One of the simplest and most important parts of a vehicle is the driveshaft. Responsible for transferring the output of the transmission to the axles, it is essentially a shaft that drives the vehicle. Though it has a basic function, it plays an incredibly crucial role in vehicle operation and is the link between a revving engine and a moving car. But before we can diagnose driveshaft problems, we need to understand some basic principles.
How Does a Driveshaft Work?
First, it’s important to know that front-wheel drive vehicles do not use driveshafts. Instead, the transmission and axle are combined into one unit called a transaxle. However, on rear-wheel and four-wheel drive vehicles, the transmission and axle are separate. This means that the turning motion of the transmission needs to be transferred to the axle in some form. Hence, the driveshaft.
Usually constructed of steel or aluminum, sometimes even carbon fiber, a driveshaft has a tubular design to ensure it is both strong and lightweight. It’s easy to imagine this as a solid piece of equipment connecting the transmission or transfer case to the axle. However, because of driveline angles and vehicle flex, sometimes the driveshaft must consist of a few parts for proper operation. Depending on the year-make-model, the design of the driveshaft may be either a single-piece or two-piece unit.
Additionally, four-wheel drive applications have two driveshafts—one that links the transmission to the rear differential and one that links the transfer case to the front differential.
Photos via The Art of Manliness
On a single piece driveshaft, you may have a solid connection to the axle while a slip yoke connects it to the transmission. The yoke slides over the splines of the transmission’s output shaft. The reason this is done is to allow the driveshaft to “change sizes” during flex. The driveshaft will be able to slide back and forth on these splines to keep from binding, while effectively being able to apply power.
You may also have a propeller shaft up front, similar to what is found on a two-piece driveshaft, which offers a fixed mounting position on the transmission but still allows the driveshaft to extend as needed.
Between the yoke and the driveshaft—and used as the connecting point for the driveshaft and the differential—are universal joints, or u-joints. U-joints are used to manage the angles. The transmission and driveshaft sit at an angled pitch and this means that the driveshaft will need to pivot as it rotates. U-joints are what make this possible.
Two-piece driveshafts do all the same things as a single-piece driveshaft, just in a slightly different way. Rather than using a slip yoke to attach to the rear of the transmission, these are mounted in a fixed position with the use of a u-joint on the output shaft of the transmission and a u-joint on the axle.
The driveshaft still needs to manage flex, and this is done between the two driveshafts instead of at the transmission. This is important for 4WD vehicles that see extreme flex—as the use of a slip yoke could actually result in the driveshaft falling out of the vehicle.
Symptoms of Driveshaft Problems
Now that we know what the parts are, we can break down driveshaft problems. It’s good to keep in mind that the driveshaft itself is rarely a point of failure. However, when it does fail, it’s usually the result of long-term corrosion and stress, so you’ll generally notice certain signs and symptoms that something is wrong.
Vibrations from the driveline are a good sign that the driveshaft is failing. This is usually caused by the driveshaft not being bolted down well or the unit being out of balance. Of course, if it has been smacked by a rock while wheeling or is rusted and rotted, issues will be evident. But in all reality, the likelihood of this being a cause for failure in a daily driver is very far off.
The most common point of failure with a driveshaft assembly is the U-joints. The U-joints use bearings inside to keep the joint moving freely. Over time, these bearings will wear out and the u-joints will need to be replaced. You can often tell that these are going bad when you hear popping and clunking coming from the driveline.
Driveshaft problems can make it difficult to control the vehicle’s ability to turn. If you feel hesitation or resistance when turning, or difficulty maneuvering in and out of parking spaces, it could be a sign that your driveshaft is on its way out.
Replacing and Repairing a Driveshaft
Replacing and repairing driveshafts and u-joints can be blended into one conversation because the process of doing either follow nearly the same steps. Jacking a vehicle up or using a lift certainly makes the process much easier, but it isn’t necessary. The exact removal of a driveshaft will vary from vehicle to vehicle, and the exact how-to can’t be covered with one single process across the board.
You will need to read into your vehicle’s exact process, but it’s never going to be a job that has an exceptional difficulty level. To remove a driveshaft, you usually only need basic hand tools, which include a socket wrench and socket set along with box wrenches. Additionally, pry bars will be needed to help encourage the driveshaft to break free after the mounting hardware has been removed.
Replacing and Repairing U-Joints
Removal and replacement of u-joints is best handled with a bearing press, as the u-joints’ caps need to be pressed into the driveshaft and yokes. However, an unorthodox method that I (and many others) use is a bench vice and old sockets. You will want to find one short socket that is slightly smaller than the end caps and one deep well socket that is larger.
You will place the large socket over one cap bracing itself on the driveshaft and the smaller socket on the adjacent cap ensuring that it can slip through the opening on the driveshaft. Then, gradually, you want to use the vice to clamp down on the sockets. As the vice comes together, it will use the small socket to press the u-joint through to push the other cap out into the larger socket. Of course, before getting started you will need to remove any retainers used to hold the caps in place. You also want to make sure that you take your time with this, so you don’t bend or crack anything. The use of a torch for heat to make the process easier is always a good idea—as long as it isn’t overkill.
The propeller shaft can also cause issues and will begin to click and pop over time as well. However, the process of replacing one falls into a higher level of difficulty than simply replacing u-joints or installing a new drive shaft. A savvy tech can take the time to break into these units to repair what’s specifically broken, but often it’s quicker to simply replace the whole driveshaft.
More Than a Daily Driver? Stay Tuned.
I want to clarify that these are issues that the everyday person with an everyday car would most likely experience. When it comes to dedicated applications like high-powered vehicles or off-road rigs, the stakes are higher and new issues will present themselves. So, if you’ve ever twisted a driveshaft into two pieces or extended it so far that it literally slid out of the transmission, hang tight—we’ll be back with some specially curated content just for you.