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Wheelchair physics -- need an explanation

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    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; 28 Aug 2012, 6:53 PM.
    stephen@bike-on.com

    #2
    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....
    "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

    Comment


      #3
      Spokes flexing I'd assume.
      DIGG.

      Comment


        #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>

        Comment


          #5
          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>

          Comment


            #6
            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.
            stephen@bike-on.com

            Comment


              #7
              Sounds like your camber plugs are way loose.

              Comment


                #8
                Originally posted by Patrick Madsen View Post
                Sounds like your camber plugs are way loose.
                Nah, they're good.
                stephen@bike-on.com

                Comment


                  #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>

                  Comment


                    #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>

                    Comment


                      #11
                      Have you recently become wider (pushing the side guards out) via extra clothing, cushion change, or weight gain?
                      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>

                      Comment


                        #12
                        It would be informative if others examined the distance between the tops of their tires relative to some stationary marker on the frame of their chair (e.g., side guards) and see if they, too, observe this phenomenon (regardless if the tires are making contact with anything). I suspect this is a near-universal phenomenon.

                        Just the same, I'll give my chair a more studied look-over tomorrow with fresher eyes.
                        stephen@bike-on.com

                        Comment


                          #13
                          Originally posted by chasmengr View Post
                          Have you checked the toe?
                          On my list for tomorrow.

                          Originally posted by chasmengr View Post
                          Have you recently become wider (pushing the side guards out) via extra clothing or weight gain?
                          I beg your pardon, sir! Even if I put on weight, which I suspect is highly unlikely, it doesn't go to my hips.
                          stephen@bike-on.com

                          Comment


                            #14
                            Originally posted by stephen212 View Post
                            I beg your pardon, sir! Even if I put on weight, which I suspect is highly unlikely, it doesn't go to my hips.
                            Just brainstorming - long distance dx is tricky.
                            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>

                            Comment


                              #15
                              Originally posted by stephen212 View Post
                              On my list for tomorrow.
                              don't waste your time, it's not that.

                              Okay, I'm not a mechanical engineer like chas. I was (and still think like) an auto mechanic. So bear with me while I attempt to put this into words.

                              Think about the force put on the tire at the contact patch while turning. It pushes the wheel out thereby causing the opposite effect at the top of the wheel where your sideguards are. How noticeable this is will depend on type of wheel and wheel spacing. I noticed from day 1 on my Oracing because of the incredibly tight wheel spacing to my solid sideguard tubing, magically it never touches but does get really close.

                              To better amplify the force I am trying to explain turn on some carpet, you will notice as you turn how much the wheel flexes and then when you come out of the turn (moving straight) the lateral force is no longer present and the wheel spacing increases.

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