Abstract: A crossmember assembly is provided for a motor vehicle having a roof with first and second rails. Each rail has a width within a range of widths, with the first and second rails being spaced from one another by distance within a range of distances. The assembly includes a crossbar having first and second ends and movable along the rails. The assembly further includes a first clamp mechanism engaged with the first end for securing the crossbar to the first rail and a second clamp mechanism engaged with the second end for securing the crossbar to the first rail. Each of the first and second clamp mechanisms includes a stanchion attached to the crossbar and having a first rail gripping surface. Each of the first and second clamp mechanisms further includes a scotch yoke, which has a second rail gripping surface and is movable between clamped and unclamped states.