A recreational board which adapted to be ridden by a user has a thermoplastic upper portion with an outer surface and an inner surface, a thermoplastic lower portion including a plurality of flanges, is bonded together by joining the plurality of flanges to the inner surface of the upper portion.

Method and apparatus for selectively adjusting the stiffness of a ski using prestressed, curved rod members. The rod members are positioned lengthwise of the ski in bores and the stiffness of the ski can be selectively adjusted by rotating the rod members about their longitudinal axes. For a stiff ski, each rod member is rotated so that the prestressed curve faces upwardly and for a soft ski, each rod member is rotated so that the prestressed curve faces downwardly. By rotating the rod members between these extremes, intermediate degrees of stiffness can be obtained. One or more rod members can be used and when plural rod members are used, the stiffness of the ski can be adjusted both lengthwise and crosswise. Indicating means are provided for showing the rotational position of each rod. The rod members can be placed directly in drilled out bores in the ski, in drilled out bores of a housing which can then be inserted in the ski, or in tubes mounted within or atop the ski. The force of each rod member can be varied along its longitudinal axes if desired and one disclosed embodiment includes prestressed, curved hollow rod members about prestressed curved solid rod members whereby the hollow and solid rod members can be individually rotated to vary the ski's stiffness. The rod members can be disposed within or atop the main body of the ski and can be adapted to adjust the stiffness of any flexible body.

A ski and a method of forming a ski. The ski includes a lower element and an upper element, each having the shape of a beam casing. The lower element has an upper surface that has either a longitudinally extending groove or a longitudinally extending projection and the upper element has a lower surface that has a groove or a projection that has a transverse cross-section that is respectively complementary to a transverse cross-section of the groove or to a transverse cross-section of the projection of the lower element, and in which the groove or projection of the lower element is nested in the projection or groove, respectively, of the upper element. Depending upon the nature of the desired response characteristics of the ski, the interior of the upper and lower elements can be selectively filled with specific materials or left empty.

The invention relates to an elongate metal body, for instance an aluminium rod with a chosen cross-sectional form manufactured by extrusion. It is a first object of the invention to make an elongate metal body stiffer and stronger without this entailing an increase in weight. In respect of this objective the metal body according to the invention has the feature that the body has at least one cavity extending at least to a considerable degree in longitudinal direction, in which cavity is received a pre-manufactured elongate reinforcing rod, of which at least the ends are coupled to the body in force-transmitting manner.

An elongate metal body, for instance an aluminium rod, with a chosen cross-sectional form manufactured by extrusion. It is a first object of the invention to make an elongate metal body stiffer and stronger without this entailing an increase in weight. In respect of this objective the metal body according to the invention has the feature that the body has at least one cavity extending at least to a considerable degree in longitudinal direction, in which cavity is received a pre-manufactured elongate reinforcing rod, of which at least the ends are coupled to the body in force-transmitting manner.