Fabrication
The The Marvel chair frame doesn’t use any straight gauge round tubes in its construction.
The technological advances in tube construction in the last decade have been staggering, and have allowed for lighter and stronger products that use a tubular construction to be manufactured in many industries, including aerospace, automotive and bicycle.
There are three main areas where the improvements have been most obvious: the shape of the tubes, the ability to vary the wall thickness of the tubes, and the heat treatment process (for aluminum tubes specifically).
The ability to shape the tubes allows the manufacturer to control the direction of the strength of the tube, a simple round tube is equally strong in every direction, an ovalized tube is strongest in the direction that the tube has been lengthened.
The ability to vary the thickness of the tubes allows this forming process to be even more improved, there are two ways to vary the thickness of the tubes; through a process called ‘butting’, which is essentially stretching the tubes in certain areas after they are extruded to thin out certain sections where the strength of the tube is less critical, or by having a custom extrusion of the tube made.
The Marvel chair frame doesn’t use any straight gauge round tubes in its construction. We use ovalized tubes in the tubes that support the axle, and a custom formed and extruded tube to support and join the seat and castor wing assembly.
This construction process allows us to maximize the strength to weight ratio of our frame, and we are currently the only wheelchair manufacturer in the world using this technology.
This tube has been formed and custom extruded. It would be strong side to side because of the thickness of the tube, and strong front to back because of the shape, while being as light as possible because all unneeded material has been eliminated.
The Marvel chair castor wing assembly is also a custom shape, using a process called hydroforming. In hydroforming, a tube is heated until it is soft, then bent and placed in a mold. The tube is then filled with water, and the internal pressure forces the tube to the shape of the mold. The part is then removed from the mold, finished, and heat treated to return the material to its original strength prior to shaping, of course the part as a whole is much stronger than it started out, because of the shape and wall thickness cross sections.
