Where does it go?
It covers and protects the CVJ (constant velocity joint) on the front (steering) wheels of the car, and on cars with independent suspension, the rear wheels also. These are the things which allow the front wheels to twist during steering and all the wheels to flex as they follow the contours of the road, and yet still remain connected to the car and (in front-wheel drive cars) transmit torque to the wheels. There are four of them on each car (one per wheel): the inside (nearside) CVJ boots (front and rear) which are on the side running nearest to the gutter and the outside (offside) ones (front and rear) which are on the side running closest to the centre of the road.
What does it look like?
A hollow, cone-shaped piece of black rubber or soft, squishy plastic about 100mm (4 inches) wide at the base and 150mm (6 inches) long. It has ridges along the outer surface which allow it to squash and deform. Up until about five years ago, they were all made from a type of elastomer (polychloroprene), but are nowadays made from a thermoplastic material, very often Santoprene.
Why is it used?
It is a semi-disposable thing designed to protect the delicate bearings within the CVJ from suffering damage from stones, dust and other particles that fly up from the road. Also, the CVJ is normally packed with grease, and the CVJ boot makes sure the grease does not spill out.
How long does it last?
YMMV. A lot of people suggest you change them every 60 000 miles/ 100 000 km, but if you often drive fast over poor roads and in the wet, or in sub-zero temperatures, then they are likely to get cut up more than if you drive exclusively on dry, well-maintained roads in warm weather. Usually the outer boot will go before the inner one.
How do I know if it needs replacing?
The inside face of your wheel and the brake area will get covered in nasty black grease, which is leaking from the CVJ. Also, you will see splits and tears in the boot which allow the grease to escape, and other foreign matter (grit, stones, glass, chemicals etc) to get into the CVJ.
How much does it cost?
You can buy kits of parts for around $15 / £15 / €15 which contain all the things you need to replace one CVJ boot (except, possibly the expertise and know-how). If you employ a professional mechanic, it will take an hour or so, and therefore add two to three times that price in labour costs. However, it makes a lot of sense to check and possibly replace the CVJ boots whenever you get the front brake pads replaced, because with the brakes disassembled, the CVJ boot is easy and quick (and therefore cheap) to change.
What happens if I don't replace it?
The grease will all leak out, and grit and other nasty stuff will get into the CVJ, which will end up making the CVJ fail. That will cost many times more to replace than a simple re-fitting of the CVJ boot. A CVJ replacement usually needs special tools and a garage mechanic. Think $300 to $400.
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