The tailshaft is an often-overlooked part of a car's drivetrain. It's only when a tailshaft starts to give reliability problems or when you make a driveline change that you begin to consider it.
But a car pumping out a substantial amount more power than factory and putting it to the pavement via big, sticky tyres is a perfect example of a car that might need an upgraded shaft.
Different engine/trans/diff conversions also often require a modified or custom tailshaft to be fabricated. This comes about because the distance from the gearbox to the differential can often be greatly different to stock (and usually shorter). A typical example of this is fitting a 350 Chev, TH350 trans and 9-inch diff into a LH Holden Torana (that's a common combo for us here in Australia). Also, a different gearbox may require a change of tailshaft yoke if it's to match-up without obvious problems.
In any case, there are two possible solutions for these scenarios; either modify the OE shaft or fabricate a custom shaft.
Modern vehicles often come equipped with multi-piece tailshafts, in order that maximum interior passenger space is achieved. A multi-piece shaft makes this possible because its sections can be set at slightly different angles, so wrapping around the underside of the cabin. Usually, the shaft leaves the gearbox at a slight downwards angle, then angling back upwards as it nears the differential. While all this sounds fine, unfortunately the more pieces a tailshaft is made from, the less reliable it is.
The two-piece shaft photographed here was pulled from a 6 cylinder VB Commodore - a car that is reputed to have common tailshaft vibration problems caused by wear in the centre coupling.
Many cars born before the late '80s have a one-piece tailshaft, which is generally reliable except for the standard universal joints that can break. While the one-piece design is certainly the strongest, it is also more likely to be older and therefore more fatigued. But it is, nevertheless, a better starting platform than a multi-piece tailshaft.
Modification of Standard Tailshafts
When powering-up a car equipped with a multi-piece design, there are some changes that can be made to the stock shaft.
Depending on the particular shaft diameter, heavy-duty joints (or larger ones from another vehicle) can usually be welded onto the ends of the factory shaft. The better types of universal joints (such as Toyo, AEC and Spicer) often have a neoprene thrust pad. Fitting these joints can be an effective solution if the car isn't too extreme, but more than likely, a full custom shaft will be more reliable. The same goes for single piece shafts - they can also have their universal joints upgraded, but how marginal is the shaft itself going to be?
If, on the other hand, you only want to shorten the shaft (in order to accommodate a longer gearbox, for example), the standard shaft can be simply cut-down, depending on its structural condition. The required amount of tubing is removed and a new universal joint carrier can then be welded into its relocated position. Note that the practice of shortening an old tailshaft can be very dangerous if it isn't in good condition.
Unarguably, the best way to overcome reliability problems or to cater for a different driveline set-up is to fabricate a custom shaft. This way, you are virtually guaranteed there will no reliability problems.
At present, the use of standard mild steel tubing is prevalent in the construction of custom tailshafts. The main reason for this is a proven track record in strength at a reasonable cost. However, the development of aluminium (and also carbon-composite) shafts is progressing steadily. The gain associated with these lightweight shafts is a reduced moment of inertia that allows engine revs to build more rapidly (thus increasing acceleration). Essentially this is a very similar concept to a lightened flywheel; it allows components to be accelerated quickly.
Unfortunately, it isn't advisable to randomly buy a length of aluminium pipe and weld the universal joint carriers to the ends. That's because the properties of that particular piece of aluminium and its ability to cope with its operating conditions are unknown. What is its resistance to shear, for example? The only way to find out is if you have the aluminium extensively tested in a proper test facility beforehand (and who really knows what is regarded as an "acceptable" grade?).
Carbon fibre and carbon-composite tailshafts are being developed quite extensively in various motorsports. Works rally cars, such as the TTE Corolla, use a woven carbon fibre tailshaft to help reduce the rotating mass of their bulky all-wheel-drive drive system. It should be pointed out though, that some breakages and reliability problems were experienced with this particular TTE shaft, but it shouldn't be long until these developmental teething problems are sorted out. Hopefully...
The assembly of a custom tailshaft is usually pretty straightforward - there's the shaft and a single universal joint inset and welded each end.
The shaft tubing comes in various diameters and wall thickness, ranging up to about 100 and 3.5mm respectively. The larger the diameter and the wall thickness of the shaft, the stronger it is. A larger diameter shaft is generally more preferable though, as it they have a higher "critical whirling speed". This is the speed at which the shaft begins to twist along its length. Remember that it's also a good idea to consider the overall mass of the custom shaft - a huge diameter/thick wall shaft may be super strong but has increased inertia. Once the tube is cut to the appropriate length, the universal joint carriers can then be MIG welded to each end.
The choice of universal joint (and its carrier) depends largely on the particular combination you are going to run. However, the main determining factors are rpm range and the type of spline coming out of the gearbox and going into the diff. There is a large range of sizes available, with many of the larger ones being replacement parts for light trucks.
For those with a flexible budget, some exotic parts like a billet aluminium uni carrier are also available.
Please note that prior to designing your own tailshaft, you should talk to a specialist in this area who should be able to recommend an effective combination. Simply tell them the approximate weight of the car, its power, rev range and application and they'll be able to steer you in the right direction.
When installing the new shaft, the optimum set up is said to be with up to a 3 degrees shaft-down angle. This is due to the differential assembly's tendency to pivot upward slightly upon take off. Very similar gearbox and differential pinion angles should be achieved when the car is under way.
After the fabrication or modification of any tailshaft, it must be thoroughly balanced. The purpose of this is to eliminate any potential vibration that might otherwise occur, which decreases the service life of the tailshaft and its associated components. If improperly balanced (or unbalanced), universal joints wear much more rapidly and their bearings can also deteriorate quickly as well.
Other driveline components such as the transmission and diff will also benefit from a well-balanced tailshaft. If there is any vibration, it can cause small amounts of chatter that are damaging to splines and gears.
The process of balancing a tailshaft is quite simple, and not unlike balancing a newly fitted tyre. The shaft is rotated on a balance machine and has any wobbles fixed by welding on a counter weight. Alternatively, depending on the wall thickness, a tiny amount of material can be ground off.
For extra powerful cars that launch extra hard, some steps can be taken to ensure the effects of a possible tailshaft failure are minimised (meaning it won't kill you!).
A tailshaft loop is required by drag racing regulations if a car is competing inside a certain ET bracket, but as a clever precaution they can be fitted earlier. A tailshaft loop is simply a heavy-duty metal loop that surrounds the shaft and has a mounting plate that gets bolted to the underside of the car approximately six inches rearwards of the front universal joint. This strategic location of the loop prevents the shaft from flying up through the floor or "pole vaulting" the car in the event of a high-rpm breakage. Sound like safe insurance to me!