joint which works on the Weiss principle is the six-ball star joint of Kurt Enke. This type uses only three balls to transmit the torque, while the remaining three center and hold it together. The balls are preloaded and the joint is completely encapsulated. Thompson joints The Thompson joint (also known as a Thompson coupling) assembles two cardan joints within each other to eliminate the intermediate shaft. A control yoke is added to keep the input and output shafts aligned. The control yoke uses a spherical pantograph scissor mechanism to bisect the angle between the input and output shafts and to maintain the joints at a relative phase angle of zero. The alignment ensures constant angular velocity at all joint angles. Eliminating the intermediate shaft and keeping the input shafts aligned in the homokinetic plane greatly reduces the induced shear stresses and vibration inherent in double cardan shafts. While the geometric configuration does not maintain constant velocity for the control yoke that aligns the cardan joints, the control yoke has minimal inertia and generates little vibration. Continuous use of a standard Thompson coupling at a straight-through, zero-degree angle will cause excessive wear and damage to the joint; a minimum offset of 2 degrees between the input and output shafts is needed to reduce control yoke wear. Modifying the input and output yokes so that they are not precisely normal to their respective shafts can alter or eliminate the "disallowed" angles. The novel feature of the coupling is the method for geometrically constraining the pair of cardan joints within the assembly by using, for example, a spherical four bar scissors linkage (spherical pantograph) and it is the first coupling to have this combination of properties. Usage in cars Early front-wheel drive vehicles (such as the 1930s Citroen Traction Avant) and the front axles of off-road four-wheel drive vehicles used universal joints rather than CV joints. Amongst the first cars to use CV joints were the 1926 Tracta, the 1931 DKW F1 and the 1932 Adler Trumpf, all of which were front-wheel drive and used the Tracta joint design under licence. The CV joints allowed a smooth transfer of power over a wider range of operating angles (such as when the suspension is compressed by cornering force or a bump in the road). Modern rear-wheel drive cars with independent rear suspension typically use CV joints at the ends of the half-shafts and