While the wheel has remained more or less round since its inception, the axle has evolved to spin in all manner of configurations and situations. Insofar as automobiles are concerned, there are more than a few different types of axles. Some are of the short and stubby variety like the spindles upon which nondriven wheels spin.
Things get more complicated, however, when the axles have to deliver power to the wheels. If a powered wheel had nothing to do but roll on a perfectly smooth surface, the solution would be simple. A straight axle with no suspension would do the job.
As anybody who has recently driven around can attest, roads are everything but perfectly smooth; thus, the axle must be able to deliver power to the wheels and travel with the suspension as it soaks up the bumps.
A CV joint is a collection of bearings and cages that allows for axle rotation and power delivery on a number of different angles and planes and enables the axle to change length as it travels up and down.
All the parts spin around in a protective layer of grease that is held inside the joint by a flexible, rubber boot. Without the boot, the grease would simply spin out of the CV joints.
The boot also keeps dirt and grime out. On the CV axle are two CV joints along with one or two sets of splines. One side of the axle gets power from the engine while the other delivers the power to the driven wheel.
Over time the rubber boots of the CV joints can become cracked, torn or otherwise compromised. Once moisture and dirt get into a CV joint axle and grease makes its way out, the CV joint’s days are numbered.
Signs of impending failures are a click-click-clicking or metallic crackling noise while turning and accelerating or a clunking upon deceleration. While one can repack, reboot and rebuild CV axles, it makes more sense for the average do-it-yourselfer to procure replacement units and swap out the old axles.
If a boot is torn or missing, it is possible to save the joint if caught early enough, but usually the damage has already been done. Replacing or servicing the entire CV joint axle assembly may be the smarter move since replacing a boot may involve removing an axle, which may also involve removing half or all of the vehicle’s brakes and suspension.