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Thread: Finding the right CG?

  1. #1

    Finding the right CG?

    Hello All,
    I am a newbie with 6 weeks in my ZRA. I am liking it so far, my only complaint is that I wish the fore/aft measurement was a inch shorter. With that said I started "Advanced Wheelchair training" with my OT two weeks ago. I had not tried any wheelies in my chair before my first session and on the first day I was pulling off 5 minute long wheelies which my OT said was pretty impressive. Since then I have been able to propel myself in a wheelie pretty much indefinitely though not terribly fast, I have been practicing wheelies down ramps, doing circles and will start doing curbs in my next session.

    With that said I am a tinkerer so I have been playing with a couple things on my chair including axle location. My chair was built with a "CG" of 2.25" which I am guessing is pretty conservative. I have adjusted the CG 4 times so far and I am about 22MM's forward of where they built the chair at. The chair is much easier to get into a wheelie as well as pulling the front end up when I am already moving to get over bumps, thresholds, etc. When going up ramps I do get a bit caster lift when I push but it is mostly comfortable. I am a T10C and still not totally comfortable leaning forward in the chair as of yet but I am getting there, I do find that even moving my head forward a few inches makes a difference getting up inclines.

    After all that I am guessing "CG" is a pretty personal thing but what is the end game when it comes to adjusting the axle location?

    Cheers,
    Jim

  2. #2
    Senior Member Oddity's Avatar
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    Absolutely, it's very much an individual setting, but also individual priorities for why to choose what configuration. It boils down to striking a balance between pushing efficiency, agility, and stability. My best push stroke needs the wheels biased forward or my shoulder line (so I don't have reach far back, which raises the shoulders), so my 'end-game' was to find the furthest forward axle position I could before either: a) I had to reach too far forward to complete full stroke, OR b) the chair became too rear tippy. I'm running 4.5" forward of the back canes now, which is probably considered aggressive, but works for me. My back rest is squeezed a bit, at ~89deg, so my functional CoG is nudged forward a bit, so 4.5" axle position isn't too tippy. I also want as short a wheelbase as possible (close to square as possible actually), for agility and tightening up the footprint.
    "I have great faith in fools; self-confidence my friends call it." - Edgar Allen Poe

    "If you only know your side of an issue, you know nothing." -John Stuart Mill, On Liberty

    "Even what those with the greatest reputation for knowing it all claim to understand and defend are but opinions..." -Heraclitus, Fragments

  3. #3
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    When I was in rehab, I was told to order my chair with the axle pin in line with the middle of the shoulder with your arms hanging straight down. That would be your safe starting point. After that it's fine tuning to what works best for you. I got carried away with pushing the axle forward to get more speed for B-ball. That led me to falling backwards a lot. It was also harder going up steep ramps at that time. With shoulder issues, I found that having the axle pin about 1.5" ahead of my shoulder offers the most comfort with no back tippiness whatsoever. That measurement is very individual to the rider.

  4. #4
    My first real chair was an AeroZ (aluminum ZRA); I loved it. Tinker away. After five years of serious tinkering, I used it's measurements to get a TR3. I love it even more, but only cuz I tinkered with the Z until it really, really fit

    Here's some pics of my tinkering for inspiration perhaps: http://sci.rutgers.edu/forum/album.php?albumid=967
    Chas
    TiLite TR3
    Dual-Axle TR3 with RioMobility DragonFly
    I am a person with mild/moderate hexaparesis (impaired movement in 4 limbs, head, & torso) caused by RRMS w/TM C7&T7 incomplete.

    "I know you think you understand what you thought I said, but what I don't think you realize is that what you heard is not what I meant."
    <
    UNKNOWN AUTHOR>

  5. #5
    Thanks Guys,
    I would say my axle is right in line with my shoulders currently. Interestingly it feels a bit tippy going up ramps currently, even on a ADA ramp I need to move my head forward to feel comfortable going up it.

    Chas,
    It looks like you tinkered with your chair a lot....LOL. What is the deal with the aluminum plate on the casters? I also love the sign on the back of your chair. I have become friendly with a guy near by who has the same injury as myself and also from skiing. When I was spec'ing out my chair he said that I should get the fold down handles so people are less inclined to push me what I do not want/need help. I did not think that would be a big deal but I quickly found out that it actually is. I find it highly annoying when people try and help when I have not asked for it.

    Cheers,
    Jim

  6. #6
    Senior Member Oddity's Avatar
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    I think leaning forward to go up ramps is 100% normal. I could loop myself pushing off from a dead stop too hard without a little forward CoG shift. Price to pay I guess. It's all a balancing act. Literally.
    "I have great faith in fools; self-confidence my friends call it." - Edgar Allen Poe

    "If you only know your side of an issue, you know nothing." -John Stuart Mill, On Liberty

    "Even what those with the greatest reputation for knowing it all claim to understand and defend are but opinions..." -Heraclitus, Fragments

  7. #7
    Ideal CG varies from person to person and also for the same person over time. Over time you will gain a better sense of balance and so you will want to move your CG as far back as possible (for best mobility) without tipping over backward. It's not only ramps and hills that you need to consider for tipping backward but also other things that shift your center of balance backward like reaching and lifting heavy items at a height. The higher you reach or lift, the more careful you need to be with your center of balance. After 35 years in a wheelchair I still fell over backward when throwing a garbage bag into a tall container after moving my CG from 3.5" to 4". Ideally, I would use both hands. But I had to use one hand on a hand rim for balance. Hence, I had to swing the bag with the other arm. In doing so the bag was behind the axle just long enough to make me tip backward. I moved CG back to 3.5" and that's where it's going to stay.
    Last edited by August West; 07-20-2018 at 04:51 PM.

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