against the wheels on the pavement through the axle gearing. As mentioned earlier, the propshaft is rotating in a counter-clockwise direction when viewed from the rear of the vehicle. This rotation and subsequent torque on the pinion is trying to rotate the axle housing. This rotation applies a downward force on the left wheel, and an upward force on the right wheel. So the load on the left wheel is increasing, and the load on the right wheel is decreasing. This is similar to the front-to-back weight transfer on the chassis during hard acceleration.
During hard acceleration, there is also a weight transfer from side-to-side across the axle based on the propshaft direction of rotation and torque reaction. We refer to this as tire jacking. Since our reference point is the ground and the wheels are attached to the ground, it may seem difficult to see what is happening. It is actually quite easy to see what is occurring, but the explanation seems backward. We see the right side of the vehicle squat down during hard acceleration. The right wheel is being pushed upward toward the vehicle body. However, since the wheels do not leave the ground and the ground is our frame of reference, it appears that the right corner of the car is lowering. If we were to put scales underneath the wheels during such an event (this is not recommended as you will probably ruin your scales), we would see that the load on the right wheel is decreasing, and the load on the left is increasing. Drag racers know this quite well, and try to combat the phenomenon by installing an adjustable air spring to apply a preload force to the lighter loaded tire on the right side of the vehicle.
These tire-jacking load conditions explain why when you take a right-hand turn it is so easy to spin the inside-right-side wheel under acceleration. During a turn, you are forcing the wheels to travel at different speeds, and under acceleration the right wheel is actually unloaded. The right tire has less contact force with the ground and the dynamics of the turn event are forcing the tire to travel slower than the left tire, so it is easy to make it slip relative to the ground. Now, try the same maneuver, but this time in a left-hand turn. The inside (left) wheel will not spin like the right did in the previous situation. As you apply more throttle to try and get the left wheel to spin, you are increasing the tire jacking load on the left wheel, and further decreasing its ability to slip. With enough power applied you end up spinning both wheels and the vehicle fishtails. Of course, I do not recommend this on public roads.
Reaction Loads on the Axle Housing
Now let’s talk about the reaction loads that the hypoid gear set applies to the entire axle housing. Chapter 6 discusses the gear reaction forces internal to the housing itself, but let’s now concentrate on the complete axle housing assembly. Imagine that the vehicle is at rest. The wheels are not rotating and the ring gear is stationary. As Sir Isaac Newton’s First Law of Motion states, an object at rest tends to stay at rest, and an object in motion tends to stay in motion at the same speed and in the same direction, unless acted upon by an unbalanced force. In other words, objects keep doing what they are doing. If they are stationary, they want to remain stationary. If they are moving, they want to stay moving (neglecting friction, of course). This sounds so simple.
Now, let’s apply it to the rear axle. The ring gear is stationary when the vehicle is not moving, and it wants to stay at rest. When we apply a torque to the pinion, it is trying to rotate the ring gear. The ring gear is attached to the wheels through the differential and axle shafts, all of which want to remain sitting still. So the pinion ends up applying a rotational load to the axle housing which mimics the pinion trying to point upward in the vehicle. The pinion is said to be climbing the ring gear.
Here, the pinion loads are applied to the complete axle housing during a hard-acceleration event. If you look underneath a lifted truck when it takes off from a stop, you can see the pinion portion of axle twist under load. As this depicts, the forces twist the axle from front to back. Since your goal is to attain maximum efficiency, you must understand all these forces to counteract and remedy these forces. After all, the goal is to maintain the driveshaft and axle alignment to transmit maximum power. (Dana Holding Corporation/Joe Palazzolo)
This pinion-to-stationary ring gear reaction actually rotates the entire axle housing counterclockwise, as viewed from the right side of the vehicle. The result is inefficient transfer of torque. This is why traction bars and pinion snubber-style devices came into existence, and were added to many vehicles. It was recognized that if this upward rotation tendency were not resisted, then the suspension would have a difficult time trying to counteract the event. The issue comes into play when this rotation of the axle is finally resisted, and the axle returns back to the correct state—back to its original, near-horizontal position. As the axle rotates downward quickly, it applies an additional rotation load on the wheels. This is very common with traditional leaf-spring-style suspension setups. In this situation, the leaf springs temporarily deform from their normal bow shape into an “S” shape and this event can be seen from the side of the vehicle. As the spring releases this stored energy, the wheels may momentarily lift off the ground with some vehicles. This is more evident on pickup trucks but can happen on cars as well.
There are many other events that can cause wheel hop. Even non-beam-style-axle vehicles can exhibit wheel hop based on the suspension geometry and overall axle drive system stiffness. The worst thing that you can do during a wheel hop condition is to stay on the throttle. As the wheels lose traction and regain traction quickly, enormous spike loads travel through the drivetrain. These impact loads can cause drivetrain components to fail quickly.
Finding the Right Parts and Service
Now that I have covered the fundamentals that apply to all rear axles, let’s review how to find a good shop for parts and service.
Let’s face it; at some point, you will need to find a place to purchase parts. After reading this text, you may even decide that you do not want to tackle the work yourself. What are some important items to consider when deciding where to buy your parts? Price is very important, and most places are competitive. However, you may want to get some technical assistance, and that costs money too. Larger businesses commonly have free tech lines or email assistance. Some very reputable places may not have the best prices, but can help you make the best-informed decision. You definitely want to get your parts from a place that moves sufficient inventory to actually have the parts on hand. Some of the smaller shops cannot afford to have stock on hand, and you could be waiting quite some time to get your parts. Not to say that the smaller shops are bad, especially if you are working on a longer-term project and have the time to wait a few weeks for the right parts. If it is your daily driver and weekend race machine, you probably want, and need, your parts quickly.
This is just one of the inventory stock aisles at Drivetrain Systems (DTS). DTS and several other drivetrain vendors offer a complete line of axle and driveline components. DTS has an entire section of the building dedicated exclusively to inventory. It stocks a wide range of drivetrain parts, and they purchase more common items in bulk. I was amazed to find pallets of Eaton posi units. (Randall Shafer)
Now, with the relative ease of shipping and the Internet, you may want to buy complete parts, a complete axle, or even send yours to a reputable shop to get it rebuilt. Some places just sell parts and do not work on the axles themselves. You want to be careful with these places. They are great to get parts from, but be cautious of any advice that they give, as they do not have daily hands-on experience with the parts and vehicles. A lot of these words of caution pertain to any work that you are having performed.
Here is a tiny sample of the quantity of hardware on hand
