Question:Why do front-wheel drive cars and minivans have
CV joints instead of ordinary U-joints?
Answer:U-joints are not used with front-wheel drive (FWD)
because they produce cyclic vibrations when operated at more than
a few degrees off-center. A U-joint will cause a change in speed
between the driving and driven shafts whenever the joint operates
at an angle. As the operating angle of the joint increases, the
speed (velocity) of the driven shaft starts to vary during each
revolution. And the greater the operating angle, the greater
the variation in speed of the driven shaft.
The driven shaft still turns at the same number of revolutions
per minute as the shaft that's driving it, but because of the
geometry of the U-joint the speed of the driven shaft alternately
increases (accelerates) and decreases (decelerates) four times
every revolution -- which causes the vibrations we're talking
about.
This isn't a concern in a rear-wheel drive application because
the U-joints on the ends of the driveshaft are positioned 180
degrees to one another to cancel out vibrations. What's more,
both U-joints always operate at the same angle. But in a front-wheel
drive application, the outer joint may have to operate at an angle
of up to 45 degrees when the wheels are steered. This is too
much of a difference between the inner and outer joints angles
for U-joints to handle. So constant velocity (CV) joints are
required.
Unlike a U-joint, a CV joint always drives the output shaft
at the same speed as the input shaft regardless of the operating
angle of the joint. Therefore, it doesn't make any difference
if the inner and outer joints operate at different angles.
JOINT GEOMETRY
In a U-joint, the four-point center cross attaches at two
points on either yoke. When the joint is bent, two of the arms
on the center cross travel in one elliptical path while the other
two arms follow a different elliptical path. This is what causes
the speed variations that result in vibration. It's hard to visualize,
but that's what happens.
CV joints handle joint angularity differently. The six balls
inside a "Rzeppa" style CV joint are positioned so they
always travel in a circular path exactly half way between the
joint angle. A circular path keeps velocity constant while an
elliptical path causes changes in velocity. So that's the inside
scoop on why U-joints won't work in FWD applications.
