The front wheel drive component most people have heard of is a CV joint. A CV (constant velocity) joint has a similar function to a conventional Hookes-type universal joint, except that it can maintain an even rate of rotation (in relation to the input shaft) through various drive angles - often up to 40 degrees in a FWD. The most common type of CV joints comprise a housing, ball cage, balls and race as shown here.
An older and now less common way to obtain a constant velocity joint is to use an intermediate shaft with a Hookes joint at each end. The irregularity of the secondary shaft velocity during an arc of travel is then cancelled out the equal and opposite irregularity of the second joint. This type of double uni joint was used on some early American FWDs.
Every front wheel drive car also has a plunge-type CV joint that is usually the innermost joint.
These are designed to accommodate longitudinal (long-ways) movement of the shaft, as encountered when the suspension moves through its travel. This longways movement, while only fairly minor, would destroy a conventional fixed-position joint. The plunger-type joint incorporates a housing in which a series of parallel channels are set to carry a tripoid bearing through a degree of longitudinal movement. During this movement, constant velocity is maintained thanks to a tricky bit of internal design that allows the ball bearing cage to position itself in a plane that exactly bisects the angle between the two driveshafts.
Are they really unreliable as some people say?
Driveshafts - in particular CV joints - should not cause more trouble than any other drive component of a vehicle. However, good maintenance precautions include cleaning and inspecting the driveshaft, its protective rubber boots and perhaps changing the grease and boot periodically. While this is being performed, the condition of the joint can also be inspected. Some joints have a lubrication fitting that will allow you to apply grease with a low-pressure hand gun. When performing this operation, ensure you don't over-fill it or let the grease flow out of the joint, as this can allow moisture to enter the bearings and can also possibly damage the seal.
But for many vehicles, re-lubricating the joints is not possible. If your car doesn't have lubrication fittings, it can be taken to a CV specialist for a routine maintenance overhaul. In this case the joint will be disassembled, cleaned and re-lubed - and this can be a good (and cheap) preventative measure, depending on the severity of the joint's operating conditions. Remember, it's always cheaper to maintain the joint rather than fix it once it has broken.
Most Common Problems
The most common cause of CV failure can be traced to a broken rubber boot, which lets grease out and dirt in. If leaking or damaged joints are detected early, you might get away with simply replacing the grease and fitting a new boot - but the chances of some minor damage having occurred is still quite high. That clicking noise that is often heard coming from CVs is also often caused in the first instance by a broken boot. Once all the grease escapes from the ball cage, the individual balls are then able to slap around inside and wear out large grooves. It is this ball bearing free-play that makes the clicking noise, which if not rectified, can lead to a complete loss of forward or reverse drive. But another cause of accelerated CV wear is an overly low suspension setting. This makes certain parts of the joint work much harder and get quite hot - which is obviously bad for the lifespan of the whole CV.
In the most severe cases it is possible to literally blow the shaft apart - though this only occurs when the shaft is treated to 100% neglect. The housing photographed here shows what can happen when the boot gets ripped and the grease leaks out. It then gets very hot, seizes and bang!
Some cars use a slightly different CV joint to the mainstream versions. Toyota's early model Tercel 4WD, for example, has a shaft with a claw cast into the end. While this initially looks strong enough to handle the torque of a standard Tercel, looking in a CV specialist's bin will usually reveal quite a few of these shafts that are well worn out. That's because the inner tracks of each claw are the main load-carrying components, and they are therefore prone to wearing an initially small amount of material. Once this happens, there can then be enough free-play in the joint to allow the claws rattle around and cause accelerated wear.
It should be noted that the factory driveshaft adjacent to both the inner and outer CV joints should not be modified in any way. That's because the strength of such a highly stressed part is severely compromised once it has been, cut welded or tampered with in general. Although it appears that doesn't stop some workshops...
A few cheapy places still persist in cutting grooves into the shaft in order to help them get a grip on it so they can separate the shaft from the joint. But thankfully, most CV joint specialists will have the proper 30-or-so tonne purpose-built machine that is designed to pull the shaft apart properly and without sacrificing the shaft's strength.
If you need a stronger shaft or one of new dimensions, always start afresh with a new chrome-moly billet shaft. The length of the desired shaft is ordered from the supplier and it is then splined to suit your CVs. As with most one-off work, it isn't usually a very cheap job though. Shaft balancing is not required since the shafts spin around 3-4 times slower than a conventional rear wheel drive's tail shaft (ie the final drive gear reduction ratio has already occurred). Also, the relatively smaller diameter of a FWD driveshaft further reduces the effects of any possible shaft imbalance.
If the lack of strength of the CV joints is creating problems in a high power FWD, there is only one possible solution - go for a larger, beefier joint from another car. This isn't a simple exercise though, since the new joint has to suit the existing shaft spline and the wheel hub requirements. For this reason, you'll be lucky if you can find an OE part that will do the trick. While you may be able to source (or make) the right shaft to fit the new CV, the main problem is found in the wheel hub area. Because there is likely to be virtually no compatibility between the new CV and the car's stock front hub, the best bet is to try and fit a complete hub assembly (possibly with larger brakes) from the same donor vehicle as the bigger CV. If not, you may be able to get a custom or fabricated hub made up - but at a fairly high cost.
In case you're thinking of going the bodge way, the CV joint/stub shaft is cast in one piece and in the name of component strength it is not recommended that you try to modify just the stub shaft to suit the hub.
A Quick Word about Driveshaft Modifications...
If you are contemplating an engine/'box swap, it's a good idea to consider the weakest link that will be in the new driveline. Sure, you may have custom driveshafts made to piece the system together, but how does the strength of the original outer CV joint compare to the beefy inner one that you probably got with the rest of the front-cut conversion? By far the best approach is to use the inner and outer joints (complete with wheel hubs) from the donor car and then make up shafts to suit. This may mean extensive suspension modification as well, but at least this way you'll get a system with no obvious weak links and you'll score bigger front brakes is the process. And that's something you'll probably need if you want to get the conversion legally approved anyhow.
Of course, if all this sounds too hard and you find yourself for ever blowing driveshafts, simply take heed of the experts' advise and "revise your driving habits"!