FASTEST CHAIR ON EARTH: Building a Bounder STMB3
STMB3 MOTOR PROJECT: Turn up the Torque!
The most hard-core bike messengers in San Francisco ride “crazybikes,” a bicycle featuring one extremely
fast gear, no freewheel, and no brakes. While most folks in the city think crazybikes are dangerous
and. . . well. . . crazy, there have been no accidents involving them. Furthermore, the messengers who
ride crazybikes can make deliveries in half the time of their colleagues on traditional bikes, and therefore
earn twice as much pay each day. Indeed, crazybikes aren’t crazy after all.
. . . So I went
to 21st Century Scientific, the manufacturer of my Bounder powerchair, and said, “In my quest for more
independence and mobility, I’d like to install the biggest motors possible on my chair, and chronicle
it on WheelchairJunkie.com.”
And 21st replied, “Sure, how fast would you like to travel in your
chair?”
And my wife replied, ”You’re wearing a helmet in that damn chair!”
And I replied,
“Cool.”
Five days later, a sixty-nine pound cardboard box was mysteriously left on my porch by
some guy in a large brown truck with no doors. Within the box was one pair of 21st Century Scientific’s
Super Torque Motors and Brakes-3 (STMB3), one wiring harness, two drive belts, two pulleys, and mounting
hardware. What a coincidence -- the box contained everything I needed to double the horsepower on my
chair, from ¼ to ½ horsepower per motor! (OK, the box was delivered by UPS, and 21st sent me the parts,
but it’s far more interesting to think of a mysterious force a work, as in Santa Clause or the Tooth
Fairy. Then again, I once put a tooth under my pillow, and awoke in a brand new QUICKIE!)
STMB3’s
are the largest wheelchair motors available, with enough torque (power) for wheelchair users weighing
up to 700lbs. (I weigh around 150lbs., and use a 16” wide seat frame.) The motors have no gears, and
feature permanent magnets, making them quiet and durable. What’s more, the motors’ brakes will hold
a 500lb. user on a 14 degree hill. Are you getting the idea?
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On the left is the original STMB to be replaced by the STMB3 on the right. As you can see, the STMB3
is much larger than the original motor.
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Oh, yes, a fine example of what your powerchair should not contain under the seat -- that is, a tangle
of wires. The first step in the project was to disassemble the chair, sorting the wires, and removing
the old motors and brakes. (Note that I already removed the armrests, joystick, and seat.)
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There, much better. After removing the motors and installing a larger curcuit breaker cable for the
new motors, I routed the wiring to its locations using "zip ties." It's important to be able to glance
under the seat each morning to make sure everything is connected properly, so there's a lot of value
in taking time to organize the wires and plugs.
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Next, I mounted the motors on the frame. The new motors are much larger than the old ones, so the motor
mounts had to be adjusted. The frame on my chair was manufactured to accomodate different configurations,
so the new motors fit perfectly once I adjusted the motor mounts. I should mention, however, that the
new motors are exceptionally heavy, 32lbs each with the brakes, and no easy task to lift onto the motor
mounts. Also, there are spacer washers that must be placed correctly -- it's always the little things
that take the most thought.
After the motors were mounted, I put new drive belts on the chair
and adjusted the clutches (belt tension). The most critical aspect of the project was aligning the belts.
If the front motor pulleys do not align with the rear wheel pulleys, the belts will quickly be destroyed,
no doubt leaving the user stranded. The motor pulleys adjust on the motor drive shaft, held in place
with two allen screws. I placed the belt on the pulleys, eyed and adjusted it to center, and then measured
the motor pulley's distance from the motor (when you remove the belt to tighten the allen screws, the
motor pulley may shift, so it's wise to use a measurement as your guide). Once the motor pulley was
tight and the belt replaced, I once again made sure it was aligned. This process was done on both sides
of the chair.
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As shown in the photo above, the brakes are seperate units that mount on the outside of the motors'
drive shafts. The brakes have grooves that mesh with grooves on the drive shafts. I've never had much
luck aligning those darn grooves -- then again, I have the small coordination of a trunkless elephant!
-- so I had Gustavo do the brakes, which took him all of five minutes.
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After plugging all of the components together, and checking the handcontrol LCD for error codes (Bounders
have the finest electronics in the industry, and monitor the chair's system, telling the user when there's
a problem), Gus and I set the rear of the chair on blocks and ran it, checking belt alignment and braking,
making sure all systems functioned properly. She was ready to roll.
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Conclusion |
After several days of use and three long road tests, I’ve found remarkable differences in the chair.
The obvious difference is that the chair is faster, though not startlingly faster. Sure, for a wheelchair
it’s moving at an astonishing speed, which really captures people’s attention. However, due to Bounder’s
design and premium electronics, the chair handles like a dream, void of any unsettling, dangerous characteristics.
The coolest thing is to run the chair at full speed in a slalom pattern on a wide, smooth, empty street
because it hugs the road and digs into the turns like a sports car. With the old motors, I loved taking
my chair out for long road rides, and the new motors make it much more fun. While discussing speed,
be aware that you must take precautions toward bumps, potholes, and manhole covers. I’m running 8” front
wheels on my chair, but I wouldn’t recommend anyone else use front wheels smaller than 9” -- that is,
9” and 10” casters are far better suited toward rolling over obstacles at high speeds. I’m especially
cautious, always watching the road in front of me, slowing for bumps and avoiding potholes. It’s not
cool to put yourself or your equipment in jeopardy.
The real value of STMB’3s is in the torque.
Let me put it this way: I could use my chair to tow a 16’ fishing boat on its trailer (but I never would,
of course). There’s a street in my town that’s unbelievably steep and slopes to one side, making it
impossible to climb in any chair I’ve tried, including an Arrow, my E&J Lancer, and my previous Bounder
set up. Not only did the STMB3’s climb the hill, I stopped in the steepest section -- the chair resting
on its wheelie wheels, nonetheless! -- and started up again, making it to the top with power to spare.
I must add that I’m running the fastest pulley configuration, resulting in the least torque, so if you
put a slower pulley configuration on the chair, dramatically increasing torque, it would climb an old-growth
redwood tree. If you live on a farm or a rural environment, a low-geared Bounder set up with STMB3’s
and large 10” casters will take you places you never imagined you could go in a powerchair. (Hey, put
a snowplow on the front of a STMB3 Bounder and make some extra money during the winter months -- kidding,
of course!)
The premium electronics -- UE1D on my chair -- work much better with the STMB3’s.
The UE1D electronics have six drive modes (programs) on the joystick controller, ranging from indoor
slow to outdoor fast, and a bunch in-between. The lower programs weren’t as responsive with the standard
motors as they are with the high torque STMB3’s (though the factory told me that this years electronics
are improved to work better with the standard motors). I now have full torque from the low end programs,
which helps toward encountering limited space obstacles like door thresholds.
The STMB3’s are
expensive at a $2,000 upcharge on a new Bounder or Bounder Plus, but well worth it when considering that
the chair may last you ten years. There’s no better chair on the market today, period.
. . .
So I dropped out of school, gave up my jobs, and now earn my living with my Bounder as a bike messenger
-- or is it, wheelchair messenger? -- in San Francisco’s financial district. Those sissy crazybikers
just can’t keep up with me.
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"I'm Molly from PBS' children's show 'Big Comfy Couch.' I don't know why this Mark fellow took all
the credit for this project? After all, as you can see, I did all the work!"
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