ATP Series

Measuring a Power Chair

How Turning Radius, Seat-to-Floor Height & Other Measurements Impact the User Experience

measuring tape


With every power wheelchair that can be demonstrated for and test-driven by a client comes a slew of numbers. Width and length of the chair, of course. But also turning radius and seat-to-floor height.

How are these measurements defined and what roles do they play in a client’s ultimate success?

What the Numbers Affect

Mark E. Smith is General Manager, PR, for Quantum Rehab. He has also been a full-time power wheelchair user since childhood.

“When we consider power chair measurements,” Smith said, “they fall into three categories: anatomical, environmental and ADLs [activities of daily living]. Firstly, measurements in the area of seating and positioning must fit the client’s anatomical needs. Secondly, the power chair’s measurements must fit in such areas as a home and a vehicle. Last but not least, such measurements as seat-to-floor height must meet ADLs. Therefore, measurements are not exclusive to a particular area, but must meet the client’s requirements, with areas of measurements often overlapping.”

Turn, Turn, Turn

Smith explained, for example, the impact of the power wheelchair’s length can have on everyday performance.

“Turning radius in the power chair industry,” he said, “is historically used to express half of the turning diameter. This is based on power chairs having two motors, casters and a pivot point, as opposed to steering like a car. The net result is that the lower the number, the smaller the space needed for turning. This makes for an all-around more maneuverable power chair in all environments.”

The location of a chair’s drive wheels greatly impacts that turning radius.

“When it comes to the impact of drive wheel placement on a power chair and turning radius,” Smith said, “it’s solely based on physics. A mid-wheel power chair has a smaller turning radius because the pivot point for turning — that is, the drive wheel — is at or near center. You can have the same length of power chair in front-, rear-, and mid-wheel drive, and the mid-wheel drive simply turns in a smaller space.

“With that said, all power chairs experience a decreased turning radius — increasing maneuverability — as the overall length decreases. Nevertheless, maneuverability is only one part of the equation. A very rugged outdoor user may benefit from front-wheel drive, or an ultra-high-speed user may benefit from rear-wheel drive, so drive wheel placement is an important consideration for each client, as well.”

Ayron King, ATP, Power Sales Specialist for Sunrise Medical, said a change in turning radius can have a huge impact on daily tasks.

“You need an open floor concept at home for a larger turning radius,” he said. “Smaller turning radiuses are huge advantages in older homes that have narrow doorways and narrow halls, so you can take a 90° turn into a room. With a more open floor plan, you’ve got room for error.”

Staying Low

Seat-to-floor height can also have an enormous impact on the power chair user’s ability to interact with his or her environment.

“Seat-to-floor height is among the most vital measurements,” Smith said, noting its impact on both positioning and ADLs. “Seat-to-floor height is measured at the front and rear of the seat pan, noting the distance from the floor to the seat pan. This determines not just seat-to-floor height, but also posterior and anterior seat angle, if varied for positioning. Although seat-to-floor height is sometimes influenced by a client’s height, such as a very tall user, it’s typically based on transfer height needs and environmental access. Fortunately, with more common use of power-elevating seating, which results in power adjustable seat height, clients now often have a low seat-to-floor height in, say, the 17.5" range, and can elevate up from there for taller transfers and environmental access, dramatically increasing functionality.”

That range of heights is important, as many common environments require lower seat-to-floor heights, King noted.

“If you have an accessible vehicle,” he explained, “you need to be able to access the steering wheel without hitting your legs. If you don’t want to have to change [the user’s] whole work setting or have the school need to purchase a different kind of desk for the end user, you can order a shorter seat-to-floor height to accommodate their current environment as opposed to additional funding needs for environmental changes.”

And clearance for the wheelchair user’s legs isn’t the only consideration.

“[Seat-to-floor height] affects their armrest height, too,” King said. He suggested that a lower seat-to-floor height gives the user “more access under the table so they can go straight on instead of sideways. Getting leg clearance and armrest clearance allows them to get closer to their [activity] surface.”

Even a small change in height, King added, can cause big trouble.

“I’ve had situations where people have had to get an entirely new vehicle because their chair [height] was a half-inch taller,” he said. “It threw off their EZ Lock [wheelchair securement] system, and the ramp that was built into their van. It’s a really important consideration.”

So keeping the seat-to-floor height low can offer a number of benefits.

“For a shorter individual who can still transfer on their own, they need the capability of being able to flip their footplate back and place their feet on the ground,” King said. “It’s just that simple. Without the seat-to-floor height low enough, they would need a caregiver to transfer them out of the chair. Independence is the key.”

It’s sometimes possible to use a lower-profile cushion on the chair to help keep the user at a lower overall height, King added.

“You can make up for [a higher seat-to-floor height] with seat cushion thicknesses, but not always. Especially if they have a custom molded seat, you’re not going to be able to cut off any thickness.”

Every Measurement Matters

Perhaps the most critical idea to keep in mind with power wheelchair measurements is that even a small change or deviation can have a huge impact in the consumer’s ability to navigate through their days as efficiently and effectively as possible.

“It’s vital to never underestimate the impact of the length and width of a power chair,” Smith said. “Indoors, even one-quarter inch of excess width may prevent access to a restroom, for example, so ensuring that a power chair’s length and width fit the client’s environments is an absolute must.”

King agreed.

“The longer the base, the more turning radius is required,” he said. “If you go from a mid-wheel to a rear-wheel drive, [the new turning radius] might mean the user has to take their legrest off their chair so they can make a turn inside the vehicle to access their steering wheel, to get into their EZ Lock system, or to access their tie-downs.

“And this could cause another instance in which they need assistance.”

This article originally appeared in the October 2018 issue of Mobility Management.

In Support of Upper-Extremity Positioning