Diagnose Rear Axle Problems

Today’s rear axle designs are not much different than they were 30 years ago so understanding rear axle problems can span many generations of classic cars. In recent years, rear axles have become so robust and reliable that most mechanics have long forgotten how to diagnose and repair them, much less modify them. The rear axle is one part of the vehicle that has evolved over time into a variety of designs but has never been replaced. We cannot say that about many other technologies in the vehicle. While ignition points, carburetors, and lever shocks have faded into obscurity as more precise and efficient devices gradually superseded them.

The most frequent complaint made of any rear axle is a gear-generated or bearing-generated noise. The next most common complaint is a fluid leak. Fortunately, actual gear failures are very uncommon on modern vehicles, so gears are typically serviced only when the vehicle owner desires a ratio change. Altering the axle ratio or swapping a limited-slip differential in place of an open differential can change the performance and feel of the vehicle.

For instance, a 500-hp big-block with a 2.73:1 ratio axle will not launch anywhere near as hard as the same car with a set of 4.10 gears. However, you need to resist the temptation to install 4.10 gears simply because it is a larger numerical ratio. First you must understand the effect of tire size on the overall ratio. And you must understand the basic structure and maintenance of axles.

The typical beam axle must provide the structure to support the gears and differential. It also needs to provide mounting points for the brake hardware and suspension and must react to loads imparted by every interfacing vehicle system.

Common Rear Axle Problems

To best diagnose a rear axle problem, it is always preferable to road test the vehicle before tearing it apart unless, of course, the part failed catastrophically and made its own exit through the axle housing. Most issues can be lumped into one of the following categories: noise, vibration, or leakage. Try to isolate the exact circumstances that exhibit the noise. Is it occurring during mild acceleration, heavy acceleration, during deceleration, or all the time? Does it get worse as you drive faster or slower? Listening is a crucial step as it can help guide your pursuit to answer the question, “What broke?”

Rear Axle Noise

Typically, noises that get worse as speed increases are related to bearings or to the wheel/tire assembly. Tire noise is usually most prominent around 60 to 80 mph. Noises that change with acceleration and deceleration are typically gear noises. (Vibrations usually point to the universal joints or to driveshaft balance issues.) Clunk noises or thumping noises in the vehicle occurring during direction changes (when shifting from drive to reverse, or when applying and releasing the throttle, or when cornering) are typically from excessive backlash in the hypoid gears, differential gears, or excessive wear in these components or bearings. Severely worn universal joints in the driveshaft can cause similar disturbances, which are often accompanied by clicking noises.

Clunks or noises that get louder during turns usually suggest that something is wrong with the differential gears or case. Unbelievably, even brand-new axles as well as improperly set up “rebuilt” axles will make noise that only gets worse with time. Interestingly, the blame for such issues is often shifted toward the gear manufacturer especially on Internet forums when the true cause can usually be traced to a flawed gear setup.

Whining noises or high-pitched sounds that vary with a vehicle traveling at speed in a straight line typically emanate from worn hypoid gears and bearings. Worn bearings usually have a lower pitched sound while worn or misadjusted gears generate a higher pitched sound. If you hear a whine during acceleration that gets noticeably quieter on deceleration, a worn pinion head bearing could be the cause. If the noise is present during deceleration and not acceleration, the pinion tail bearing could be the cause.

Wheel-end bearings are unique for semi-float axles because the inner bearing race is the axle itself. Over time, this surface can wear, and when it has significantly worn, this type of bearing or surface problem produces a rumbling or roaring noise while the vehicle is in motion. You may also notice that the tone changes as you turn back and forth.

Rear Axle Vibration

Vibration can be one of the trickier issues to diagnose. You want to first rule out a driveshaft problem; or it could even be something as simple as a wheel out of balance. You may remember that cool, sideways burnout you did coming around the corner you remember, the one that ended with a hard slide into the curb. Well, in this situation, you may have bent your axle. This is far more common than you might think. One uncomplicated way to check if the axle flange is bent is to measure runout with a dial indicator.

Mounting a dial indicator on the axle housing and placing the indicator on the flange is the easiest method for measuring axle flange runout. As you rotate the axle shaft, make certain that shaft does not plunge in or out. I like to push the shaft inboard and then take the measurements. Typical values should not exceed 0.030 inch.

If the axle shaft is bent, it is harder to detect. One indication is that the shaft is difficult to remove. You can also support the axle with V-blocks and measure shaft runout. Do not try to straighten axles that have been bent, as they were overloaded in the first case, and now you are just going to overload them more trying to get them straight. This continued overloading weakens the shaft. This is like bending a paperclip back and forth a few times; it ends up breaking in half. They need to be replaced.

Remember that time spent diagnosing a problem is time well spent. The reason to isolate the source of the noise allows you to eliminate the culprit for good. Even if you have already decided to replace all the bearings and seals (which is highly recommended), you need to look for other problems that may have caused the bearings or seals to go bad in the first place, such as a bent axle or a worn seal surface.

Axle Leakage

Leaks are easy to spot; just look very closely for any signs of fluid leakage. The easiest leak to repair is the rear-axle housing cover. Any of the radial lip seals that are on the pinion flange or wheel ends require the axle to be partially disassembled. Usually, dirt and debris stick to the leaking fluid, making it easy to spot the source of the leak. As always, exercise common sense. I have worked on some vehicles where the engine has leaked so much oil that the entire floor-pan is soaked with 30-weight engine oil. Most of the time, the source of the leak is obvious. Other times it may not be as obvious, such as when oil is pumped out of the axle vent. Unintentional overfilling of the axle, or ingesting water during operation, can overfill the axle and cause it to leak.

Finally

We have made huge advances in materials to increase strength, developed better mounting structures to control deflection and noise, and designed even better bearings and seals to ensure long-term durability. We have even developed suites of sophisticated tools to analyze and predict the math and physics behind gear reaction loads. But the same basic function and operation is still evident. Even with all that improvement some basic rear axle problems can still exit, and understand where and what the issues is will save you time and money to fix your classic car and get it back on the road.