The Straight Axle DictionaryPosted in How To: Transmission Drivetrain on July 1, 2012 0) (
With over a century of 4x4 truck history, there’s been a lot of time for innovation. Light trucks use one fixed and one steering axle to drive four tires. Over the decades, changes have been made to 4x4 axles and the aftermarket has really stepped up to offer stronger, more robust assemblies for hardcore use. However, the basic straight (solid) axle assemblies and their construction have survived for decades. There are several basic types used in four-wheel drives. The designs can vary in load handling, axle support methods, and differential gearing configuration. We’ll take a look at the various designs and discuss their differences.
Semi-Floating vs. Full-Floating
There are two types of rear axles found on light-duty 4x4s: semi-floating and full-floating. There are pros and cons to each type.
A semi-floating axle is the most common in use on the rear of most 4x4s. Here, an axleshaft on each side is splined on its inner end where it mates to the differential and has a wheel flange with lug studs at the other end. This axleshaft assembly usually mates to the end of the axlehousing using some type of bolted flange. The axleshaft also rides on a large roller or ball bearing out at the end of the axlehousing.
The axleshaft in a semi-floating assembly serves two purposes. First, it attaches to the wheel and supports the weight of the vehicle. It must also serve to transmit rotational torque from the spinning differential out to the wheel.
In contrast, a full-floating axle uses its axleshaft only to transmit power out to the wheel flange. Its axleshafts may be splined at both ends, or splined on the inner end with a drive flange on the outer end. The shaft mates to the differential in the same way as one does on a semi-floater. Each end of the axlehousing has a hollow spindle where a wheel hub rides and where the wheel attaches. The spindle and its bearings support the weight of the truck. Since the full-floater shaft does not have to support the truck weight like a semi-floater, a full-floater of similar size is a considerably stronger axle assembly.
Advantages of a full-floater include being able to remove a broken axleshaft from the axlehousing and still have the ability to keep a rolling tire on that corner of the vehicle. If the axle has manual locking hubs, it may be possible to unlock the rear hubs for towing a disabled vehicle on the trail or for flat towing over the road.
Full-floating axles are often found on the rear of larger ¾- and 1-ton trucks rated at a higher load capacity. There are also some kits on the aftermarket that allow owners to convert some semi-floating rear axles to full-floating. All front straight axles would be considered full-floating because the axleshaft “floats” in the spindle and does not support the vehicle weight.
C-Clip vs. Pressed Bearing Designs
On a semi-floating axle, each axleshaft must be retained to the housing by some means. One method uses a C-clip inside the differential, and the other uses a pressed bearing out at the wheel end of the axleshaft.
With a C-clip-style axle, the shaft rides on roller bearings and it is held in the axlehousing by a C-clip inside the differential assembly. This clip fits in a small groove machined near the end of the axleshaft. To remove this clip requires removal of the differential cover, and may require partial disassembly of the carrier itself depending on the specific type of differential in the axle. Once the clip is removed, the axleshaft can be slid out of the axlehousing. On this type of setup, the brake assembly usually remains bolted to a flange on the outer axlehousing. Examples of C-clip axles include most Jeep Dana 35, Ford 8.8, and GM 10- and 12-bolt rearends.
On an axle using a pressed bearing setup, the shaft is held in place by the pressed-on wheel bearing and sometimes a pressed collar and/or retaining clip adjacent to the bearing. The bearing assembly usually fits into a flanged cup that bolts to a mating flange on the outer axlehousing. To remove the axleshaft on this type of axle typically requires removal of the hardware securing the flange. The brake assembly is sometimes captive on the axleshaft behind the axle ball bearing and is removed with the shaft. Examples of pressed-bearing axles include most Ford 9-inch and Dana 44 rearends.
Both axle retention methods have their good and bad points. Access to the differential area is necessary to remove a C-clip-type shaft. However, the pressed bearing-style sometimes requires removal of the brake line and dealing with fluid loss and bleeding upon replacement. C-clip-style axleshafts are known to try to separate from the vehicle should a shaft break, as the inboard clip is all that holds the shaft in the axlehousing. On a pressed bearing axle, the wheel and tire will usually remain intact with the bearing pressed to the axleshaft holding the assembly together. We’ve even seen Toyota trucks with a broken rear shaft limp along for some distance without the axleshaft separating from the housing.
Here again, the aftermarket offers some upgrades for C-clip axlehousings by offering a conversion to a pressed-bearing setup. The factory roller bearings are removed and replaced with press-on ball bearings. A pressed collar and shaft flange are added to the axleshaft and the assembly is bolted to the housing flange through the newly added shaft flange and the holes on the brake backing plate.
Cast Center Housing vs. Third Member
There are two types of differential housings used in straight axles. A cast center housing holds the differential carrier bearing caps and uses a bolt-on inspection cover. This is common on many domestic axles. All ring-and-pinion setup must be performed at the axlehousing, which means it often is performed under the vehicle. Examples of axles with cast centersections include Dana 30, Dana 44, Dana 60, and GM 10-, 12-, and 14-bolt axles.
An alternative method of mounting a differential carrier is in what is called a drop-out centersection, or third member. This type of axle is common on many imports and on the venerable Ford 9-inch housing. In this case, the carrier is mounted in a cast housing that bolts up to a flanged hole in the axlehousing. These are also known as banjo-style axlehousings.
One disadvantage to a third member axle is that there is typically no easily removed inspection cover. The entire third member must be removed to view or setup gears, and this requires removal of the axleshaft ends from the differential. However, once removed, the third member can be carried over to a workbench where all gear work can be performed. Gear setup is more convenient with this method. Trail swapping a broken third member is also a realistic option.
Low-Pinion vs. High-Pinion
Most often, a straight axle will have what is referred to as a low-pinion design, meaning the pinion gear meshes with the ring gear below the centerline of the axle. High-pinion designs are ones where the pinion gear meshes above the centerline of the axle.
The big advantage of running a high-pinion axle is that it keeps the driveshaft higher, improving ground clearance. This is especially helpful where tall lifts or short driveshafts are in use, and it provides lower driveshaft angle when compared to using a low-pinion axle. However, moving the pinion flange or yoke upward on the axle means that there may be more clearance issues with other vehicle parts during full up-travel.
When used in a front axle application a high-pinion axle and gearset is generally 20 to 30 percent stronger than its low-pinion counterpart. However, when used as a rear axle, a low-pinion axle and gearset is 20 to 30 percent stronger than the high-pinion version of the same axle.
Fixed Spindle vs. Unit Bearing
On a front axle, you’ll encounter two types of bearing assemblies: fixed spindle and unit bearing. For many years, 4x4 axles used a hollow fixed spindle on which a wheel hub spun on tapered roller bearings. In recent years, more OEM manufacturers have started to shift to the use of unit bearing assemblies. On these, a bearing set and spindle reside in a sealed, non-serviceable assembly that is bolted to the axle as a unit.
Traditional fixed spindle assemblies offer the ability to clean, repack, and replace individual components as needed. They are also typically a stronger unit than a comparable unit bearing assembly. For some applications, there are aftermarket kits to convert from the use of a unit bearing to a fixed spindle for longer wear life and the ability to perform maintenance.