Reply to "Cv conversion"

U-joints have three characteristics that must be accounted for when an engineer is using them.

First, drive shafts and half shafts equipped with u-joints must be set-up in the vehicle to operate with a small constant angle in order to prevent driveline vibration and to improve the life of the u-joint. Achieving this "small constant angle" is accomplished with independent rear suspension having "half shafts" by off-setting the axial alignment between the differential or transaxle (gearbox) output shafts and the rear wheel stub-axles. The radial axis of the two components connected by the half shafts must remain parallel to each other, but off-set. The amount of axial off-set between the two components connected by the half shafts is critical in order for the u-joints to operate within the acceptable range of angles; that acceptable range is ½° to 3-½° (nominally 2°). The reason u-joints must be set-up with at least ½° of operating angle is because u-joints require some angle or “movement” to minimize wear on the needle bearings. Excessive wear leads to a type of bearing damage known as “brinelling”. This angle appears to have been built into the Pantera chassis by mounting the ZF gearbox slightly rearward in relation to the stub axles. Thus the half shafts operate at a small constant horizontal angle.

Secondly, there is a maximum limit to the amount of angle that a u-joint" should operate at. Exceeding this limit places undue stress on the u-joints, and the bearings that support the gearbox output shafts and stub axles. When the Pantera's suspension deflects upward, such as when it encounters bumps in the road, the vertical angle of the half shafts increases. During extreme deflections (large bumps) the short length of the half shafts causes the u-joints to operate at extreme angles exceeding the prudent limits to their operation. This increases the stress upon the u-joints and stub axle bearings from time to time.

Lastly, and most significantly, the u-joints at either end of a driveshaft or half shaft should not have operating angles differing by more than ½° from each other. A difference in operating angle greater than ½° tries to make the tires speed-up and slow-down during each rotation; the tires don't want to rotate at a constant speed. But of course due to the traction of the tires and the inertia of the moving vehicle this can't happen, so the parts bind, bend and deflect. This is all happening at the "output end", i.e. at the stub axles. The result is harmonic vibration AND severe stress upon the u-joints and stub axle bearings. This is where two problems arise in independent rear suspension applications. (1) Any amount of rear wheel toe shall horizontally misalign the axis of the differential/gearbox output shafts and the stub axles. Any amount of rear wheel camber shall vertically misalign the axis of the differential/gearbox output shafts and the stub axles. Yet for best tire performance the rear wheels must be adjusted for a small amount of both toe and camber. The misalignment on the horizontal and vertical planes combine to change the angle of the u-joints at the stub axles in relation to the angle of the u-joints at the output shafts of the differential/gearbox. (2) As the rear suspension travels up and down the rear wheel camber changes significantly thus increasing the vertical misalignment between the axis of the differential/gearbox output shafts and the stub axles. This is where the stress upon the u-joints and stub axle bearings is most severe, and it is why the Pantera's stub axle (rear wheel) bearings wear as fast as they do. This is also why CV joints make the vehicle operate so much smoother.

CV joints would have been a better choice in this application because (1) CV joints are designed to operate at more extreme angles AND (2) they are designed to cope with differing angles at either end of the same shaft. The Maserati Bora (1971 – 1978), a mid-engine sports car closely related to the Pantera, was equipped with CV joints. The half shafts installed in the last version of the Pantera (i.e. the Pantera Si) were also equipped with CV joints.

If its in the budget a CV joint conversion is a wonderful improvement for a "driver" Pantera or a Pantera race car. It is the "real fix" for the fast wearing rear wheel bearings.