Simplified Strength Testing of Manual Wheelchairs (#1)

Many small manufacturers of wheelchairs throughout the world's developing countries do not have the resources and technical background to test their products for safety and durability. This paper suggests some essential tests which they can easily perform. The tests are based on fifteen years of experience with the Whirlwind type wheelchairs (which are manufactured and used in developing countries), and on the ISO International Standard for Wheelchairs.

The paper is intended for use by manufacturers in developing countries who are working to improve wheelchair quality.

Background
There are many small to medium size wheelchair manufacturing shops throughout the developing world. In order to successfully market their chairs and serve their customers these manufacturers must provide chairs that are safe, durable, and good performers, and that are producible at affordable prices. This is an enormous challenge where money and materials are very limited. Although many wheelchair designer-mechanics demonstrate a high degree of skill and innovation in building chairs with available materials and tools, they often lack the background -- either educationally or experientially -- to evaluate their designs for safety and durability.

In recent years, a standard for evaluating "Western" wheelchairs has been established. It is the ISO International Standard for Wheelchairs, dated 1997 (1,2). (Previously, it was the similar ANSI/RESNA Wheelchair Standards.) These extensive Standards cover many subjects including: A uniform description of the basic seating dimensions of chairs so that consumers and health workers can accurately place orders; ways of determining tipping stability; minimal strength requirements to assure that the chair will not bend or break due to occasional high forces; and standard methods for testing long term durability where the chair is subjected to the equivalent of a lifetime of riding on rough surfaces and being dropped.

There are several facilities in the United States and Europe equipped to perform the tests (3). Ideally, manufacturers in developing countries should submit sample chairs for testing at one of these facilities. However, the cost of transportation and testing is often prohibitive

Problem Statement
Manufacturers in developing countries need simplified standards for testing their chairs. The testing methods, and minimum specified values, need to be appropriate for their testing capabilities, and their customers' riding environments. They need to include at least the tests essential to preventing catastrophic or serious functional failures.

Tests and Testing Method 
Fortunately, the following essential strength tests can be performed by manufacturers using simple equipment and procedures. However, the durability tests, and some of the impact tests, specified in the ISO Standard require complex machines that small manufacturers cannot afford. The issue of durability will be covered later under Results and Discussion.

The tests, testing methods and minimum values have been developed by Ralf Hotchkiss (see Acknowledgements), and others, during the more than fifteen years history of Whirlwind-type wheelchairs. They are based on mechanical analysis, laboratory testing to ISO Standard, and extensive field testing by Ralf Hotchkiss and other riders. They found that a chair that passes these simple static or impact force tests is likely to pass the ISO tests. More importantly, it will also survive active use in rugged Third World riding conditions.

The diagram below shows the static and impact strength test forces a wheelchair of any design should withstand without breaking or sustaining permanent structural deformation. Tests which are in addition to those in the ISO International Standard for Wheelchairs, Part 8, or that have more demanding test values, are marked with an "*". The rest are similar, but not necessarily identical, to those in the ISO Standard. Some test load values are followed by an "e" to indicate they are estimates of dynamic values. The related Paragraphs in the ISO Standard are in brackets, [PX.X]'s. A description of each test follows its minimum test value.

All the tests can be performed using a tape measure, large square (e.g., a wood or metal sheet with a square corner), spring scale, and a lever-arm made of strong pipe for measuring moments. For impact velocities, the tester can practice propelling the chair at 1.5 meters per second by, say, learning to traverse 12 meters in 8 seconds -- a fast walking speed.

The lettered arrows (vectors) on the diagram show the direction and point of application on the test forces. A 91 Kg (200 lbm) active adult rider (either a dummy or willing person with a weight in their lap) is assumed.

To check for permanent deformations, before-and-after test measurements should be made using the tape to measure the distance from:

- Rear wheel-to-rear wheel at the front and rear (toe-in or toe-out). Better, back the chair against a flat wall. Use a large square and tape to measure perpendicularity of each rear wheel to the wall.

- Rear wheel-to-rear wheel at the top and bottom (camber). Better, set the chair on a flat surface and use the square and tape to measure the perpendicularity of each rear wheel.

- Caster axle to a rear-most point on the bottom of the each side frame. Measure when the casters are in the rolling-straight ahead position. 

- Foot rest mounting points on the side frames to rearward points on the frames.

- Side-to-side between side frames at the foot rest mounting points, caster pivot bearing barrels, rear of bottom frame, and push handles, and between the front and rear of the seat tubes.