Rear swing arm of motor-bike with tapered rectangular pipe member and method of manufacturing tapered rectangular pipe for vehicle body

A rear swing arm for a motor-bike comprising a rectangular pipe, the rectangular pipe having a front end which is pivotally supportable to a vehicle body and a rear end on which a rear wheel at a rear wheel supporting part is supportable, the rectangular pipe being tapered at least partially in a longitudinal direction and having sectional surfaces which are generally perpendicular to the longitudinal direction, the rectangular pipe having a longitudinal rectangular shape formed by upper, lower, left-side and right-side walls, at least a portion of one of the upper, lower, left-side and right-side walls having an irregular thickness which is thicker than that of other walls and one longitudinal end of the rectangular pipe being thinner than the other end, the lower side wall of the tapered rectangular pipe being substantially straight and the upper side wall being tapered, a rear part of the upper wall being tapered such that the rectangular pipe is narrowed to form a rear wheel supporting part and to accommodate a brake caliper arranged thereabove, at least one partition wall dividing an interior of the rectangular pipe into at least two chambers, each of the at least two chambers having side walls and one of the chambers having a tapered wall surface and one of the chambers being without a tapered wall surface, a side wall of the one chamber without a tapered wall surface being thinner than a side wall of a chamber having a tapered wall surface.