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using tractive rolling contacts. Some of the most promising con-tinuously variable transmission ~CVT! designs depend on tractive rolling. In traction drive CVTs, one can expect deviations of the speed ratio from the pre-set transmission ratio that grow steadily as the torque load increases and eventually give way to gross slip. These performance limitations are largely responsible for the low success rate of traction drive CVTs in demanding applications. The behavior of a traction drive CVT, including deviation and eventual breakdown in its transmission laws, depends on the me-chanics of contact at each internal rolling interface. A physically realizable rolling constraint is not impervious to the tangential forces ~also called tractive forces or simply traction! transmitted across the contact between rolling surfaces. Traction induces elas-tic deformations and changes in the regions of sticking and slip-ping within the finite size contact patch, causing the rolling con-straint to deviate from that dictated by the nominal wheel and rolling surface geometry. In this paper, we investigate deviation of the speed ratio from the transmission ratio setting as a function of transmitted torque in a particular CVT design. Our CVT uses tractive rolling between two cylindrical wheels ~known as drive rollers! and a rotating sphere. An additional pair of wheels ~known as steering rollers! acts to orient the rotational axis of the sphere and thereby set the transmission ratio. A prototype CVT is shown in Fig. 1. The CVT their role in the design and construction of cobots. Cobots are a new class of robotic device, designed to carry out manipulation tasks in cooperation with a human. While the cobot and human simultaneously grasp an object to be manipulated, the human pro-Contributed by the Mechanisms Committee for publication in the JOURNAL O