Axles: We use them and abuse them, every 4x4 has them, and changing parts on them is what we do to turn bigger tires, climb steeper hills, and crawl massive boulders. But what are all those parts in an axle and how do they work together? Axles do the job of taking torque from the engine, transmission, and transfer case and redirecting it to the tires. This month we'll address the multiple components including the housing, ring-and-pinion, and the differential. Next month we'll reveal the secrets of axle-shafts, various bearings and seals, and front axles that have some sort of steering joint, stub shaft, and steering knuckles. If all this sounds like mumbo jumbo to you then hopefully these stories will help clear the fog.
There are two main styles of axles: independent and solid. The tried-and-true off-roader will swear by his solid axle, and considers independent units nothing but junk, better suited for grocery-getters and sissified dork-utes. But with nearly 20 years of independent axles being delivered from the OEM factories they are definitely common and being used on many trails, especially at high speeds.
Independent Suspension Axle An independent suspension axle has the housing solidly mounted to the frame and has half shafts running from the housing to knuckles or end housings at each wheel which can move independent of each other (thus the term independent). Most independent axles are found in the front of '87-and-newer 4x4s and are referred to as independent front suspension, IFS for short.
Solid Axle Under any 4x4 the most basic style is a solid axle (also known as a straight-axle). A solid axle is compromised of a housing that solidly attaches the wheels from side to side such that when one wheel articulates (moves up and down) the other wheel moves in the same or opposite direction. This housing is also attached to the vehicle's frame via some sort of suspension (spring), allowing the entire axlehousing to travel up and down with the tires following suit.
A solid axle often has more unsprung weight (this refers to any weight not supported by the suspension) than an independent suspension axle and less ground clearance. When doing high-speed driving, minimal unsprung weight is beneficial and thus independent suspension is chosen in most motorsports, but currently there is not an affordable independent suspended axle design that can match the strength and reliability of a solid axle for severe off-road use in rocks (though we would like to see one). Since most four-wheelers desire a solid-axle vehicle, we will be concentrating on them for this story, though many of the components are universal to both.
The axlehousing is basically a metal container that protects and holds all the important moving parts inside. The most important job of the housing is to keep the ends of the axle in a straight, rigid line, thus preventing gear or bearing failure or leaks at the seals. There are two major types of housing: the integral carrier version (also known as the Salisbury or Dana style) and the removable carrier version (also known as the Hotchkiss or drop-out style).
Integral Carrier housing An integral carrier housing usually consists of a cast centersection with tubes pressed in that are then plug-welded or riveted through holes in the centersection. In an integral carrier housing the differential must be assembled within the centersection and is supported by two carrier bearings that are clamped to the internal edges of the housing with bearing caps. Common versions of this housing include all the Dana or AMC axles found in Jeeps and other SUVs and trucks as well as the GM 10-, 12-, and 14-bolt. This type of housing requires that the gears and differential be set up within the housing, and as such it requires a heavier cast centersection.
Removable Carrier housing The removable carrier style housing is commonly a stamped- or forged-steel assembly that a cast third member, which holds the gears and differential, is then bolted into. Common versions of this style axle are found in Toyota trucks as well as Suzuki Samurais and older GM 3/4-ton trucks. The most common and well known, however, is the Ford 9-inch. This style axle is known for being lightweight, yet not usually as strong as a comparable integral carrier style unit. However, they can be made very strong with gusset reinforcements or completely fabricated aftermarket housings.