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Extending the life span of CV joints

By unknown | Jul 11, 2007 | COMMENTS [ 0 ]

Front-wheel drive (FWD) is all the rage at the moment, at least for small and medium-sized sedans and small bakkies.

Front-wheel drive (FWD) is all the rage at the moment, at least for small and medium-sized sedans and small bakkies.

And for good reason - a front wheel drive system is cheaper to manufacture and install than a rear-wheel drive (RWD) system.

FWD saves weight by eliminating a heavy prop-shaft and bulky transmission components in the rear axle.

It makes space available in the passenger compartment and boot, and it provides better traction in slippery conditions.

But there are no gains without pains, because FWD also has disadvantages compared to RWD.

It is easier to work on the components of a RWD system than FWD. Rear-wheel drive is more robust because its "universal" joints are less trouble-prone.

And yet the CV joints on FWDs are over-specified and should last for 150000km if you look after them. Why then do they have this unfortunate reputation? Why do we need them?

Whenever a rotating drive shaft transmits power from an input shaft to an output shaft, and the centre lines of the input and output shafts don't lie along the same straight line, flexible couplings are needed at the ends of the intermediate shaft.

These flexible joints must be able to swivel in all directions because they will be flexing while the drive-shaft is spinning.

In the traditional RWD layout, this job was done by simple two-yokes-and-a-cross universal joints on the prop-shaft.

A flexing U-joint has the property that its output end will have a slightly jerky movement as it goes around, and the bigger the flexing angle, the more pronounced the jerkiness becomes.

On RWDs the flexing angles of the joints on the prop shaft are small, so the jerkiness is slight, and it can be smoothed out by fitting two U-joints, correctly aligned, on the prop-shaft.

It's a different story when we come to FWD.

Here we find shorter drive shafts, instead of a long prop- shaft. As the front wheels move up and down, the flexing angles will be considerably bigger.

What's more, the outer end of each drive-shaft is joined to a hub carrying a drive wheel. And the angles through which the front wheels must swivel exceed the flexing angle on a RWD prop shaft.

So the joints on a FWD drive shaft need more work than the simple U-joints.

This is where CV joints come into the picture.

They are designed to ensure that during rotation the output side will maintain a constant angular velocity equal to the input side, irrespective of the flexing angle.

As mentioned before, these are tough components on modern cars. But they still have an Achilles heel, namely the rubber bellows also called a boot, gaiter or dust cover enclosing them.

The rubber boot serves two purposes: firstly, it keeps the vital, special CV joint grease with which the joint is packed from being thrown outward and lost as the joint spins; secondly it protects the joint from road grime, dust and water getting into it and causing rapid wear.

As long as the dust covers are intact and tightly clamped to their sealing faces, your CV joints have every chance of doing their job flawlessly for 150000km.


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