Clash of the Titans

Aluminum vs. Titanium

For Ultralightweight Chair Users, Is There a Clear Winner?

• By Laurie Watanabe
• Mar 01, 2010

In the chatroom entry, a consumer is asking for advice on aftermarket tires for his new ultralightweight wheelchair. While tire brands are his main question, he pauses to take a swipe at his new ride. He mentions the brand and model of the chair, which is aluminum.

“That’s the best I can get out of Medicaid,” he typed. “I wanted titanium.”

If you’re a complex rehab provider or clinician who works with ultralight chair users, you’ve probably heard that comment over and over. You may even hear it from industry professionals: “We want titanium!” But when aluminum and titanium go toe to toe as today’s materials of choice to create ultralight chairs, is one material clearly, truly, always better than the other?

Introducing Our Contenders

Titanium’s discovery in the late 1700s is commonly credited to German chemist Martin Heinrich Klaproth, who named the metal after the Greek Titans, a clan of powerful gods and goddesses (including Zeus’ mom and dad, Rhea and Cronus). Among ultralight users today, titanium continues to get the royal treatment. In other observed chatroom conversations, ultralight chair users described titanium as “sexy” and – in one humorous case – a “manly man” metal. Adding to titanium’s seemingly elite status is its higher cost and its relative rarity, at least compared to aluminum.

Aluminum, in contrast, is earth’s most abundant metallic element, according to the Mineral Information Institute. While titanium’s name was inspired by deities, aluminum’s origin is simpler. It comes from the Latin word alum, a kind of mineral, in combination with the “-ium” suffix commonly given to elements named in that same time period (the 1820s). It’s “aluminum” in the United States, but “aluminium” in Britain and other parts of the world.

Once upon a time, aluminum was called the “space-age” metal – back in the days when steel was the other material choice for wheelchairs. But that anecdotal honor has since been taken over by titanium.

So is titanium merely getting the credit it deserves, or is aluminum perhaps underappreciated and misunderstood as a medium for ultralightweight chairs?

We took our collection of assumptions – widely held and/or anecdotal beliefs about titanium and aluminum – to a group of industry experts and asked their opinions. Participating in our “search for truth” are…

• Jim Black, Invacare Corp./Top End
• Mike McCarthy & Mike Zablocky, Pride Mobility Products
• Tom Whelan, Sunrise Medical
• Josh Anderson, TiLite

Currently, Invacare, Sunrise and TiLite all offer both aluminum and titanium ultralight chairs. Pride does not currently have a titanium ultralight in its lineup, though Mike McCarthy says, “Titanium is definitely something that is talked about internally, in addition to some other hybrid materials.”

Assumption #1

Titanium Is Lighter

Perhaps the most common rationale for choosing a titanium chair over an aluminum one is weight: Titanium chairs have historically been (or have been presumed to be) lighter in weight than aluminum chairs, and for the consumer who self-propels many miles in a lifetime, having to push a chair that’s heavier than necessary can become a significant physical burden.

So our first question: Is titanium lighter than aluminum?

Answer: No… and maybe.

“The perception out there is that titanium is lighter,” says Jim Black. “Titanium is not lighter than aluminum.” As a raw material, titanium actually weighs more than aluminum, though both metals weigh considerably less than steel.

“Aluminum has one-third the density of steel,” says Mike Zablocky. “Titanium is half the weight of steel, so because it has a lower mass, it can be just as strong (as steel) at half the weight.”

Then why are titanium wheelchairs so routinely assumed to weigh less than aluminum chairs if titanium weighs more than aluminum? Chalk that up to the metals’ different properties and how they’re used in the ultralight wheelchair manufacturing process.

Imagine that a wheelchair manufacturer is creating two identical ultralight frames – one aluminum, one titanium – and wants to make each frame as strong as a steel counterpart would be.

“You need more aluminum to equal the same amount of (strength as) titanium,” Zablocky explains, noting that with titanium, “you can use thinner wall tubing to yield the same amount of strength.”

So aluminum as a raw material weighs less than titanium, but because larger amounts of aluminum have needed to be used, titanium chairs have historically been lighter in weight than aluminum counterparts.

“You just have to use more aluminum to get to your outcome,” Mike McCarthy says, “whereas with titanium, because of the strength capabilities, you can use a thinner wall material. I think that’s where the perception issue arises.”

But as manufacturing processes and wheelchair designs have evolved, Tom Whelan says, it’s no longer a given that a titanium chair will be inherently lighter than an aluminum one.

“In the past, all the lightest-weight chairs were titanium,” Whelan acknowledges. But he references the bicycle industry: “The parallels are just compelling, because 90 percent of what you see in wheelchair technology as far as base material and material management flows from the bike industry. As aluminum bike frames have gotten lighter and stronger, they’ve caught up with titanium.”

And now, Whelan says, evolving manufacturing techniques are making it possible to work with aluminum in ways that weren’t possible before. “There’s a technique called hydroforming, where you can make an aluminum tube where it is thinner and thicker in the walls along the length of the tube. I might need a slightly thicker wall where I have to join two tubes with welding, but in the length of the tube, I might not need that wall thickness.”

This ability to vary wall thickness can allow manufacturers to use more aluminum only where they need to, and reduce the amounts of aluminum they use in other areas. “Titanium can’t be hydroformed,” Whelan says, “so (it requires) a uniform wall thickness throughout. That’s a technique that’s helped aluminum catch up….You just can’t make that claim anymore that titanium is lighter, therefore it’s going to be a better chair.”

Assumption #2

Weight Is Everything

This one is closely related to the first assumption that titanium makes for a lighter chair. Assumption #2 is that weight is extraordinarily important to the ultralight chair user, to the degree that even shaving a pound or two from the chair’s weight will make all the difference in the world.

So is it true that weight is all important? Our experts largely agreed: Yes for one particular situation, but for most other applications – no.

Let’s start with the situation for which the answer is yes.

Unlike most other wheelchairs, ultralights are routinely transported in the passenger and back seats of cars, often by users who transfer from their ultralights into their cars, then haul their chairs in after them. For transportability – when users or caregivers are hefting chairs into vehicles – the weight of the chair is key.

“The only argument about (weight) that makes sense and that you’ll hear from therapists is the lifting weight,” Black says. “If someone is pulling it in and out of the car, that’s important.”

Beyond transportability, however, our experts didn’t buy the simple “weight is everything” argument. Instead, they focused more on judging a chair’s overall performance, including the quality of its ride and how efficient it is to propel.

Aluminum and titanium both have inherent characteristics that have made the metals favorite choices among engineers.

“What makes aluminum a good material for building a wheelchair frame is that it is very lightweight and strong,” Josh Anderson says. “It is also very stiff, which is good because it maximizes your energy transfer – energy transfer is the movement of energy from one place to another, in this case, from your body to the wheelchair. Doing this as efficiently as possible means that more of the energy you expend goes into moving you forward.”

Anderson calls titanium “the ideal metal for ultralightweight manual chairs,” noting, “The titanium alloys used for wheelchairs were developed for the aerospace industry and have some of the highest strength-to-weight ratios of any metals.” He also lauds titanium’s vibration-dampening abilities: “Most metals transmit vibrations, but titanium absorbs vibrations. This means that a titanium chair will provide the smoothest ride. There are at least two benefits to the smooth ride. First, a smoother ride is more comfortable for the user. Second, the less the chair bounces around, the more efficient the propulsion. And the more efficient the propulsion, the less fatigued the user will be.

“In part, this vibration-dampening effect is due to the fact that titanium is not as stiff as some other metals, such as aluminum, which is often why you see larger-diameter tubes on titanium cantilever frames. That largerdiameter
tube has more surface area and gives a titanium frame that needed stiffness to maximize energy transfer.”

Overall, the experts found characteristics to praise in each metal, even while differing as to whether titanium or aluminum gets the edge in ride quality.

Black, who has worked internationally with elite wheelchair athletes, says, “If you look at the sports products of the world, there’s not a titanium sports product that any world-class athlete uses. It’s all aluminum. And the reason for that is aluminum you can make lighter, and aluminum you can make stiffer, so it has a more responsive, energy-efficient ride.”

As an industry, Black adds, “We’re so concerned about weight. Weight is relevant, but it really has nothing to do with how the chair performs. So when you’re in the chair, especially these new openframed designs, these monotube frames that everybody has now, they flex so much that you lose energy. And when you put it in titanium, they even flex more, so you’re losing energy. That means you have to spend more time on your handrim, and you’re pushing longer. So the reality is you’re losing your rollability of the product.”

Assumption #3

Aluminum Rides Better

Does that mean that aluminum actually holds an ultralight performance advantage over titanium?

While Black says world-class wheelchair athletes don’t choose titanium chairs for competition, he also adds, “The design has more to do with it than anything. So a box frame or a frame that has some triangulation or welds that come together as opposed to being open is a better choice when you choose titanium, because titanium moves so much.

“One of the things that other industries do, say the medical industry when they replace joints – they use titanium because titanium has a lot of the same properties as bone, for example. So it moves and twists, and your bones are moving and twisting. But in a wheelchair, you want something that’s going to be stiff and hold the energy and make it more responsive. That’s why aluminum’s probably the better choice, and it’s less expensive. I’d rather put more money into the components of the chair than put it into the material of the chair.”

Speaking of components, when it comes to ride quality, Black suggests taking a closer look not necessarily at the metal making up the frame, but at caster and wheel choices, such as “a rear wheel that’s stronger and has less flex so you’re more efficient.”

As for titanium’s advantage in the vibration dampening department, Black says, “If I was being pulled behind a car every day at 30 to 40 mph, it would make a difference. But that’s not the case. Justification (for an ultralight) should be designed around the options you choose on a chair.”

On the subject of whole-body vibration, Whelan says, “If you’re traveling over a rough road surface, you’re transmitting shock in the form of vibration into the chair from going over pavement that has gravel embedded in the pavement. Now the chair is seeing this vibratory shock. It’s an oscillation that’s going on, and that vibration is being absorbed by the body. There’s very clear evidence that supports that wheelchair users will suffer from whole-body vibration. The primary negative effect of absorbing that shock is fatigue.

“The issue of whole-body vibration is an interesting issue to study. For the titanium material to have a positive effect, it has to have a relationship to the frequency and magnitude of the shock. If the vibration was a certain frequency, the material might actually amplify the vibration. If it has a certain relationship, it might attenuate the vibration.”

Whelan says an ongoing challenge for the assistive technology field is to be able to scientifically measure outcomes such as wheelchair ride and performance.

“If you had two chairs that were exactly identical in design and the only variable was the material, then the material might make a difference,” he says. “Then your question becomes how substantial is the difference that it makes.… We have to deal with ‘What is the measure that predicts performance?’ and ‘What are the performance characteristics that are important to it?’”

Whelan adds that a range of factors – including a chair’s weight, setup, wheel and tire selection, and rigidity – all contribute to how well a chair rides and how efficiently it can be propelled.

“What has never existed in our industry and today still doesn’t exist is an actual way of comparatively measuring a chair’s propulsion efficiency,” he says. “There’s technology like the SmartWheel, where I can put a wheel on a chair, and I can put a user in a chair, and I can start theorizing that if I make changes to the chair or compare two identically set-up chairs with identical wheels and tires, that using the output of the SmartWheel, I might be able to start comparing that.” But Whelan adds that conducting such a test would assume “the person is able to put the same, identical energy into the systems for that comparison. I haven’t met the perfect human yet who can sit and push two chairs with the identical same force two times to create that measurement.”

Whelan is enthusiastic about an ongoing Mobility Rehabilitation Engineering & Research Center (RERC) project at the Georgia Institute of Technology that could help provide answers. “The RERC which is at Georgia Tech has instigated a project called AMPS (Anatomical Model Propulsion System),” he says. “They’re going to build a functional robot that can sit freely in a wheelchair and propel it.”

That robot could enable a researcher to “remove all the variables and literally compare two chairs as far as their efficiency,” Whelan says.

Assumption #4

Titanium Is Stronger…

This is one of the few more straightforward questions about the two metals.

“Titanium does have a better strength-to-weight ratio than aluminum,” Whelan says. In a practical sense, that can equate to better durability and easier upkeep for consumers.

“Titanium does not corrode or rust,” Anderson points out. “Therefore, titanium does not need to be painted like aluminum to protect it from the elements. If you scratch it, you can simply buff the scratch out with a Scotch-Brite pad, like the kind you use to clean your dishes. That makes keeping a titanium chair looking like new much easier than any chair that needs to be painted.”

From an overall usage standpoint, Anderson also notes that titanium “unlike aluminum, does not work-fatigue. Work-fatigue means that over time, the material becomes brittle. For this reason, eventually all aluminum frames will become prone to breaking. This is why aluminum parts on airplanes must be replaced at specified intervals. Because titanium is so strong and does not work-fatigue, durability is greatly improved.”

Whelan agrees titanium has greater fatigue strength, but questions the practicality of it: “If your goal in design was to build a wheelchair that would be very, very light and last forever, you would lean towards titanium, because the fatigue strength of titanium can be greater than the fatigue strength of aluminum. However, let’s face it: No wheelchair user wants a chair that lasts forever, because they want to benefit from changes in design and changes that go on in the industry.

“There are very few wheelchair frames out there more than 10 years old. So you are always in design identifying a life cycle that you design to. And exceeding that life cycle may not have any real value.”

Assumption #5

…But Aluminum Is Easier to Work With

For all its strength, titanium requires careful handling in a carefully controlled work environment.

“When you’re working with titanium, it is far more challenging than aluminum,” Zablocky says. “The bending and grinding and welding of that thinner tube and that wall thickness is a very delicate process that requires some skill. That’s not the case with aluminum – it’s very easy to weld.”

Environmentally, titanium “is very susceptible to contaminants during the manufacturing process,” Zablocky adds, “and you can end up with a term they call embrittlement. So if you don’t take care, you can end up with a stress point that will fracture because of poor manufacturing. It’s a very thin wall, and it’s a very delicate process. It’s definitely more difficult to fabricate.”

But cosmetically, Zablocky says, titanium gets the edge. “If you weld it properly and you handle it and it does come out right, you get a much smaller weld,” he explains. “It’s very clean, while typically with aluminum, because the wall thickness is bigger, you get a slightly larger weld bead.”

The message here may be that providers and clinicians who recommend and sell titanium chairs should work with manufacturers who are experts in handling the metal.

Says Anderson, “I don’t think any other frame material can match its ride quality, durability and performance. However, it is difficult to manufacture, so the company producing a titanium chair needs to have some specialty knowledge when it comes to welding and putting the frame together.”

And the Winner Is…

Five assumptions, plenty of praise for both metals, and possible advantages for each. So is there a clear “winner” between titanium and aluminum?

Says McCarthy, “Having come from nearly two decades on the provider side, as a former practicing ATP, I’ve had consumers that have ridden both types of chairs, one constructed of aluminum and the same consumer riding a chair that’s constructed of titanium – with probably unnotable changes in ride characteristics.”

He calls the two metals “comparable” in their ultralight results, adding, “I believe in my heart of hearts that titanium and the use or the application of titanium in any instance – whether it be manual mobility, custom mobility or shopping carts – it’s definitely a perception issue. I think if I were to blindfold an end-user and put them in identically configured rigid chairs, one constructed of titanium, one constructed of aluminum, they would not be able to tell the difference.”

In fact, about many consumers’ vehement preferences for titanium, he says, “I think I would get different opinions if I were to poll consumers – two consumers, one of which was recently injured and one that was 15 years post-injury. I think that if I were to put them in two separate rooms, I would get two totally different responses. Titanium hasn’t been around that long, number one, and it is a perception thing. I think the word titanium is an exciting sounding word. I think for these younger folks that are recently injured, it’s something that they may ride during their rehab stay.”

Black, in his work with Top End, frequently speaks to wheelchair users at consumer events.

“Everybody talks about weight, and it just consumes our decisions,” Black says. “It just consumes them. It’s evident in the bicycle industry, too, because they talk about one or two lbs. difference. (Titanium) is thousands of dollars’ difference in cost, but what really true advantage do you have? The reality is on flat surfaces, there’s no true value. It’s all in the wheels and the components on that bike. It doesn’t have anything to do with the material.”

In fact, Black suggests, while weight is undeniably important, choosing the best ultralight for a particular client is much more complex than simply choosing one metal over the other or selecting the lightest chair available.

“You can lose five lbs. and be more efficient,” he says. But he believes the real significance comes from “how that chair is square, and how you’re fit into that product. In my opinion, that’s the frontier of the industry: Where our dealers and our clinicians have a lot of strength is being able to sell the concept of a proper fit. It’s way more important than the product itself.”

“If you really want to solve problems,” Whelan says, “you look at design first and materials second. I know I keep coming back to this, but I think it’s a very important point so we don’t mislead the industry into thinking that material is a magic bullet.”

Whelan, like the other experts interviewed for this story, says aluminum and titanium each has its advantages.

“There is no way you can say that one is clearly better than the other, especially if you bring cost into the mix,” Whelan says (see sidebar on Aluminum v. Titanium: The Funding Question).

“Titanium has inherent properties that may be taken advantage of through design to be better than aluminum, relative to vibration. Aluminum may actually have advantages, especially if you consider costs, relative to stiffness.

“A stiffer chair is going to be easier to propel, but it’s going to likely transmit more vibration. A chair that transmits less vibration is inherently going to absorb more energy that was meant for propulsion. It’s ‘What’s your design intent, what are you looking for?’ If I’m an end-user who lives in an environment where I’m primarily on hard, smooth surfaces, I may want propulsion efficiency and not worry about vibration.”

Whelan agrees that ultimately, the question of “Which metal is better?” might be better phrased: “I think the other aspect of this is ‘Who’s the user?’”

Anderson leans toward titanium for its performance qualities, but for certain applications, he says the field is more level. “As far as transfers and portability, in my opinion both of these are more a factor of design than the actual material of the frame.”

He, too, notes the need for ultralight chairs to fit the needs of individual clients. “These are custom-made ultralightweight wheelchairs and should be thought of more as a prosthetic. An amputee can’t just take someone else’s leg and put it on and expect it to work perfectly. The same holds true with this class of wheelchairs. The user’s ability to transfer or the portability of the chair will be dependent on the user’s abilities, the design or shape of the frame, overall weight and a variety of other factors. It is really not dependent on material.”

The “battle” between aluminum and titanium seems to be too complex to say that one metal is always the better choice for all clients and all applications, and the experts instead urged providers, clinicians and consumers to view ultralightweight wheelchairs as the comprehensive seating & mobility systems they are.

“The chair is the soul of the person,” Black says. “That whole thing has to work as one for it to work correctly.”

This article originally appeared in the March 2010 issue of Mobility Management.