There are over 8 million trucks and SUVs produced each year in North America. If 50 percent of those vehicles are four-wheel-drive or all-wheel-drive, a total of 12 million axle assemblies are needed to supply those vehicles. Twelve million axles per year equal 32,876 per day, or 1,370 per hour, or 22.83 axles per minute. What are the chances that every axle is set up correctly? The chances are pretty low. I tell my students, “Just because it is new, does not mean it is set up correctly.”
Over the years, I have received many questions related to what has become known as “Chevy Shake” vibrations on later model Chevrolet, GMC, and Cadillac trucks and SUVs. Similar vibrations have occurred on the Ford Mustang the Ford F-150, and Ram 1500 and 2500 trucks. The frustrating thing for the customer is that nobody can diagnose and repair them, or even worse, they are told it is normal.
A tale of two trucks
I have been personally involved in diagnosing two vehicles that were purchased back from the customers for unresolved vibration problems. The first vehicle is a $90k 2015 Cadillac Escalade 4x4 SUV. The second is a $55k 2014 GMC Denali 1500 Series 4x4 Crew Cab short-bed pickup truck. Both of these vehicles had the exact same cause of the vibration; a rear axle that was not set up correctly.
|Figure 1 - Measuring total turning preload on a rear axle|
2015 Cadillac Escalade 4x4 SUV
In the spring of 2016, I was teaching a manual drivetrain class at the university where I work. As part of that class, we always go through the complete inspection, disassembly, repair and assembly process of a rear axle. We had three vehicles on the hoists and five axles on the workbenches. I love to work on vehicles with real problems, so I gathered my students around the 2015 Cadillac Escalade 4X4 on the hoist and told them about the vibration problem that resulted in the buyback of this vehicle. I told them we would carefully inspect the 9.75” rear axle as we disassembled it to see if the rear axle had any troubles that may have contributed to the vibration.
Following the same diagnostic procedure I have taught for years, one of the very first steps you should perform when diagnosing and disassembling a rear axle is to measure what is known as the “Total Turning Preload” of all the preloaded bearings that hold the ring and pinion gearset in place inside the axle housing. This measurement should be done at the pinion nut with the driveshaft, wheels, and tires, and brake rotors removed. Using a flexible beam type or dial-type inch-pound torque wrench, a technician should rotate the pinion nut and see what the constant rotational torque (effort) is. Typically, the measurement is at least 1.7 - 2.8 Nm (15 - 25 in*lbf) on an axle that is set up correctly, and we measured zero! I could not believe it! The only time I had ever read anything close to 0 Nm (0 in*lbf) of rotational torque was on a 35-year-old severely worn axle with high mileage on it. This axle is in a one-year-old Cadillac; how could this be?
|Figure 2 - The 2015 Cadillac Escalade with an “unfixable” vibration|
2014 GMC Denali 1500 Series 4x4 Crew Cab short-bed pickup truck
Next, we moved to the 2014 GMC Denali 1500 Series 4X4 Crew Cab short-bed pickup truck and performed the same measurements. It also read 0 Nm (0 in*lbf) of rotational torque for the “Total Turning Preload” measurement. The truck had the same rear axle housing except is had a 9.5” ring gear rather than the 9.75” ring gear in the Cadillac. Having two vehicles with zero bearing preload is unheard of; there must have been a problem with the axle setup, the lubricant, or something was very wrong here from the factory. My suspicions in 2016 were confirmed earlier this year (2019) when a friend of mine, who is also a GM Field Service Engineer, confirmed that “many of the axles out of an assembly plant in Mexico had problems.”
There is no excuse to have an improper setup axle on a new vehicle anymore. Axles have been mass-produced for over 100 years now. Sure, there have been improvements in materials and machining, but the same setup process has been in use all of that time. A properly set up axle is silent under all operating conditions and temperatures; it outlasts the lifespan of the rest of the vehicle.
Causes of axle vibrations and noises
Improper Bearing Preloading — Proper bearing preload on the ring and pinion gear set prevents the gears from moving vertically, horizontally, or diagonally as they rotate. Improper bearing preloading allows the gears to push away from each other as you accelerate your vehicle and pull together as your vehicle decelerates. Any gear movement can cause the gear backlash to become too small and have the gears bind with each other as they try to rotate. These conditions cause noises, vibrations and oil leaks.
- The ring gear rotates at tire speed and can mimic a tire speed-related vibration.
- The pinion gear rotates at driveshaft speed and can mimic a driveshaft speed-related vibration.
- Improper pinion bearing preload can cause what appears to be a pinion seal leak, but the seal is fine, the pinion gear is moving vertically, horizontally, or diagonally as is rotates. No seal can hold on oil under those conditions.
|Figure 3 - Differential case side bearing preload|