Toyota-Lexus-Subaru-Honda CV boots (also called constant velocity joint boots, or axle boots)

The front axles (and some rear axles with independent rear suspension) have flexible joints on the inner and outer ends of the axles to allow your wheels to get power smoothly even while going around corners or when going up and down relative to the vehicle.

This rubber boot is intended to protect the constant-velocity joint from losing its grease and from being damaged by water or road dust.

Eventually, after flexing through millions of rotations, these boots tend to crack and split open. When this happens, if the axle joint is still good, we routinely replace the boots and repack the joints with fresh grease. As long as the boots are intact, the original Toyota joints tend to hold up so well that I prefer replacing the boots over replacing the entire axle.

If the joint has been damaged, then the entire axle needs to be replaced. As the damage to the axle joint progresses, it can eventually impair your ability to corner safely. Although rare, in extreme cases it can even break.

When the axle breaks, at minimum the car abruptly loses power to the wheels and won’t drive. Sometimes it can damage other surrounding parts. A friend of mine once had an axle break and it ripped loose his brake lines so that he simultaneously lost the ability either to accelerate or brake. He had just come down out of the Pyrenees Mountains-off a steep, winding road that ran along cliff’s edge-and out onto a level, relatively safe place to have it happen. Whew! Close call.