Last month we touched on some basic properties of axles ("EZ Axle Info," Feb. '06), the different styles, housings, gears, and differentials, but those bits are just the tip of these underframe icebergs. This month we'll tell you a bit more of internal parts that take torque and turn tires. Plus we'll drop some knowledge about what we consider valuable upgrades to keeping your tires trundling up rock-infested climbs and down muddy two tracks.
`Running from the differential out to the wheels within the housing are the axleshafts (on an independent axle you can usually see the shaft as the housing does not run all the way to the wheels). Axleshafts usually have small grooves or splines on one end that key into the sides of the differential, and either a flange if it is a rear axle or a yoke if it is a front axle at the other end. Very rarely are the axleshafts the same lengths side to side, even on rear axles where the differential is centered. The job of the axleshaft is to turn, yet also allow some bit of twist in order to counteract aggressive torque being applied to them. In this way axleshafts are very similar to torsion-bar springs that have some twist to them. In fact, many motorsports teams will replace the axleshafts after a season finding that they can twist up to 180 degrees from their original spec.
Size, Spline, and Material Depending on the type of axle there are different types of axleshafts. Heavier-duty axles usually require a thicker shaft with more splines. Dana 60 and Dana 70 1-ton axles may have 35 splines, where Dana 44 1/2- or 3/4-ton axles will more likely have a 30- or fewer-spline axleshaft. Of course there are always exceptions such as the GM Corporate 14-bolt that only has a 30-spline axle, though it does have the same 1 1/2-inch diameter as the Dana 35-spline shafts. Another misconception is that thicker axles are always better. An axle may be 1 3/4 inches along the shaft, be made out of standard OEM material and neck down to a 1 1/2-inch major diameter at the splines, while an aftermarket chromoly shaft that is made with the shaft body thickness equal to the median measurement of the splines (this is halfway between the major and minor diameter at the splines) will be able to twist and spring better while still returning to its original spec.
In a rear axle there are many different styles of axles, full-floating flanged axles, full-floating two-piece axles, semi-floating flanged axles, semi-floating two-piece axles, C-clip axles, and retainer-plate axles.
Full-Floating A full-floating rear axle is designed to support the most weight. This approach has a spindle on the end of the axlehousing and a hub that has the wheel studs pressed in and is supported by two bearings that rotate on the spindle. There are usually two spindle nuts that lock the hub on, and even with the axleshaft removed, the hub and bearings will support the weight of the vehicle. In this application the axleshafts are only required to transmit the rotation of the ring gear to the wheels. Most of these axleshafts are one piece with a flange that bolts it to the hub and splines at the other end and they are most commonly found under 3/4- and 1-ton 4x4s. There are a few aftermarket versions that use a two-piece full-floating axleshaft with splines on both ends and a drive flange that splines into the hub and the shaft.
Semi-Floating A semi-floating axle not only transmits the rotation of the ring gear but also bolts directly to the wheel via pressed-in wheel studs so that it must also support the weight of the vehicle. Most semi-floating axles have a forged flange as an integral part of the axleshaft and these are known as flanged axles or one-piece axles, but some older versions came with a two-piece axle comprised of a shaft that was splined and keyed into the flange with a nut that held the two parts together. We recommend upgrading to a one-piece axle if your 4x4 has two-piece shafts since they are prone to coming loose and shearing the key, not to mention that the design doesn't counter the leverage on the flange like a one-piece axle does.
C-clip vs. Retaining Plate Within the semi-floating axle design there are two ways to keep the shafts in place-a C-clip that sits in a groove in the end of the shaft within the differential and a retainer plate that captures a bearing that is pressed onto the shaft and then bolts to the end of the axletube behind the brake backing plate. The C-clip axle is inferior due to its design. First, in order to remove the axleshafts you must open the differential cover and remove a cross pin within the differential that locks the C-clip in place (thus you will only find a C-clip axle in an Integral Carrier housing). Secondly, if you break a C-clip axleshaft on the trail, the end with the wheel attached is likely to slide out and take your wheel and tire with it, which is also inconvenient. If we were going for ultimate strength in a rear axle we would choose a full-floater, though if light weight is our goal, we would choose a semi-floating flanged axle with retaining plate.