Universal joints or “U-joints” is a rather simple mechanical device that allows one or more rotating shafts to be linked together, allowing the transmission of torque and/or rotary motion. However, these rather simple components of your classic car that is key part in driving your car. If you suspect an issue with your u-joints diagnose u-joint failure on your car.
U-joints are a small, simple component – in fact, they have changed much since the very first automobiles. However, on any vehicle with a driveshaft, its ability to move depends entirely on the u-joint being there. You will often hear “maybe it’s the u-joints” as a solution offered for problems with a vehicle – but what are they, what do they do, and how do you know if yours is bad?
A u-joint, or universal joint, is a component that connects the driveshaft to the differential, and sometimes connects the driveshaft to the transmission or two driveshafts to each other. It is a cross shaped piece with a bearing at each end, that allows for lateral and side to side movement of the driveshaft.
A u-joint allows for slight positions changed between the driveshaft, transmission, and differential. In other words, the driveshaft connects the transmission to the differential, but it cannot be a static, solid connection. Since the differential moves independently from the rest of the vehicle over terrain, the transmission and differential do not stay perfectly always aligned with one another. The u-joint compensates for motion, keeping the driveshaft connected and spinning freely.
That vibration you have managed to ignore for weeks has finally gotten to the point at which it is hard to keep from spilling your morning coffee on your pant leg. You resolve to make two stops on the way home from work this afternoon–one at the dry cleaners to pick up clean pants, and a second at the mechanics to investigate the vibration. As you accelerate away from the dry cleaners, there is a big change in the vibration. It is followed immediately by a lack of power and big clanging noises, the latter due to your driveshaft falling to the pavement. You have had a universal-joint failure.
Lack of Lube
Most of the time, loss of lubrication is the reason U-joints fail. Original equipment joints typically have no grease fittings (zerks), and even replacement units often have zerks that you cannot reach with your grease gun. The tiny needle bearings inside the cups over the ends of the trunnions rust and eventually crumble, which results in clearance where there should be none.
How do you know if it is bad?
First, it is important to remember bad u-joints only really apply to rear wheel drive cars, trucks, and SUVs. To a technician that knows what they are looking for, a bad u-joint is obvious, since it displays specific symptoms. A bad u-joint can cause a clunking sound or jerkiness while driving, particularly when letting off and pressing the accelerator. A bad u-joint can also cause vibration at certain speeds, emanating from the center or rear of the vehicle. If a u-joint is excessively worn, there is an easy test you can perform. With your foot on the brake, shift the car into reverse. Then, shift into drive. If the vehicle makes a clunking sound, or you can feel it knock one time after engagement, a u-joint is likely the culprit. As it wears out, it does not make a tight connection to the differential and driveshaft, allowing for too much play at the joint.
Fortunately, a u-joint is a straightforward and inexpensive fix. Get it done sooner rather than later – if it breaks completely, the driveshaft will disconnect from the vehicle, which can damage your transmission, the underside of your car, and anything else around it – plus, the vehicle will not be going anywhere until it is fixed.
Another destroyer of U-joints is an excessive angle of operation. This may be due to an overloading condition, which makes the tail end sag, or a set of helper springs or air shocks that puts the rear of the car up into the stratosphere. In either case, leveling the vehicle is the only way to increase the universal joints’ life span.
The driveshaft itself is a simple-enough component. Unless it is bent from contact with, say, a railroad track or is out of balance due to the loss of a weight, it will keep spinning just about forever.
Rules of Thumb to Determine U-joint Failure.
Vibration caused by a U-joint or driveshaft usually occurs at 25 to 35 mph, about 60 mph, or while braking at low speeds. That “body boom” that hurts your ears probably is caused by a bent or unbalanced driveshaft. A clunk when the transmission is put into gear, or when “floating” at about 10 mph, is either the result of damaged U-joints or a differential problem.
To find out if a vibration is coming from the driveline or the engine, attach a tachometer if you do not have one on the dash, run up to the speed at which the roughness is felt, and note the rpm. Then, shift into a lower gear, go back up to the same rpm and see if the vibration has diminished. No? Then you have found engine trouble. If, on the other hand, the vibes change considerably at the original engine speed, the U-joints or driveshaft are probably at fault.
Next, raise the vehicle and support it under the rear axle or suspension control arms so that the springs are compressed with a normal amount of weight and the driveshaft is at the proper angle. Grab the shaft near either end and force it up and down, then twist it hard back and forth. Any play you can see, or feel is reason for replacement.
Once you have diagnosed u-joint failure that the U-Joints need replacement then you can either have a shop replace them or you can jack the car up and do the work yourself. Nex,t we will discuss the DIY work on U-joint replacement on your classic car.
