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Wheelchair physics -- need an explanation

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    Originally posted by stephen212 View Post
    While stationary and pushing my wheels in toward my body I am not able to get the top of the wheels as close to my side guards as when occurs when I'm turning.

    Spoke flex.

    I also have my skirt guards attached to the tube to keep them in place.

    Straight & no rub then turning & rubbing with no bearing play or lees mounting hardware = spoke flex.
    Steve Garro.


      First I read this thread and informative.
      The "wheel scrub" is actually the result of lack of lateral stiffness. In a quest for lighter wheels one has to deal with torsional forces, vertical forces, and Lateral forces. The goal is to get the best benefit from all without sacrificing too much.

      Turning the chair deflects the rim. Factors making the rim stiffer include...
      spoke lacing. (Radial lacing pattern usually provides the best lateral stiffness.) Also benefits from the lightest design.
      Use of a stiff and deep rim which will come with shorter spokes.
      Larger spokes.
      Finally increase the bracing angle of the spokes. (Hubs with flanges.)
      Finally High tensioned spokes which will delay the moment when spokes on the opposite side of the load lose tension.

      All of this comes with some sacrifice. I would also consider bearings going bad and axles that are flexible can add to the lateral movements.

      I have noticed on my chair when turning on surfaces that have a lot of traction in relationship to the rubber and the road I can hear the spokes make noise. Also when the tires are lower in pressure the rubber can take up some of those lateral forces.


        Originally posted by The_Wheel_Life View Post
        There's no good way to get around the physics of the wheel "scrub." When turning in place, it generates forces that create deflection. Essentially, the only way to eliminate the aforementioned scrub would be to add a LOT of camber to the wheels. When wheels are straight up and down, they like to go in the direction in which they are pointed. Think about being on carpet and turning in place while using a basketball or tennis chair. How would it compare to making the same turn in your everyday? The cambered wheels on the ball chairs are sort of always in a "turn." So, you'll feel a lot less scrub when you're turning in place.
        You are describing what is known in bike terms as trail & flop, here is a good tool/description:

        We also use caster angle (kind of analogous to a bike's head tube angle) and I'm not sure if we use Ackerman in chairs due to the casters……….but the rear must, as a multi-wheeled vehicle excites a turn the insides of the wheels transcribe a tighter circle then the outsides, check out this video………but I still think your spokes are just loose……...
        Steve Garro.


          I think outside the box. At least on hard turns your body twists. Maybe your butt is pushing the side guards out against the wheel if you have a chose fit. I kept my manual chairs as narrow as possible and this happened to me with plastic side guards. I solved the problem by going to a metal shop and getting a pair of side guards made out of 3/16" thick aluminum. Solved the problem. The guys in the shop were proud of doing a good deed for a crip and only charged me for the metal.
          You will find a guide to preserving shoulder function @

          See my personal webpage @