Wear and Tear on the Chair

Care and feeding of the sports machine

The sport of extreme racing has wheelchair pilots powering at high speed through the great outdoors.
Article Highlights:
  • Power wheelchair sports are growing in popularity among people with disabilities, but power wheelchairs are exposed to more extensive wear and tear than is the case with typical use.
  • For example, wheelchair motors, controllers and batteries can be subjected to stresses not originally intended by manufacturers, and the metal structure of the chair also can be bent, twisted and broken.
  • To combat against these issues, the author recommends that power chairs intended for rigorous sports can benefit from using specific types of equipment, ranging from bigger batteries to shock absorbers and specialized tires for different kinds of surfaces.
by Bill Norman on April 1, 2009 - 4:16pm

QUEST Vol. 16, No. 2
Power soccer wheelchairs often have structural and mechanical modifications.
In power hockey, players sometimes tape their sticks to the wheelchair frame.

Power wheelchair users are a growing presence in the world of sports. Players (and parents) need to be aware of the unique stresses that power chair sports exert on the chair’s framework and machinery.

Common power chair sports include:

Power soccer. Two four-player teams square off against each other, typically on a basketball court. Players use their chairs (equipped with an add-on plastic or metal foot guard) to kick an oversized ball toward the goal. They also use their chairs to intersect or deflect an opponent’s kick.

Power hockey. Similar to power soccer except players slap a puck (usually a plastic 2”- or 3”-ball) with lightweight hockey sticks. Sticks may be used by hand or taped to the wheelchair and the chair’s power provides the “swing” energy.

Dog agility competition. Dog owners race alongside as their specially trained dogs navigate an outdoor obstacle course, striving for fastest time and accuracy.

Rural racing. Also sometimes called extreme racing, this sport entails competitors – usually equipped with protective clothing – striving for fastest time when negotiating multi-mile race courses that can include dirt, mud, water and other obstacles.

All of these sports are high speed and chairs get a real workout. In particular, wheelchair motors, controllers and batteries can be subjected to stresses their manufacturers may not have intended. The metal structure of the chair also can be bent, twisted and broken.

Heat is enemy No. 1

Mark Smith, creator and operator of Wheel Chair Junkie Web site, says the main issue in power soccer is “thermal foldback,” where the constant acceleration demands and turning resistance run very high amperage through the controller, overheating the chair.

“Power chairs have been custom modified for individual users from time to time, but sacrifice everyday reliability, as there’s long-term harm to constantly running power chair components at very high amperage and excessive heat,” says Smith. “I’ve seen serious European players with custom-modified, unlimited electronics go through a controller and/or a set of motors every few games by eliminating thermal protection — it makes for a very dangerous, expensive sport.”

Adam Elix, program coordinator for Far West Wheelchair Sports in San Jose, Calif., a nonprofit organization that promotes and organizes wheelchair sports, says power chairs can be modified to a certain extent for sports.

“We have a mechanic who can adjust the gear ranges — gear one for street cruising, gear four for athletes. We’ve done a couple of things to deal with heat problems. One parent [of a soccer player] is a tech guy who hooked up a personal fan to blow on the battery compartment. We’ve also used ice packs on top of the batteries, and we’ve tried blowing [canned] compressed air on them. It comes out of the can very cold.”

Component factors

Power chairs intended for rigorous sports use can benefit from utilizing specific types of equipment:

Belt drives rather than direct drives. A belt-drive chair, in conjunction with wheelchair motors that have a brake, can be abruptly switched into reverse gear from full-speed forward, says Dick Roberts, customer service representative for 21st Century Scientific, an Idaho firm that manufactures the Bounder brand of power wheelchair. He notes, “If you try that with a direct drive, you’ll have parts flying everywhere.”

6-pole motors rather than 4-pole. Motors have magnetic poles, just like simple magnets, but their poles are creating by winding magnetic wire in bundles. Motors with fewer poles spin faster (1,800 revolutions per minute, in the case of four poles) and have to work harder. Six-pole motors spin slower (1,200 rpms), but they have more torque (the twisting force that produces rotation) and can propel a chair at greater speeds. The 6-pole version also runs quieter.

John Cross, president of ASI Technologies, which designs and manufactures drive systems for wheelchairs, says another advantage of 6-pole motors is that “a lower mechanical gear ratio could be used with the higher pole motor.”

However, apart from specialized applications like sports, Cross said, “The added complexity and cost of 6-pole vs. 4-pole is usually not justified in a wheelchair motor.”

Bigger batteries are better. Wheelchair batteries usually come in three group sizes: 22, 24 and 27, with the larger numbers signifying larger external dimensions. Group 22 is standard for wheelchairs; group 24 is utilized to power heavier chairs; 27 is best for frequent chair use (as in sports) and traveling long distances over rough terrain. Larger batteries also often have the greatest power, reflected in their ampere (“amp”) hour rating. Before attempting to move up in battery group size, chair users must ensure there’s adequate space in the battery compartment and that their other electrical gear, such as the chair’s controller, is compatible with a boost in power.

Mike Serhan, product manager and vice president of sales for Drive Medical Design and Engineering, said gel cell batteries cost more and usually don’t have the range of wet cell batteries, but they do provide more torque. Some wet cell batteries can leak or spill acid; gel cells are sealed (as are some wet cells), and that can be important in sports where impacts and steep terrain can sometimes knock batteries (along with the wheelchair) onto their sides.

Shock absorbers. Most wheelchairs don’t have them. Roberts said optional nitrogen-filled shock absorbers surrounded by coil springs greatly reduce the impacts felt by a wheelchair user when traveling over rough terrain.

Digital controls rather than analog. “The read-outs on your controller screen look the same with both types, but digital is more precise. You get more joystick sensitivity and ability to make a fine distinction between drive speeds,” Roberts said.

TIG welds for frames. The welds that hold metal wheelchair structural members together can be subjected to front and sideways wrenching and twisting forces strong enough to break them apart during intense sports action. Tungsten insert gas (TIG) welds are stronger than those produced by common arc welding.

Different tires for different surfaces. Roberts said low-pressure wheelchair tires that are taller (14 inches vs. 12 inches) and wider (6 inches vs. 4 inches) offer a smoother ride and more flotation (less tendency for the tires to mire down on wet or muddy terrain). Serhan also suggested that flat-free wheelchair tires may be good for outback cruising, while pneumatic tires are preferable on the court.

Heading for the woods

John Mryczko is president of Extreme Chairing and the Power Wheelchair Racing Association, based in the Chicago area. Both organizations organize outback wheelchair treks and races. Extreme Chairing, a 501(c)3 nonprofit group, aims to expand the popularity, corporate support and geographic range of events for a sport that is just getting off the ground.

“We’re pretty much using everyday power chairs in the events right now, but competitors usually have their chairs’ controllers reprogrammed so they can get more speed. When you first get up to 10 miles an hour, it seems really fast, but after a few miles it just seems normal. Sometimes I even wonder if I’m slowing down when I’m not,” Mryczko said. He hopes they’ll succeed in boosting the speed to 20-plus mph.

Some specialized all-terrain power chairs have four-wheel drive; others have literally replaced wheels with smaller versions of the metal treads used on tracked vehicles.

In that regard, the Wheelchair Site, an independent consumers’ guide to wheelchairs, scooters and accessories, says of all-terrain wheelchairs, “Sure, the most rugged models will travel over just about anything, but they drive like tanks!” The site suggests renting an all-terrain before buying, and notes that many resorts throughout North America offer all-terrain rentals to vacationing guests.

Shopping considerations

Abledata, which provides information about assistive technology products to the National Institute on Disability and Rehabilitation Research (part of the U.S. Department of Education), also has some guidance about power chair drive systems.

Power wheelchairs have either rearwheel, front-wheel or mid-wheel drive. The drive type has a significant effect on maneuverability and handling.

Rear-wheel drive is better for outdoors travel, said Drive Medical’s Serhan, while mid-wheel drive is better on a court (soccer or hockey) setting. “But mid-wheel drive is a little squirrely if you’re going to be outdoors,” he said.

The Abledata Web site, contains an exhaustive list of wheelchair manufacturers and distributors; a wheelchair-related organizations resources list; and information about approved wheelchair feature standards approved by the American National Standards Institute in cooperation with the Rehabilitation Engineering Society of North America.

For power chair resources, see InfoQuest.

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