How Differentials Work: How Complicated Can It Be?

How axles work, what are they and what do they do?

At Houston Rebuilt Axles, we sell rebuilt axles. Are you asking yourself, “what is a rebuilt axle and why would i want one?” You may have heard of the rear axle, rear end or differential of your vehicle but you don’t know much about it. It can also be hard to troubleshoot axle problems. In this series of articles, you can learn all about axles, the various types, how axles work and how differentials work.

Doesn’t it just turn the wheels? How complicated can it be?

Actually, correct function of the rear axle is of major importance to the vehicle operation and thus the safety of its occupants. An improperly functioning rear end assembly can be catastrophic at high speeds. At a best case scenario it will just leave you stuck on the side of the road.

Throughout these articles, you will gain an appreciation for how complicated axles actually are. This will help assure you that having professionals work on your axle is the best option for you and your vehicle.

How does the axle work in relation to how differentials work?

The axle is commonly referred to as the rear end or rear differential. Did you know that referring to it as the differential is a common mistake as the differential is only a part of the rear axle? How a differential works is a complicated process and will be addressed in steps. There are a variety of differentials and how differentials work depend on the type. We will discuss them in other articles.

So what is the rear axle?

The rear axle is the final leg of the drivetrain. It is the final point where rotational speed from the driveshaft is applied to the rear wheels. The basic design of axles has been adopted by all manufacturers with several variations. Despite these variations, internal operation works under the same basic principles.

What are the different types of rear axles?

Axles can be classified into two broad groups:

Solid axles incorporate rigid non-flexing drive axles and axle tubes. Due to this, they are referred to as solid-axle rear suspension. This means that both wheels will move in response to bumps on the road.

Independent rear suspension incorporates jointed drive axles so that each has a point for movement. A bump on one wheel does not affect the other wheel. This system does not incorporate axle tubes and means each wheel has independent axle movement. It is thus referred to as independent rear suspension.


Let’s start with the Solid rear axle Solid Axle Housing showing axle components


The main components are the rear housing, differential assembly and the axle shafts. In heavy duty applications i.e. when a rear end is subjected to heavy loads, the wheel hubs or spindle ends can be considered part of the housing as they are mounted/attached to the axle tubes. However this is not the case in light duty vehicles due to their added cost and complexity. The rear housing contains the differential assembly. The axle shafts span from the differential assembly through the axle tubes to the axle flange/hub/spindle end and wheel.

So, what is the benefit of the solid rear axle?

Even when subjected to heavy loads, the solid rear axle housing is of rugged construction and seldom fails. It not only acts as a housing for the differential and axle shafts, it also forms a reservoir for the rear end lubricant, without which, the differential components would overheat and fail in just a matter of miles.

Additionally, the housing also supports attachment points for various suspension components such as springs, shock absorbers and control arms. In some cases, the rear brake assemblies may also have support points on the axle housing.

What are the parts of the rear axle housing?

The large somewhat circular middle part is known as the central housing or differential carrier. As the name suggests, this is where the differential is housed. The long tubes extending to the wheels are the axle tubes which in some applications have the spindle ends attached. With both parts made most commonly out of steel, it is understandable how these components are so durable. Other features on the differential carrier include a fluid drain plug, fill plug and pressure relief vent associated with pressures related to differential gear movement. This vent will be located towards the top of the housing.

So what parts of the axle need repair?

The most common failures include a variety of bearings throughout the axle and the ring a pinion gears. The bearings inside the housing which support the rotating internal components of the differential assembly, axle shafts and pinion gear (explained later) are worn out over time simply due to mechanical wear. Another common failure are the ring and pinion which transfer drive power from the drive shaft to the axle shafts. This process will be discussed in detail in an upcoming article. There are other components that fail such as the clutch discs in a clutch-type differential. Components like this are specific to certain types of differential but are essential in how differentials work. It is no surprise considering the forces put on the ring and pinion gears and bearings that their failure is common, however, regular maintenance will prolong the time before failure. Ask us about preventative maintenance!

Bent Axle and Spindle End Repair

Despite the ruggedness of the rear axle housing, other problems can occur. One such example is spindle ends. As mentioned previously, in heavy duty applications (some ¾ ton axles and almost all 1-ton axles), the spindle end is the mounting point for the wheel and thus it carries the weight of the vehicle (and its load). This is to prevent the load being placed on the axle shaft as in other light duty vehicles which would result in its premature failure. However, spindle ends can also get damaged and bent due to excessively heavy loads and impacts endured by the wheels as a result of significant bumps and holes in the road.

If spindle ends are attached to the housing, can spindle ends be repaired?

Yes but by very few people! We are one of only a few companies in the country who can manufacture and install spindle ends. Both manufacture and installation are highly specialized techniques requiring specialized equipment and of course, the experience and know-how. See an article on this process!

Occasionally people seek junkyard axles which can end up costing you even more money. A slightly bent axle tube or a worn out spindle end can upset the entire function of the axle. This is why it’s better to rely on axles prepared or repaired by professionals with warrantied work to OEM standards.

So is that all there is to know about how axles work?

No, we have only just begun. Next we will talk about what happens inside the differential to make the wheels turn at different speeds and why! This is essential in understanding how differentials work as this is the basis by which they all work and how axles work.