Rear Axle Tech
What's Spinning in Your Truck's Housing?
There are several basic rear axle types used in 4x4s, and each have specific design pros and cons. Axle designs can vary in load handling, bearing and flange type, and gearset configuration. In this article we'll take a look at rear axles and discuss the benefits of various designs.
SEMI-FLOATER vs. FULL-FLOATER
There are two types of rear axles found on light-duty 4x4s: semi-floating and full-floating. Each has its advantages and disadvantages.
A semi-floating axle is very common on the rear of most 4x4s. It consists of an axleshaft on each side that is splined on the inner end where it mates to the differential and has a wheel flange where the wheel studs mount at the other end. This assembly typically mates to the end of the axlehousing using some type of flange arrangement. The axleshaft also rides on a large roller or ball bearing out at the end of the axlehousing.
1. This Dana 35C semi-floater rear axle from a late-model Jeep TJ uses a flanged outer axle housing (brake backing plate and components removed). The shaft and flange that holds the wheel studs are all one piece.
The axleshaft in a semi-floating assembly serves two purposes. First, it attaches to the wheel and is used to support the weight of the vehicle and its cargo. Second, the axleshaft must transmit the rotational torque from the differential out to the wheel.
A full-floating axle can be found on the rear of some 4x4s, but it is generally reserved for vehicles that are designed for severe duty, or are intended to carry heavy loads. This type of axle uses an axleshaft on each side that is simply splined at both ends or splined on the inner end and has a drive flange on the outer end. The shaft mates to the differential in the same way as a semi-floater. However, the outer end of the shaft differs. Here, the splined end of the shaft slides into a locking hub or an internal splined steel drive plate that bolts to a hub cap, similar to what is found on a front axle. In some cases, the drive flange may be part of the shaft itself. In either case, the axleshaft is allowed to float in the system.
For a full-floater system, the axleshaft only serves to transmit the rotational torque from the differential out to the wheel. It does not carry the weight of the vehicle like a semi-floater does. On a full floater, a spindle is attached to the outer end of the axlehousing. The hub's cap is attached to this spindle and rides on tapered roller bearings. It is this assembly that carries the vehicle weight. As such, a full-floating axle system is considerably stronger than an equivalently sized semi-floating system.
2. A full-floater axle is easily recognized externally by the drive flange or locking hub that is evident in the center hole of the wheel.
For those of you who carry heavy loads, this means your axle load capacity is greatly increased with a full-floater. Load ratings for similar vehicles with the two different axles are usually significantly different. If you do hard-core 'wheeling on big tires, a full-floater means that your axleshafts can also handle much more loading than a similar semi-floater could because it now must only handle torque loading.
Further advantages of a full-floater include being able to remove a broken axleshaft, yet still have the ability to keep a functional rolling tire on that corner of the vehicle. This can be done since the wheel actually bolts to the hub that rides on the spindle attached to the axlehousing. 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.
It is now possible to convert some semi-floater rear axles to full-floaters using aftermarket kits. These kits allow an owner to easily upgrade the axleshaft strength of his axle. However, such a kit does not upgrade the differential assembly, so axles with this portion as a weak link would not benefit much from such a conversion.
3. This is part of a Toyota full-floater bolt-on conversion kit. A spindle is attached to the rear housing flange, and a wheel hub is supported using tapered roller bearings, such as those typically found in the front axle.
C-clip vs. PRESSED BEARING
When it comes to holding the axleshafts in a semi-floating axle housing, there are two methods. One uses a C-clip inside the differential assembly, and the other uses a pressed bearing out at the wheel end of the axleshaft. On a C-clip-style axle, the axleshaft rides on roller bearings and is held in the axlehousing by a C-clip in the differential assembly. The clip fits in a small groove machined near the end of the axleshaft. To remove this clip requires the removal of the differential inspection cover, and may require partial disassembly of the carrier itself depending on the specific type of limited slip or locker used in the axle. Once the clip is removed, the axleshaft can be slid out of the axlehousing.
On an axle using a pressed bearing setup, the axle is held in place by the pressed-on wheel ball bearing and possibly a pressed collar 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. This type of axle uses ball bearings because the bearings must support both radial and axial loads (perpendicular and parallel to the axleshaft).
4. Here you can see the axleshaft and end plate of a typical full-floater that uses an attached drive flange. Other full-floater axles may have the shaft and drive flange as separate pieces.
There are advantages and disadvantages to disassembling each type of axle. The C-clip variety requires access to the differential area, but the press bearing variety requires brake line work and brake bleeding. Another difference is that when an axleshaft on a C-clip assembly breaks, there is nothing left holding the axleshaft in the housing so the tire and wheel assembly will readily separate from the vehicle. On a pressed bearing-type axle, the wheel and tire will usually remain intact, with the bearing pressed to the axleshaft holding the assembly together.
For some axles that are factory-equipped with C-clips, there are kits designed to eliminate the C-clips and convert the setup to a pressed bearing setup. These eliminator kits solve the problems associated with standard C-clips. The stock 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.