When it comes to off-road vehicles, 4×4 axles are a crucial part of the equation. These axles come in various shapes and sizes. In which each has its unique qualities that can make a big difference in terms of performance. In this article, we will cover the basics of different axle types, from solid axle housings to removable carrier housings, gear sets, and more.
The most commonly used axle type in the 4×4 world is the solid axle, also known as the “straight axle.” This type of axle housing cannot cycle independently from one another, unlike the Independent Front Suspension (IFS). When one wheel travels or articulates over an obstacle, the other wheel travels in the same or opposite direction. Solid axle housings often consist of a cast differential that has steel tubes pressed in and then plug-welded. The diameter of the tubing varies from axle to axle, as does the weight. Other housings are one-piece with a dropout-style center section for the differential.
An integral carrier housing is where the differential carrier, along with the attached ring gear, is supported by two carrier bearings and must be assembled inside the housing. Integral carrier housing is commonly used by Dana, AAM, and GM. This type of housing is often viewed as stronger than most factory removable carriers because of its one-piece design. However, the overall housing and strength tolerances are different between different carriers.
Solid Front – IFS Axles
Since the mid-’80s, most pickup and SUV platforms have converted from front solid axle housings to IFS. These IFS platforms use both cast aluminum and steel differential housings and are center-mounted within the frame rails. Half-shafts equipped with CV (constant velocity) joints are used to get the power from the differential to the wheels. This setup allows the half-shafts to move independently from the differential as the suspension A-arms cycle up and down. IFS components are generally lightweight and offer more ground clearance over solid axles. Though they are still not considered as strong as the traditional solid axle housing.
Removable carrier housing is the carrier type found under many Toyota pickups and Suzuki Samurais. The famous Ford 9-inch is also a removable carrier style. Also known as dropouts, removable carriers can be set up outside the axle housing. Dropout-style carriers are known to be lighter, but the housing they attach to is not as strong. Companies such as Currie Enterprises and Spidertrax make beefy aftermarket housing that can be used with factory dropout units.
The differential housing supports the carrier, a ring gear, and a pinion gear set. The pinion is the part that bolts to your driveshaft. As the pinion rotates, it drives the ring gear, which is bolted to the differential carrier. The ring gear has two distinct sides: drive and coast. Due to the way the ring gear is cut, the drive side of the gear is engineered to be the stronger side. The number of teeth the ring gear and pinion have determines the gear ratio of the differential. For example, a 5.13 gear ratio is considered a lower gear than a 4.56.
Low-Pinion vs. High-Pinion Axles
Low-Pinion
Another important factor to consider when looking at your 4×4 axles is whether they are low-pinion or high-pinion. The low-pinion design is the most common and uses a hypoid-style gear cut, which applies pressure to the stronger drive side of the gear when placed in the rear of a 4×4. However, the pinion sits below the centerline of the axle tubes, which reduces ground clearance. In the front of a 4×4, the low-pinion is less desirable because it places drive pressure on the weaker, coast side of the gear.
High-Pinion
On the other hand, the high-pinion design, also known as a reverse-cut differential, is ideal for the front of a 4×4 because it applies the driving force to the stronger, drive side of the ring gear. Additionally, the higher pinion position helps with ground clearance and driveline angles. However, in the rear of a vehicle, the high-pinion is not as strong because it rides on the weaker, coast side of the gear. It’s important to note that high-pinion gears cannot be used in a low-pinion housing.
Floaters Axles
Full-Float
Another design to consider is whether the axle is a full-float or a semi-float design. A full-float axle is designed for heavy vehicles and loads. It uses dual-bearing hub assemblies that are fitted over fixed-end spindles attached to the outer portion of the axle tubes. This spindle and bearing setup requires periodic maintenance and adjustment. They are similar to the spindle and bearing combo that’s found on the front of many older 4×4 vehicles. A full-float axle doubles and enlarges the bearing support within the hub and separates the axle duties. This design allows the shafts to solely be used to move the rig, and the massive hubs can handle the weight. To secure the hub in place, most use two spindle nuts.
Sime-Float
On the other hand, the semi-float axle uses a bearing and axleshaft combination to both support and propel the vehicle. The outer ends of a semi-float axle are generally fitted with a forged flange from which studs protrude, and the wheel bolts onto it. In between the mounting flange and the bearing often rests a backing plate that’s used to bolt the axleshaft assembly into the housing. Semifloating axles are great for lightweight rigs that are not hauling a large amount of weight.
C-Clips or Bearings
One other design to consider is whether the axle uses a C-clip or a bearing to secure the axleshaft. The C-clip axle is still a semi-float, but instead of a bolt-in style shaft security system, a C-clip shaft uses a C-shaped clip that secures the shaft from inside the differential carrier on the end of an axle. The two most common rear C-clip axles are the Dana 35 and the Ford 8.8. When a C-clip axle fails, there’s nothing to keep the shaft in place. To help with this less-than-desirable axleshaft security, companies such as Superior Axle & Gear make C-clip eliminator kits.
Found under most modern 4×4 front ends is a set of sealed unit bearings. A unit bearing is a one-piece hub and bearing combo that uses closely spaced opposed dual-wheel bearings. Unlike a spindle bearing setup, unit bearings are non-serviceable, but they are slated for long-life intervals. Using a splined center section, the inner part of the unit bearing serves as a fully engaged hub,. In which case the front drivetrain components to constantly rotate.
Another important aspect of the axle system are the ball joints or kingpins that connect the steering knuckles. Ball joints consist of a metal case crimped around a ball with a threaded shaft protruding from it. On the other hand, kingpins are solid pins that rotate around either a bushing or a bearing. Traditional closed knuckles use the kingpin or pivot-style setup. While the strength advantages of one over the other are still debated. They both OE manufacturers and aftermarket axle builders such as Dynatrac and Currie Enterprises now use ball joints. Even though some believe that the kingpin design is stronger, the future currently seems to rest with ball joints at both the aftermarket and OE levels.
Conclusion:
As you can see, the world of off-road vehicles is complex , with many types of axles to choose from, each with its own unique features and qualities. While solid axle housings are the most commonly used axle type in the 4×4 world. The other types such as integral carrier housing, independent housings, and removable carrier housing have their own advantages. Differential housings support the carrier, ring gear, and pinion gear set, which determine the gear ratio of the differential. Low-pinion and high-pinion housings are two common types of differential housings, with high-pinion being ideal for the front of a 4×4 and low-pinion for the rear. The full-float and semifloat axles are two types of axles commonly used in heavy and lightweight vehicles, respectively. Finally, ball joints and kingpins are used to connect the steering knuckles.
Understanding the basics of these different types of axles and their unique features is essential for any off-road enthusiast who wants to build and customize their 4×4 vehicle for optimal performance. Whether you’re looking to upgrade or replace your current axle system or simply want to learn more about the world of off-roading, knowing the basics of axles is a great place to start. With this knowledge, you can make informed decisions about which axle system is best suited for your needs and build the off-road vehicle of your dreams.