last updated 5-26-10- “What makes a good soccer chair”
Background
I have been modifying motorized wheelchairs since about 2007, mostly in support of my wife, Kendra’s, advancement in the sport of power soccer. In November 2009 she was selected for a spot on the 2011 Team USA. She and eleven other athletes will travel internationally to defend the World Cup, first won by the US in 2007.
I hope that this page can be of some help to people who are interested in power soccer, or in repairing and modifying their power wheelchairs.
You may be asking, “What is power soccer?”. Power soccer is a sport very similar to soccer (or “football” if you are outside of the united states), except it is played by a team of four players using motorized wheelchairs on a basketball court. The chairs are limited to a forward speed of 6.2 mph, enforced by speed testing before each match. Although hard or intentional contact results in a foul, some contact is unavoidable.
Many people play power soccer in the same chair that they use every day. The power wheelchairs that people rely on may cost over $10,000, and repairs can be very slow and very expensive. The powerchair means independence, mobility, and safety. It lets them go to work, get around the house, and do a lot of things that walking people take for granted. Repairs can be very expensive, and can take days or weeks to happen. This is one reason why many people have a second chair that they use mainly for soccer.
Another reason that many people use a dedicated soccer chair is that many of the things that make a chair good for soccer are at odds with what are needed for daily use. I will go into some detail regarding specific details as I expand this page.
What Makes a Good Soccer Chair
Whether you’re shopping for a new chair, updating your old chair, or reviving something from the classified ads, here are a few things to consider that will make a chair better for soccer.
-Most of the weight should be on the back tires. This is very important for traction, as well as allowing you to pivot in place, giving a much easier spin kick.
-The seat should put your personal center of mass right over the wheels. If your body pivots in space, you fly around less, and the chair doesn’t have to work as hard to turn quickly.
-The seat should be as light, low, and far back as possible. You can have powered or customized seating in a soccer chair, but in my experience it tends to move your center of mass forward and up. Too far forward, and you lose traction and skid everywhere. Too high, and you’re more prone to wheelies and tipping over sideways. Obviously the seat must be able to be adjusted to a configuration that is safe and comfortable, but it must be able to do so while putting your physical position on the chair at the desired spot. The right spot will lessen the need for straps and restraints, and decrease fatigue while playing.
-Good mounting points for a guard. A properly adjusted P200 will drive almost like a mid-wheel drive, but most chairs sold as mid drives lack solid places to mount a foot guard. A guard needs to be rigidly connected to a strong structure that holds the weight of both the batteries and the player.
-Good access to programming. As a soccer player practices and gains skill, they gain improved control of their chair, and will be able to use higher acceleration and speed settings. A good soccer chair must be able to have all aspects of its drive characteristics changed easily, with wide ranges of allowed settings and fine control to avoid having to pick between “way too much” and “not nearly enough”.
-Better than usual preparation for air travel. Games and tournaments are often hundreds or thousands of miles away, and most people fly. Batteries absolutely must be non-spillable gel or AGM, loose and fragile parts should be replaced or taken off the chair, cables should be very well out of the way from getting cut or snagged, and things must be very clearly labeled. Another note- keeping weight to a minimum works in your favor. Airports don’t always have elevators at every step of the way, so sometimes baggage handlers have to manually raise and lower the chair between levels. I have frequently seen airport people physically lift (and sometimes drop!) a powerchair with as few as three people.
-Maximum speed between 7 and 8 mph. Unless you are extremely light in weight, having a chair with gearing that will go much faster than the game’s forward speed limit (6.2 mph) will just give you less torque and acceleration from the maximum current that your electronics can deliver. You can play with increasing and decreasing tire size to change your effective gear ratio to help this. The “Tentacle Combat” calculator linked below really lets you explore what I’m talking about with this. Divide the “maximum speed” in the calculator by the “time to maximum speed” that it gives you to get an acceleration rate. In the calculator, the NPC T64 motor is equivalent to a stock P200 motor. Note- it’s important not to have your maximum chair speed at only 6.2 mph, because experienced players drive backwards faster than forwards (no speed limit backwards), and will outmaneuver you for court position. There’s little need to go much faster than 8 mph in reverse, because in the second or two that it takes to accelerate to full speed you’ll have already traveled several chair lengths. I’m waiting for someone out there to prove me wrong on this, but I haven’t seen it yet.
-Rear wheels as widely spaced as possible. The spin kick is a rotational acceleration of the chair, and the wheel force can be thought of as imparting a torque on the chair about the vertical axis. Moving the point at which the wheels apply that force farther out from the vertical axis reduces the force required for a given rotational acceleration. This lets you have shorter windups and more powerful stationary kicks from the same motors and electronics. You still need to fit through doors and over ramps, so 26 inches is often the practical limit. My math says it’s near a 10% improvement to go from 24 inches wide to 26 inches wide. This is definitely contrary to what you’d want from an ordinary wheelchair, but it will help your kick and you might have less tendency to tip over sideways.
-Good batteries. With chair settings turned up, and with upgrades in place, it is possible to draw anywhere from 80 to 200+ amps from the batteries. A spin kick can use this current for a fraction of a second, or when two players are pushing against each other through the ball, the power can be sustained for multiple seconds. All of the power in your game comes from the batteries. Some of my links below go into good detail about what makes a good battery.
-Cooling. The drive electronics have an output limit based on how hot they get. Adding a fan can let you play harder for longer without having your chair slow down. Removing a plastic fender can also be easy an effective, though it can affect how attractive and safe the chair is.
-Reliable, repeatable, stable. A baseball player isn’t fretting about the condition of the bat, a basketball player isn’t constantly worrying that the sole of their shoe is about to fall off, and a hockey player doesn’t stop the action to sharpen their skates. The chair and everything on it should be able to stand a whole day of practice without becoming the center of attention. Of course a top athlete will want to push the limits with equipment, but when it’s time for the game the only thing that can be on the athlete’s mind is the game.
Power Soccer Links
United States Power Soccer Association- The United States organization for power soccer
Power Soccer Team USA- The US National team for international competition in power soccer
Xable.com- Media network for the disability community- many important power soccer matches have been webcast here, are and are available for viewing.
Powersoccershop.com- source for balls and pre-made guards. The guards are good, hard to build much better for the weight. The main weakness at present is the mounting system for many chairs, which is not that hard to work around.
Manufacturer and Vendor Links
Sunrise Medical – manufacturer of Quickie wheelchairs, including the P200 and S646
Penny and Giles Drive Technology – manufacturer of drive electronics used in Quickie, and other powerchairs
Dynamic Controls- manufacturer of drive electronics, owned by Invacare, and used in various powerchairs
Eurton Electric- motor rebuilder, able to refurbish most common wheelchair motors at a much lower cost than buying new motors. Also able to rewind motors for improved performance characteristics.
Robot Marketplace- not a wheelchair company, but carries many parts and materials useful for modifying, building, and upgrading powerchairs.
NPC Robotics- also not a wheelchair company, but manufactures some useful parts, including a gearbox compatible with the Quickie P200 motors that will increase top speed by about 43%.
Roboteq- manufacturers of a high power DC motor controller which has been used as a powerchair controller.
The Aftermarket Group- a source for those hard to find replacement parts. I tend to scavenge used parts from Ebay and Craigslist for much lower prices, but if you need it you need it.
Individual Builders
Wheelchairdriver.com- aka Burgerman, from the UK, this individual is serious about powerchair modification. His site has a lot of detail regarding his quite capable modifications to various powerchairs, as well as his tests and reviews of batteries, lift and ramp vans, and other products. Henry Taber- a few good pages worth of information on improving an Invacare and two Quickie chairs. Other resources http://www.rehab.research.va.gov/cluster/wheelchair.html – The US Government funds some wheelchair related research, and makes the reports available online for free. A wealth of information that our tax dollars paid for. Battery University- good source for in-depth information regarding the inner workings of batteries, a critical part of powerchairs. Windsun Battery FAQ- another good source for information about batteries. Tentacle Combat torque calculator- A handy online tool for examining “what if?” scenarios for electric vehicle design. Tip- the NPC T64 is equivalent to the Quickie P200 stock motor, and the NPC T74 is the up-sized version. This utility really lets you see how you trade speed for acceleration, and how much weight will rob you of acceleration. Keeping in mind the simplifications, I found this site to be very useful. Interesting High Speed Video Clips- this may be of some interest while visualizing what happens during the instant when the ball hits the guard. Whirlwind Wheelchair International, Testing Standards- If you want to modify a commercially built wheelchair, it’s a good idea to have some understanding of what kinds of things a wheelchair is supposed to be able to endure. This page is geared exclusively to manual wheelchairs, but the addition of motors shouldn’t decrease how safe it should be. It’s true that some changes that are beneficial to power soccer will not be good for driving over uneven ground- but you still need to leave the court to get to the game. Papers of Interest I find that by reading what has been patented by the manufacturers, as well as research papers in loosely related subjects, we can gain insights into how all of this works. Invacare G-Trak- An add-on module that uses a MEMS gyroscope (like a Wii remote) that senses chair rotation, and uses feedback to improve the ease of driving in a straight line with alternative controllers like sip/puff. Wheelchair Caster Shimmy- a paper on the “flutter” effect when one of the front caster wheels starts vibrating. Are Composite Bats better than Aluminum Bats?- an interesting read when thinking about what makes a good foot guard. Electric Wheelchair with Improved Speed Control- Invacare, 1982. Send email for any comments, questions, or suggestions- jzoe@jzoe.com