Until you need to change its configuration or it falls on the ground in a twisted mess, most people don’t give their tailshaft much thought. However, breakages are both inconvenient and downright dangerous — even lethal!

First published in the November 2007 issue of Street Machine

Driveline Services has a wealth of experience in manufacturing and supplying the high performance and racing fraternities with a wide range of driveline and suspension components, so we asked the team for the low-down on a bulletproof tailshaft.

The most common failures are universals. Larger and beefier universals can be fitted to stock yokes, the biggest being the 1350 series — standard issue if you’re making more than 500hp. Note the relative difference in size of the six cylinder, typical V8 and 1350 (above) unis. Billet retention caps are preferable to the standard U-bolt style.

Most competition categories require a front loop that circles the tailshaft — about 150mm back from the uni. In the event of a front uni failure, it prevents the tailshaft from tearing up through the floor or dropping down and acting as a pole-vault.

As for the tube itself, the larger the diameter and the greater the wall thickness, the stronger the shaft is. Generally speaking, it’s better to go for a larger diameter than a thicker wall. Also the shorter it is, the stronger it is. Typical tube diameters are 2½in (standard six-cylinder), 3.0in (hot V8 and mild racing) and 3½in for more than 600hp (shown above).

Seamless mild steel is standard, while far more expensive chrome-moly and aluminium (above) are employed in extreme applications. Upgrading to a bigger, stronger tailshaft is all well and good, but keeping the tailshaft as light as possible will help reduce inertia losses.

Yokes can fail by snapping in half but can also lead to uni failure by spreading. Stepping up to a high-strength forged steel or billet gearbox slip yoke (above, left) and diff yoke (above, right) will address both problems. Similar in strength to forged steel, the billet style is more expensive but nicer looking. Most billet diff yokes are chrome-moly.

If your tailshaft needs to be shorter due to an engine, gearbox or diff swap, it’s possible your original shaft can be cut down and the yoke re-welded into the tube. This may require a step-down collar, which is a commonly available item (above). But if you’re also upping the horsepower considerably, it’s probably best to have a new shaft custom made.

Your tailshaft fabricator will tell you how to make certain measurements so they can recommend and manufacture the appropriate shaft. They’ll also want to know specs like vehicle weight, engine power and torque, rev range, and general application.

Shortening or manufacturing a custom tailshaft requires special equipment and is definitely not something that can be accomplished at home, especially when it comes to checking straightness and balance. An unbalanced shaft will rattle your fillings out and quickly destroy unis, as well as potentially damaging your gearbox, differential and engine as well. Tailshafts are balanced by welding on small tabs (above), or grinding metal away.

To ensure your tailshaft stays balanced and straight, don’t let things like handbrake cables rub on it. Watch out for overly close exhaust systems, as localised heating can warp the tube. And if you’re doing a diff conversion, be sure to check the diff yoke doesn’t smash into the floor at full bump.

Due to higher tailshaft speeds and non-optimal angle ranges, many late-model cars run two-piece tailshafts, which are known to give trouble. Centre bearings and CVs (above) become worn, causing significant driveline vibrations. Driveline Services offers heavy duty CVs and centre bearings for high-horsepower applications.

When setting up your tailshaft, the universal joints at either end must be exactly parallel to each other. If a uni fails or wears out, it’s best to replace both at once — they’ve both been under exactly the same stress and have rotated the same number of times. Excessively worn unis will result in driveline vibration (sometimes at very specific road speeds) and a characteristic clunk as you move off the mark.

With quality brands like Mark Williams, Strange, Ozdiff, Drive Pro and genuine Dana Spicer parts on the shelves, Driveline Services will be able to solve your driveline issues, regardless of application. Info: www.driveline.com.au, (07) 3275 7600.