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Off-Road Truck Axle Basics

Posted in How To: Transmission Drivetrain on July 1, 2011
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Photographers: Pete Trasborg

4x4 axles come in all shapes and sizes for recreational and competition wheelers. From the heavy-duty 21⁄2-ton Rockwell to the classic lightweight Dana 30, each axle has its own definitive qualities. These axle qualities can make some people either want to swap the said axle out for a better unit or spend a little dough and beef it up.

In this issue we gave you the rundown on differential lockers and how the traction aids can greatly improve the performance of your 4x4. While the locking differential is an extremely important part of the axle equation, it is just a part. From IFS housings to removable carriers, the wide world of 4x4 differentials is extremely complex and varied. Here we have compiled some axle basics to better inform you of what’s likely sitting under your Jeep, truck, and/or SUV platforms. For more axle info be sure to visit us at, where we’ve compiled years of our most in-depth axle-tech articles for your browsing pleasure.

Solid AxleHousings
Commonly referred to as the straight axle, the solid axle is the most commonly used axle type in the 4x4 world. Unlike IFS (independent front suspension), the wheels on a solid axle cannot cycle independently from one another. Rather, when one wheel travels or articulates over an obstacle, the other wheel travels in the same or opposite direction. Many solid axlehousings are comprised of a cast differential that has steel tubes pressed in, which are then plug-welded or riveted to secure them in place. The tubing diameter varies greatly from axle to axle, as does the weight. Other housings are one-piece with a dropout-style centersection for the differential.

Integral Carrier Housing
An integral carrier housing is one where the differential carrier, along with the attached ring gear, is supported by two carrier bearings and must be assembled inside the housing. An Integral carrier housing is commonly used by Dana, AAM, and GM. Integral has also become common in the majority of the light pickup and SUVs on the road toady. The integral housing is often viewed as stronger than most factory removable carriers because of its one-piece design. Though this doesn’t mean that your 9-inch is weaker than a Dana 27, it does mean that the overall housing and strength tolerances are different between the two.

Independent Housings
Since the mid-’80s, most pickup and SUV platforms have converted over from front solid axlehousings to IFS. These IFS platforms use both cast aluminum and steel differential housings and are center mounted within the framerails. To get the power from the differential to the wheels, half-shafts equipped with CV (constant velocity) joints are used. This setup allows the half-shafts to move independently from the differential as the suspension A-arms cycle. IFS components are generally lightweight and offer more ground clearance over solid axles. Though they are still not considered as robust as the traditional solid axlehousing, the aftermarket support and performance technology are rapidly making IFS-equipped trucks better suited for off-road use.

Removable Carrier Housing
This 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 axlehousing. Dropout-style carriers are known to be lighter, but the housing they attach to is not as strong. Companies such as Currie ( and Spidertrax (www. make beefy aftermarket housing that can be used with factory dropout units.

The differential housing supports the carrier and a ring gear and a pinion gear set. The pinion is the part that bolts to your driveshaft by way of an externally mounted yoke. 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, if the pinion has eight teeth and the ring gear 41, it creates a 5.13 gear ratio, as that is the number of rotations the pinion will make for the ring gear to spin just once. The lower the gear ratio, the higher the number. For example, a 5.13 gear ratio is considered a lower gear than a 4.56.

The most common housing type used is a low-pinion, also known as a standard-cut differential. The low-pinion differential uses a hypoid-style gear cut, which when placed in the rear of a 4x4 applies pressure to the stronger drive side of the gear. The main drawback of a low-pinion design is that the pinion generally sits below the centerline of the axletubes, which hurts ground clearance. A low-pinion is less desirable when used in the front of a 4x4 because it places drive pressure on the weaker, coast side of the gear. Flipping a low-pinion upside-down does not make it a high-pinion and will actually make the wheels turn the opposite direction! Also, a low-pinion gearset will not work in a high-pinion differential.

The high-pinion housing is also known as a reverse-cut differential, as the gear teeth are cut opposite of the standard direction to allow the axles to turn in the proper direction. In the front of a 4x4 the high-pinion is ideal because the reverse-cut gearset applies the driving force onto the stronger, drive side of the ring gear. The higher pinion positing also helps with ground clearance and driveline angles. In the rear of a vehicle the high-pinion is not as strong because it rides on the weaker, coast side of the gear. High-pinion gears cannot be used in a low-pinion housing.

A full-float axle is one that is designed for heavy vehicles and loads. Found in the rear of most 3⁄4- and 1-ton trucks, a full-float axle uses dual bearing hub assemblies that are fitted over fixed-end spindles attached to the outer potion of the axletubes. This spindle and bearing setup requires periodic maintenance and adjustment similar to the spindle and bearing combo that’s found on the front of many older 4x4s. Unlike a semifloat axle, which uses a single axleshaft and bearing per side to support and propel the vehicle, 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. Though not as common, variations of a front full-float axle use unit bearings along with a selectable hub.

Semifloat axles are very common and can be found in the rear of most Jeep and 1⁄2-ton 4x4s. The semifloat axle uses a bearing and axleshaft combination to both support and propel the vehicle. The outer ends of a semifloat axle are generally fitted with a forged flange from which studs protrude and the wheel bolts onto. 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.

While the semi- and full-float mounting flanges are straightforward, solid designs, the C-clip axle is a bit different. The C-clip axle is still a semifloat, but instead of a bolt-in style shaft security system, a C-clip shaft uses—you guessed it—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 Ford 8.8. When a C-clip axle fails there’s nothing to keep the shaft in place. Superior Axle & Gear (, among other companies, makes C-clip eliminator kits to help with this less-than-desirable axleshaft security.

Unit & Spindle Bearings
Found under most modern 4x4 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 that are preloaded to factory specifications. Unlike a spindle bearing setup, unit bearings are nonserviceable, but they are slated for long-life intervals. Using a splined centersection, the inner part of the unit bearing serves as a fully engaged hub, which causes the front drivetrain components to constantly rotate. A spindle bearing wheel end can use a selectable hub, which will allow you to disengage the front end. Disengaging the front end components will save you money, time, and wear on your expensive drivetrain parts. Though traditionally the spindle bearing and selectable hub has reigned supreme, the compact and versatile unit bearing has removed the classic selectable front hub from the front of most modern 4x4s.

Open & Closed Knuckles
Solid front axles have two steering knuckle designs, closed or open. A closed knuckle is common on older solid-axle vehicles and is designed to completely enclose the front axleshafts and seal the differential components. The idea behind a closed-knuckle setup is to better protect the front shafts and differential internals from damage and debris. Most modern 4x4s have gone away from the closed-knuckle design in favor of the open knuckle. Open knuckles use outer and inner axle seals to keep mud and debris out of the housing. Open knuckles also tend to be easier to service and offer better sight into potential steering shaft issues, such as damaged U-joints.

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Ball Joints & Kingpins
To connect the steering knuckle, an axle will either have ball joints or an upper kingpin with a lower pivot point for the knuckles to rotate on. A ball joint is a metal case that is crimped around a ball, and the ball has a threaded shaft protruding from it. A kingpin is a solid pin that either a bushing or bearing rotates around. Traditionally, closed knuckles only use the kingpin or pivot style setup. Though the strength advantages of one over the other are the subject of ongoing debate, most OE manufacturers as well as aftermarket axle builders, such as Dynatrac ( and Currie Enterprises (, now use ball joints. Though many insist that the kingpin design is stronger, it’s clear that the future currently rests with ball joints at both the aftermarket and OE levels.

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