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From: oneill@lll-crg.ARpA (Neil J. O'Neill)
Newsgroups: net.auto.tech
Subject: Re: wheel bearings
Message-ID: <974@lll-crg.ARpA>
Date: Fri, 8-Nov-85 01:58:44 EST
Article-I.D.: lll-crg.974
Posted: Fri Nov  8 01:58:44 1985
Date-Received: Sat, 9-Nov-85 05:47:37 EST
References: <2427@sunybcs.UUCP> <204@ucdavis.UUCP> <2180@amdahl.UUCP> <114@emacs.UUCP>
Distribution: net
Organization: Lawrence Livermore Labs, CRG, Livermore Ca
Lines: 42
Summary: what are CV joints


> OK, what is the difference between a bearing an a CV joint.  My
> 1981 Dodge Omni, (yup readers, the same one) has "worn front CV
> joints".  On one of them (drivers side) the rubber bag that holds
> the grease is ripped.  The grease can disappear.  ...

CV or constant velocity joints serve the same purpose in a front wheel
drive car that U-joints serve on a rear wheel drive car.  That is, they 
allow a flexible connection between the transaxle ( or transmission) and
the drive wheels.  On a fwd car there is a CV joint at each end of each
drive shaft.  One connects to the transaxle and the other to the wheel.
They are called constant velocity joints because they provide for the
linear transfer of angular velocity through the joint.  That is, if a shaft
on one side if the CV joint is run at a constant 3000 RPM a wheel connected
to the other side will run at a constant 3000 RPM regardless of the angle
of the joint.  If you run a constant 3000 RPM into a U-joint you will get an
output that only averages 3000 RPM but actually speeds up and slows down
during a single revolution (play with the U-joints in a socket-wrench set
and you will see what I mean).  This non-linear transfer of rotation
depends on the angle of the U-joint.  For a straight joint the effect is
zero.  And for the small angles encountered in rear wheel drive it
is negligable.  However, it cannot be neglected in the angles encountered
in a fwd car. The angles are greater because the drive shafts are so much
shorter and the wheels also have to turn to steer.  So fwd cars use CV
joints so that their drive trains don't self-destruct.

The CV joints have ball-bearings inside of them, but these are not the
front wheel bearings.  The front wheel bearings are the ball|roller|
taper bearings that allow the front wheels to spin smoothly inside
the wheel housing.  

If the CV joint protective boot is cracked then grease can get out and 
water can get in and it would not be suprising if the joint wore out
in short order.  So called "split-boot" kits can be obtained which allow
you to replace the boot without taking off the drive shaft -- the boot
is split and you glue it back together after wraping it around the shaft.
I have one of these on my Rabbit and it seems to be holding up.  It is 
possible that your CV joints just need to be repacked with grease; you
might want to ask your mechanic about this.


oneill@lll-crg.arpa