Ultralight Research, Part I

Manual Chair Propulsion: New Insight Into Maneuvering Efficiency

Manual Chair PropulsionManual chairs are mechanical systems just like bicycles. Wheelchairs and bikes have several things in common. Both are propelled by the occupant or user, who weighs more than the mechanical system. This means that the occupant has a great influence on the forces imparted onto the vehicle and on the forces required to propel the vehicle.

Conversely, many significant differences also exist between bicycles and wheelchairs.

A wheelchair is much less efficient than a bicycle. This means that the occupant must expend more effort to perform the same task. This requirement of greater propulsion effort can have consequences for wheelchair users.

Greater effort can lead to difficulty in achieving desired speeds, a higher probability of fatigue over long bouts of mobility, and difficulty negotiating inclines. Over time, the accumulation of this greater effort can increase the potential for injury in the upper extremities.

One of the reasons for this difference in effort lies in the means of propulsion. Bikes use cranks and often have gears to increase the mechanical advantage. Another reason is due to turning. A bike is turned using handlebars that the operator can easily turn while pedaling. In distinction, a wheelchair user turns by propelling forward on one wheel while slowing down the other wheel. This is obviously inefficient because the user is expending energy and slowing down simply to change direction. However, this type of differential steering makes the wheelchair much more maneuverable than a bike. You could never maneuver a bike with the same precision as a wheelchair.

Inertia, Friction & Wheelchairs

Propulsion effort, whether on a bike or a wheelchair, is dictated by two principals of physics: inertia and friction.

Inertia is inherent to all things with mass and reflects the resistance to any change in motion, including changes in speed or direction. Your body weight is representative of your translational inertia. For a wheelchair, this translational inertia is due to the occupant’s mass and the mass of the entire wheelchair system comprised of the base (frame, two drive wheels, two casters and two caster forks), seating system (cushions), and accessories (headrest, backpack, supplies).

Maneuvering a wheelchair also requires overcoming the inertia due to turning and the rotational inertia of the drive wheels and casters. Therefore, one can think of a wheelchair as a system with translational, rotational and turning inertia.

You’ve probably heard of Newton’s First Law of Motion: A body at rest stays at rest, while a body in motion stays in motion with the same velocity — until acted on by an external force. Well, for wheelchairs (and bikes) the external forces that influence (or impact) the ability to change velocity are the propelling forces and friction. Friction is the force that slows down a wheelchair. It is ever present and is dependent on a number of factors, most significantly the weight on the wheels, the type of wheels and tires, and the surface upon which one is rolling.

Given this background, one can deduce that many choices that are made about wheelchair components and configuration affect both friction and inertia.

Frictional forces are influenced by the mass of the occupied wheelchair, the mass distribution (the amount of weight on the drive wheels and casters), and the types of wheels and tires that impact rolling resistance and tire scrub. Rolling resistance is the frictional force that slows down a rolling wheel. Without this friction, the wheels spin on the surface, so we need friction to move about.

Tire scrub is the frictional force that occurs at the tire-surface interface during turning. Front casters will scrub as they swivel into alignment with the path of travel. In distinction, the drive wheels will scrub throughout the turn so, in fact, may have a greater frictional influence on the effort of propulsion.

Inertia of the wheelchair system is also influenced by the weight of the occupied wheelchair, the weight distribution and the rotational inertia of the drive wheels and casters. When initiating wheelchair movement, one must exert greater force (i.e., perform more work) compared to that required to keep the wheelchair moving.

You have experienced this every time you ride a bike. Think about how hard you have to pedal while starting compared to how hard you pedal to maintain a certain speed.

This explanation should leave you with the first two important points of this article:

1) Decisions made about a wheelchair and its configuration will impact either inertia or friction, and most often, will impact both. So, by extension, decisions made about a wheelchair or its configuration impact propulsion effort.

2) Maneuvering a wheelchair throughout the day involves a series of starts, stops and turns, and these changes in velocity require more effort than that needed to keep moving in a straight line.

We measured how full-time wheelchair users move about during everyday life. The data from 68 wheelchair users consists of about 60,000 bouts of activity and shows that people move in short bouts with 85 percent of bouts lasting less than 60 seconds. Just as importantly, wheelchair users perform, on average, 90 bouts of mobility every day, with each bout involving accelerating the wheelchair. Some users perform over 200 bouts of mobility a day. This fact served as our motivation for developing a system that was capable of measuring forces during changes in momentum — a change in momentum is a change in the speed or direction of travel.

Two Manual Wheelchairs, One Robot

Measuring propulsion effort of wheelchairs during maneuvers is not a trivial task. One must measure the torque applied to the wheels during freewheeling maneuvers, something that treadmills and rollers will not allow. One must also measure a wheelchair without changing its inertia or frictional parameters, so adding mass to the chair or inertia to the wheels will not suffice.

Therefore, a wheelchair-propelling robot was designed at the Rehabilitation Engineering Research Center on Wheeled Mobility (mobilityRERC), a project funded by the National Institute of Disability and Rehabilitation Research.

The robot was designed with anthropomorphic characteristics in order to replicate the loads imparted by the occupant onto the wheelchair. It was designed to mimic the 50-percent male in height and the 95-percent male in mass or about 100 kg/200 lbs. The robot propels the wheelchair by applying forces to a special geared handrim using motors. We are able to program certain maneuvers and have very precise control over the acceleration, speed and direction. Therefore, the robot offers the ability to measure propulsion torque during wheelchair maneuvers in a very repeatable and accurate manner.

To illustrate the influences of wheelchair design and configuration on the effort to maneuver a wheelchair, the robot was programmed to perform both straight and turning maneuvers. These maneuvers were designed to highlight the varying inertial and frictional influences that govern propulsion effort. The straight maneuver consisted of accelerating the chair to 0.7 meters/second (m/s) in 2.5 seconds and then continuing at that speed for another 2.5 seconds. The turning maneuver was a fixed-wheel turn in which one wheel is driven forward while the other wheel is fixed in place. This maneuver accelerated the wheelchair to 0.5 m/s in 2.5 seconds and then maintained the trajectory for another 2.5 seconds. Therefore, both maneuvers had an acceleration phase and a steady-state phase. Furthermore, these maneuvers were performed on tile and low-pile carpet surfaces.

We selected two wheelchairs that represent disparate designs and configurations.

The standard folding-frame wheelchair with 7" front casters, 24" drive wheels and 1" solid tires had a mass of 17.6 kg (i.e., about 39 lbs.), and when loaded with the robot, had about 55 percent of the system mass loaded onto the drive wheels.

The second chair was an ultralightweight rigid-frame wheelchair with 5" casters and 24" wheels with 1 3/8"-wide pneumatic tires. It had a mass of 12.1 kg/26-plus lbs. with 70 percent of the system mass on the drive wheels. The specific configurations, including the type of tires and mass distribution, are very important to consider when comparing systems. Our methods include complete measurement of the inertias of all components as well as the frictional parameters as measured using a simple coast-down test. These parameters will be discussed as we review the results of the study.

The results are shown in Figures 1 and 2. The values in the figures are the torques required to either accelerate the chair or to maintain the respective speed during the maneuver. Each graph depicts the torques for both phases on tile and carpet. One graph illustrates results from the straight maneuver, while to the other is for turning. Torque is reported in N-m and reflects measured work.

Ultralight vs. Standard Torque

Editor’s Note: See the January issue for Part II of this series.

This article originally appeared in the November 2014 issue of Mobility Management.

About the Author

Stephen Sprigle, Ph.D., PT, is a professor of Applied Physiology, Bioengineering & Industrial Design at the Georgia Institute of Technology in Atlanta.

In Support of Upper-Extremity Positioning