Simplified Strength Testing of Manual Wheelchairs (#2)

Test Values and Descriptions
The order in which the tests are generally listed is determined by the clockwise progression of the lettered force vectors around the diagram. Please note that this is not necessarily the order in which the tests should be performed. 

* - Indicates tests which are either in addition to or have more demanding test values than those in the ISO wheelchair standards, part 8.
e - Indicates estimated values of dynamic forces.
[Px.x] - Indicates related ISO standards paragraph number.

Test A, Downward Force on Footrests 
1,000 N (225 lbf)[P8.5] -- Force of rider's leg pushing, or a "hitchhiker" standing, on Footrest. Test by holding the chair so the Footrest is parallel to the ground. Stand on it. Test both footrests if there are two.

 
Test A

Test B*, Footrest Rolling Impact 
1,800e N (400e lbf)[P9.6] -- Impact force on one footrest or front of chair running into a solid object. Test by hitting a solid step at 1.5 m/sec (4.8 ft/sec) -- a typical rolling speed, at about 90 degrees and 45 degrees. Test both sides.

 
Test B

Test C*, Caster Wheel Rolling Impact
1,800e N (400e lbf)[P9.5] -- Peak force when one Caster Wheel with a soft rubber or pneumatic tire impacts a solid step. Test as for B except a 7 cm (3 inch) thick block is against the curb. This tests both the caster fork and casterwheel. Impact each caster wheel at about 90 degrees and 45 degrees to the block. 

 
Test C

Test D*, Caster Force Bending Strength 
195e N-M (140e lbf-ft) -- Moment on a Caster Fork due to C. Component test using a vise and lever-tube over the stem. About 40 mm (1.5 in) of the fork should extend above the vise.

 
Test D

Test E*, Wheelie Drop Off Inclined Curb 
950e N (200e lbf) -- Approximate sideways force on a rear wheel when doing a wheelie off a 18 cm (7 in) step, with the chair tipped sideways 5degrees, pneumatic tires on rear wheels, or rolling off a level step at an angle so that the rear wheels don't hit simultaneously. Test by doing a wheelie off an inclined plane. It is also, the approximate force that a well-spoked 24 inch rear wheel should withstand without collapsing. 

 
Test E

Test F*, Rear Axle and Mount Bending Strength 
280e N-M (200e lbf-ft) -- Moment on Rear Axle due to E. Component test using a lever-tube over the axle while the side frame is held securely against a vertical reference surface.

  
Test F

Test G*, Upward Force on Push Handles
1,140 N (250 lbf)[P8.10] -- Lifting force on Push Handle when pulling chair up a 18 cm (7 in) stair with one hand. Test by doing so.

 
Test G

Test H, Handgrip Attachment 
1,000 N (225 lbf)[P8.7] -- Rearward pulling force on Handgrip during G - difficult to test. Use a good rubber/plastic-to-metal-to-glue and skip testing. 
> Go to Test Selection Area

Test I*, Inward Force on Seat and Seatback Tubes 
280e N-M (200e lbf-ft) -- Moment on Folding Mechanism or Frame Cross Members when doing a wheelie off a 18 cm (7 in) step. It is best to test a complete loaded wheelchair by doing a wheelie off a step as in tests E and L. 

For a frame assembly component test lay the chair on its side and squeeze the push handles together with the force necessary to cause the specified moment about the axle position.

 
Test I

Test J*, Seat Back Tube Bending Strength 
610e N (135e lbf) -- Pulling force of Seat Back fabric on top of Seat Back Tubes during test I. Do not test since this is redundant with Test I.

  
Test J

Test K*, Downward Force on Armrests
1,140 N (250 lbf)[P8.4] -- Rider's hand force on armrest during a pressure relief "push up", or when boosting up to sit on the armrest to reach high. Test by applying load to armrest.

   
Test K

Test L*, Wheelie Drop Off Level Curb 
2,670e N (600e lbf) -- Distributed force on the seat and seatback during wheelie off a 18 cm (7 in) step so that both rear wheels hit simultaneously. 

  
Test L

Test M*, Frame Twisting Strength 
Twisting moment on frame cross members when only two diagonal wheels are supporting the wheelchair on rough ground. Test with rider seated, tester holds one footrest down while lifting the opposite side-frame with force of 355 N (80 lbf).

  
Test M

Test N, Upward Force on Footrests
440 N (100 lbf) [P8.9] -- Upward force on non-folding footrest. Folding or removable footrest must fold or remove at less than 44 N (10 lbf) applied at the free end.

  
Test N

Test O, Upward Force on Armrests
895 N (200 lbf) [P8.8] -- Upward force on Armrest. If the armrest swings away or is removable, this must occur at less than 90N (20 lbf).

  
Test O

Test P, Handling Drop Test
1m (39in), empty -- Impact during handling. Drop from the height on to each wheel when opened and folded.

 
Test P 

Results of employing the tests
The Whirlwind II design wheelchair passes these tests. Two such chairs were tested to, and passed, the ISO Standard tests, including the minimum 200,000 cycles on the two-drum durability tests [P10.4], and 6,666 cycle horizontal 50 mm (2 inch) drop test [P10.5]. In addition, thousands of Whirlwind chairs have been built to the established specifications, and are withstanding rigorous riding throughout the world. This attests to these tests being appropriate.

Therefore, passing the above static strength and impact tests is a good indicator of a chair's ability to also pass the ISO Standard durability tests. 

Discussion
It is recognized that passing the above strength and impact tests does not directly test fatigue failure of the structural materials or joints. For that reason it is recommended that chairs be built of materials that don't become brittle with repeated flexing, and any welds or brazed joints be of high quality. 

For example, the Whirlwind chair uses malleable (mild or low- carbon) steel for the frame and other mechanical parts. This steel experiences minimal work-hardening when flexed well below its elastic stress limit. Also, when parts are inadvertently over loaded, they will bend rather than break, thus, avoiding a catastrophic safety hazard. (For example, 16mm (5/8") diameter low-carbon steel axle bolts are used.) Also, it is generally braze-welded (gas-welded with brass) because the steel at the joints has less tendency to become brittle, and a broken weld can be easily and reliably welded by local bicycle repair person.

The Whirlwind chairs have the design features needed by a large segment of the wheelchair riding public in developing counties. That is,they are designed for countries where the chairs will be used in both city and village environments where rough riding conditions exist. Also, the materials and techniques used to manufacture and repair the Whirlwinds are appropriate for these countries.

There are many good wheelchair designs, besides the Whirlwind, being manufactured. Also, shops trained to manufacture the Whirlwind often modify the design to meet requirements important to them or their customers. It is recommended that all current and future designs and modifications be strength and impact tested as suggested in this paper. Doing so should help improve the quality of wheelchairs.

References 
1. "ISO International Standards for Wheelchairs (Final Draft), ISO/FDIS 7176-8, Part 8, Requirements and test methods for static, impact and fatigue strengths", Copyright ISO 1997. 

Available from:

American National Standards Institute
13th Floor
11 West 42nd St
New York, NY 10036
Attn: Customer Service
Ph:212/642-4900
Telfax: 212/398-0023
e-mail: info@ansi.org,
WWW: http://www.ansi.org 

or

RESNA
1700 N. Moore Street,
Suite 1540
Arlington, VA 22209
Ph 703/524-6686
WWW: http/www.resna.org (Contact Persons/Publication) 

2. Peter Axelson, MSME, A Guide to Wheelchair Selection, How to 
use the ANSI/RESNA Wheelchair Standards to Buy a Wheelchair,
Beneficial Designs Inc.
2240 Meridian Blvd, Suite C
Minden, NV 89423-8628
775.783.8822 ph
775.783.8823 fax
mail at beneficialdesigns.com

3. Peter Axelson, MSME, List of Wheelchair Testing Facilities (an
informal, unpublished list of addresses and test
capabilities)

Acknowledgements 
The information contained in this paper is primarily the results of tireless efforts and experience of Ralf Hotchkiss on behalf of people who need wheelchairs throughout the world. He is Technical Director of:
Whirlwind Wheelchair International
SFSU
1600
Holloway Ave.
San Francisco, CA 94132,
(415)338-7734.

The author is also grateful to Peter Axelson,MSME, Beneficial Designs, Inc.; and to Rory Cooper, Ph D., and Brad Lawrence, B.S., Human Engineering Research Laboratories, University of Pittsburgh, for critiquing draft descriptions of these tests, and the latter for testing Whirlwind chairs to the ISO Standards. 

Note: This paper first appeared in RESNA 1996 Proceedings, Pages 414-416. This revision includes corrections, comments, and changes that reflect the current testing practice of Whirlwind Wheelchair International (formally, Wheeled Mobility Center). We intend to publish a simplified version for dissemination to wheelchair builders worldwide. It will use diagrams and require minimal English comprehension.