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Carbon Fiber Racing Wheelchair

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    "Standards can be good, don't get me wrong. But, Minimum standards yield minimum compliance, often, and the point of it all is more to manage liability than to make sure we get chairs best built and designed for our needs, as users."

    Certainly something to think about. In fact, once the conditions of a standardized test are known to manufacturers, it's generally pretty easy to design a product intended more for passing the test than performing the task of say being a really good wheelchair.

    Of the various components of the RESNA wheelchair tests, the double drum fatigue test is clearly the most demanding.

    In many of the studies I've read where chairs of various types have been subjected to the DD test, the majority of frame failures could have been prevented entirely or at least delayed significantly by changing accessory components on the chair rather than making any design, material selection or manufacturing process changes to the frame. The easiest of these is simply to add a larger diameter front caster and/or choosing a caster with a softer rolling surface or pneumatic tire.

    Probably the single most effective design change to ensure long term durability of the frame is to build one with suspension. IMO a suspension frame creates many more problems than it solves, but it certainly reduces stress on the frame. Suspension caster forks would probably be a good compromise.

    Many frames that do experience failures during this test, do so as the result of poor planning with respect to stress risers in the frame. Understanding the direction and magnitude of the forces encountered by individual components of the frame can quickly tell a designer that drilling holes through a frame tube; in such a way as to remove material from the tension and compression sides of any structural beam, moves the location of the outermost continuous fiber closer to the neutral axis of the beam. Doing this will significantly reduce not only the total tensile strength of the beam (generally not a concern in wheelchair frames), but greatly reduces it's bending stiffness. The result of this is that when subjected to bending forces, the beam will deflect more in the area of the hole during each load/unload cycle which accelerates work hardening in materials sensitive to this, such as metal alloys. If a hole is required, orienting it such that material is removed from the tube through the neutral axis rather than the tension and compression areas will have much less impact on bending stiffness and reduce the progression of work hardening in that area significantly.

    An interesting side note to this is that if you haven't engineered a so called "weak point" in the frame by drilling a hole in it, then the weak point has been moved somewhere else. If you can't reasonably predict where this new potential failure point is, then destructive testing would be required to identify it.

    An easy way to visualize all this is to look at steel I beam construction in structures like buildings. When connections between horizontal beams and the vertical columns are made, the bolts pass through the web of the horizontal beam rather than the flanges. The direction of the applied force is known and the beam is oriented such that the flanges are the tension/compression fibers. The web; while providing some additional strength in the areas close to the flanges, simply spaces them apart some distance to achieve the required bending stiffness. Drilling the connection bolt holes through the flanges would be the same scenario as drilling holes vertically in the seat tube of a wheelchair frame. It makes a convenient mounting arrangement for seat upholstery, but removes material from the load bearing areas the tube.

    A round tube is still just a simple beam that is equally capable of resisting bending forces from any side.