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Thread: Wheelchair physics -- need an explanation

  1. #1

    Wheelchair physics -- need an explanation

    Lately the tires on my ZR2 have begun rubbing into the side guards when turning my chair. (The wheel spacing is 1" -- I have a few rubber washer/spacers separating the guards from the frame.) I've tightened the side guard mounting screws and have achieved clearance again, but I would like to understand the mechanics that explain why the wheels approximate closer to the frame of the chair when turning than when simply going straight.

    Chas, this is probably right up your alley, but all expert opinions (SCI-OTR, ChrissyOT) are welcome to chime in. (Please, no big words.)
    Last edited by stephen212; 08-28-2012 at 06:53 PM.

  2. #2
    Senior Member DaleB's Avatar
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    Me too! This happens with my Mg. It's running .5" spacing, 559 rims.

    The inner wheel is rotating less than the outer, turning, maybe this causes some torsion?!?

    Wondered about this, myself....
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  3. #3
    Spokes flexing I'd assume.
    DIGG.

  4. #4
    More details, please, and I'll offer an opinion.

    What brand of wheels (what kind of spokes)?
    How long has it been since wheels were trued?
    what camber?
    when did the rubbing start? gradually, suddenly? (after making changes?), always (since new), ?
    which wheel rubs (inside or outside of turn)?
    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
    Quote Originally Posted by DIGGER View Post
    Spokes flexing I'd assume.
    My guess, too.
    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>

  6. #6
    Quote Originally Posted by chasmengr View Post
    More details, please, and I'll offer an opinion.
    What brand of wheels (what kind of spokes)?

    • Round Betty Dinos, but I've experienced this with other wheels as well.

    How long has it been since wheels were trued?

    • A few weeks ago.

    what camber?

    • 2 degrees. I will have to check to see if the camber tube is still in proper alignment.

    when did the rubbing start? gradually, suddenly? (after making changes?), always (since new), ?

    • Recently. I suspect that the side guards may have pulled away from the frame as the mounting screws loosen.

    which wheel rubs (inside or outside of turn)?

    • Both simultaneously.

  7. #7
    Sounds like your camber plugs are way loose.

  8. #8
    Quote Originally Posted by Patrick Madsen View Post
    Sounds like your camber plugs are way loose.
    Nah, they're good.

  9. #9
    If not loose camber plugs, something else. Check everything for proper tightness.

    Also swap wheels from side-to-side; does the rubbing still occur? If so, swap axles so they enter each wheel from the opposite sides of the hubs, and try the wheels on both sides again. (Note: I've never seen Dinos up close, so I don't know for sure if their hubs are symmetrical, but they appear symmetrical in pictures)

    Regarding physics, turning imposes lateral (sideways) forces that the chair doesn't experience when traveling straight. Because the wheels' bottoms are on the ground, lateral forces push the chair's frame sideways; if something be loose, the tires rub. I say loose because the rubbing started recently, and both sides rub. Your wheels are trued, so they should be okay. (I presume they were trued by an experienced person so they are properly tightened, actually true, and centered, not dished.)
    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>

  10. #10
    Have you checked the toe?
    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>

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