweight chair has an extremely light (and specifically defined) frame weight, so how does the choice of materials impact that already low number? And is frame weight sometimes an over-emphasized part of an ultralightweight chair’s functional success?
“While weight is important for function, the evidence doesn’t support that weight is particularly important for propulsion in a straight line,” said Tina Roesler, PT, MS, Motion Composites. “However, if you consider how a person uses a wheelchair in everyday life, they usually experience short bouts of propulsion and lots of stops and starts. It is in these tasks — stopping, starting, turning and lifting the chair — where weight does matter.”
Srinivasan said, “Frame weight is important, but we teach our referrals that fit is the most important aspect to the overall success of an ultralight chair. Our objective is to enable users with the best functioning product that fits correctly, as that is the greatest predictor of comfort and independence, which over the long term can reduce the likelihood of secondary injuries.”
While weight is probably the most-mentioned trait of an ultralight chair, Srinivasan said, “Weight is overemphasized in our market. The reality is all ultralightweight rigid frames on the market today are very light, and therefore the relative benefits of weights of different materials is very little. Fit and efficiency make the biggest impact to the client’s function and health.
“A chair can be lighter, but if it is poorly fitted to the user, it will feel less efficient than a heavier chair that is correctly fitted to the user.”
While the weight of the ultralight’s frame tends to get the most scrutiny, Ibarra said the components that go with that frame are just as critical.
“Considering that the frame weight is only 20 to 25 percent of the average total weight of the chair, the type of material you choose is just ounces of a difference,” he explained. “So it’s not a key success driver that [will determine if] you will have a highly functioning ultralight chair. The overall functionality/success will come down to the option selection. You can have the lightest carbon fiber frame, but if you start adding footplates, wheel locks, aluminum backrests, etc., the overall weight of the chair will be way different.”
“We see the accessories as the more important factor of weight because the components that are added to the frame are generally what make a chair drastically heavier than needed,” Srinivasan said. “We actually have our education managers travel with what we call a ‘naked’ frame to make that point. It’s just a frame with no components attached, and referrals are shocked when they pick it up because it looks like an almost complete chair. It really gets the audience thinking about the accessories chosen like wheels — or even backpacks loaded up with things — and how much they add to the overall equation.”
Admittedly, the one area in which an ultralight’s weight is key is during transfers, such as when the ultralightweight end user is lifting the chair into and out of his or her car several times a day.
“The biggest effect I see where weight of the chair is very important to consumers is with transfers,” Ibarra said. “Picking it up and moving it vs. sitting in the chair and riding along.”
Said Simoneau, “An ultralight wheelchair will shine mostly during transfers, because users might have to lift their wheelchair, and every pound of weight saved will matter. Weight is an easily quantified attribute and serves to improve the overall performance of a wheelchair. Every pound saved means less energy required to propel the wheelchair, and [could] reduce the user’s injuries due to the propulsive effort.”
Propulsion as the End Goal
Rather than focusing too intently on an ultralight chair’s weight, the focus should be on propulsion efficiency, Simoneau said.
“Weight is an important aspect, but the main objective of an ultralight wheelchair is to achieve a high propulsion efficiency, which is what changes lives of users,” he noted. “A light wheelchair is not necessarily efficient at propulsion, but an efficient wheelchair won’t be a heavy chair! Efficiency can be achieved with a combination of characteristics, like weight, well-designed structures and ergonomics to allow the user to travel forward with the least energy possible.”
“From a functional and clinical perspective, the ability to dial in the configuration can be more important than the weight,” Roesler agreed. “While it is nice to have a lightweight wheelchair, the configuration and the distribution of the weight are more important. For example, weight added to the back of the chair (such as backpacks) will have more negative impact than weight under the seat when the wheelchair is configured correctly. So, the adjustability and customization of lightweight chairs is as important, if not more important than weight alone.”
Asked what other characteristics could make or break an ultralightweight chair’s functional success, Ibarra said, “Frame stiffness, which is tied to the efficiency of the push, and end-user positioning in the chair, which is going to affect how the chair functions/maneuvers.”
“We strongly feel that fit and balance are key considerations because they impact the user in huge ways,” Srinivasan said. “The better the fit is for the user, the better balanced they will be in the system. There are many factors that contribute to good fit, but being in control of every aspect of that fit is critical to optimizing the functionality.
“Once these concepts are learned, they can be applied to any manual chair frame, including a folding frame that is less customizable. We are trying to educate the market on a regular basis about these concepts because many are still misunderstood. The degree to which you can control, understand and apply these concepts is the key variable. That is why we have chairs that can be adjusted for optimal fit or built to an optimal fit without adjustments that can both be successful.”
Beauty That’s in the Eye of the Consumer
While high-grade aluminum, titanium and carbon fiber all can offer great strength at low weights, titanium and carbon fiber especially are also known for their showy good looks. So how much attention do manufacturers pay to the appearance of their ultralightweight wheelchairs?
And are there other characteristics or traits that should be considered just as critical to ultralight success?
“Weight and appearance are indeed two important factors,” Simoneau said. “A light wheelchair will generally offer more performance, and a well-designed product will be visually more attractive. Other very important factors are propulsion efficiency, comfort and durability. Lightness should not be a compromise with quality of wheelchair components, but being a result of innovation and optimization of overall assembly!”
Ibarra said he agrees that weight and appearance are important to consumers, and thereby worthy of manufacturer attention. “But it also comes down to the type of user that’s acquiring the chair,” he noted. “If you have a very active user (two to four years after injury) that is completely dialed in on his needs (chair configuration), then weight and aesthetics will be their main concerns.
“But for a [newly injured client], where he still needs to review between going to a rigid frame or a folding frame, [wheelchair] adjustability will be an additional factor.”
“We do believe that appearance is highly important, as it appeals to the user, and they use it every day,” Srinivasan said. “But that also doesn’t mean having 52 colors to choose from. We see clients often choose the same groupings of colors and material finishes that go well together. It’s about the package look and how well the system looks as a whole that allows them to feel like they’ve personalized the chair to their style.”
In the end, perhaps an ultralight owner’s overall happiness with his or her wheelchair is less about weight or a certain frame material than about functional success — the chair’s ability to perform well because it fits its user properly and is ideally equipped with the right components and accessories.
“It is important that the client is the center point when they enter a room, and not the equipment,” Srinivasan said. “You see the same things in high-end auto manufacturers: They spend a lot of effort putting packages together that all blend nicely. They sort of do the work for the consumer while still offering a personal touch. So, while weight and a chair’s appearance should get attention, the focus should be on how to achieve the best possible fit and function, allowing users to prevent upper-extremity injuries to shoulders, wrists and elbows over time while using the chair.”
“In the end, the consumer’s opinion is the most important,” Roesler said. “Clinicians need to work with the client to be sure all the needs are met, including appearance! If someone feels good and looks good in their wheelchair, they are more likely to use the chair and function at a higher level. We really need to relate everything back to client-centered prescription.”
This article originally appeared in the January 2018 issue of Mobility Management.