First published in the December 2007 issue of Street Machine

Rick Luly, proprietor of Queensland Performance Axles And Centres (QPAAC) is a driveline specialist, with more than 15 years’ experience. Rick’s also responsible for developing the super-tough range of Aussie-made AP Axles (below), so we asked him what’s what when it comes to axles. There are two types of axles: cast and forged. Many factory axles are cast, while virtually all aftermarket (or custom) axles are forged.

With forged axles, the forging machine grabs the length of raw bar-stock, then a massive hammer-like weight comes down and smashes into one end, mushrooming it out to form the flange, a process called upset forging.

While some factory castings can be quite strong, all aftermarket axles are superior in strength due to their forged manufacture and higher quality material. AP also equips its axles with thicker flanges, which give the wheels greater stability. Thinner factory flanges tend to crack from the wheel stud holes.

Custom axle manufacturers are coy about material specifics but Rick did say that there’s a high manganese content in the steel, which makes it a better spring. A completely rigid axle would be brittle and simply shatter. Axle length also affects springiness — longer axles can wind up further (and spring back) than shorter axles.

Case hardening is important to ultimate strength. A hard outer with a softer centre results in the best compromise of strength and durability. Here’s where you run into problems re-splining factory axles. Most stockers neck down just before the spline; when shortened they must be cut down past this section, towards the larger diameter main shaft. Machining this section down to the spline diameter eats deep into the hardened outer layer, resulting in an axle that’s often weaker than the original.

An alternative method requires softening (annealing), cutting, re-splining and re-hardening But this process often results in through-hardening. Without the softer centre, the axle becomes overly brittle and failure prone. Since custom axles are relatively inexpensive, it’s no longer worth re-splining factory axles.

The splines themselves are all the same size and pitch, so as you step up to a stronger, larger diameter axle, you also go up in spline count — 25 and 28 are stockers; 31 is high performance (factory and aftermarket); 35 is ultra-tough street car up to Doorslammer territory; while 40-spline is for extreme and specialist applications. The bigger an axle’s diameter gets, the heavier it becomes, which is bad. To reduce weight, axles can be gun-drilled, an expensive procedure that adds a couple of hundred dollars per pair.

Not all custom axles are created equal. Many American-made units don’t cater for the small-bearing Australian brakes, which covers the Ford, Holden and Chrysler families. Rick also says that to save money, some companies machine their 28 and 31-spline axles out of the same stock. While the case hardening on a custom axle is thicker than a stocker, it’s only so thick and when machined down to make 28-spliners, the case hardening can end up overly thin. AP manufactures specific diameter cores (above) to suit each diameter and spline-count.

There are two schools of thought on whether cut or rolled splines are stronger. Rick believes it’s much of a muchness. Cut splines have earned a bad reputation due to the old practice of re-splining factory axles. Cut splines (above) maintain the same diameter as the major diameter, whereas a rolled spline slightly increases in diameter.

Multi-fit custom axles are a budget option. While not as good as purpose-made custom axles, they’re far stronger than stockers. Generally they come with about 75mm of spline, which is cut down to suit the application (only about 25–30mm of spline goes into the diff’s spider gears). Generally, they’re also multi-drilled, so they don’t have a socket access hole for getting into the bearing retainer plate bolts. The beauty of a custom axle is the flanges come as blanks (above), which can be machined to any PCD and spigot diameter and still have access holes.

When they fail, axles break in two main places: where they come out of the carrier or spool (1, 2 & 3) or where they step up, next to the bearing (4). Twisted or broken splines are material or diameter issues; a forged axle that incorporates a generous radius between the shaft and the bearing step (below) will address outboard breakages. Insufficient fillet radius creates massive stresses that invariably lead to failure.

New axles should be accompanied by new seals. Oil leaks due to incorrect seal installation are a common issue with streeters using Ford 9in diffs. Never knock the seal all the way in until it bottoms out on the seat — it should end up flush with the bevel at the end of the housing. One trick QPAAC employs to overcome persistent leaks is to opt for a bearing that incorporates an O-ring seal.

Thanks to QPAAC/AP Axles (www.qpaac.com.au) for assistance with this story.