Whatsamatter U?

Every truck shudders and shakes to a certain degree but when vibrations persist at a specific road speed, engine rpm, or gear position, it’s time to see a mechanic who can thoroughly check your driveline and address the cause.

As inspections go, checking a driveline is a straightforward job. Just about everything is out there in the open, easy to reach, grab, tug, and service. It’s also an important job because of where the driveline is situated: between the transmission and the drive axle, two very complex and expensive pieces of machinery. Any shakin’ going on in the driveline can fatigue the internal workings of your transmission as well as the bearings in the rear-axle differential.

The driveline really is a system of components that includes the drive shaft, universal joints, and flanges or yokes. The number of drive shafts and U-joints you have on your vehicle will depend on its wheelbase length and the type of transmission and rear axles it has. If your vehicle has more than one drive shaft, it will use a centre bearing for support.

The most basic driveline assembly is a hollow steel drive shaft with a U-joint at each end. As the drive shaft aims down from the transmission to meet its connection to the differential, the U-joints allow the angles of the shaft to change while still transmitting all that torque and power from one end of the driveline to the other. A slip joint spliced into the shaft provides a way for it to change length and absorb movement when the suspension flexes.

You may not know what any of these components are or how they work together, but you can help your mechanic diagnose your vibration problem more efficiently by describing the symptoms in detail. Make note of when the vibration starts, where it’s felt, and what the road conditions are. Does the shaking start under high-torque situations? At a specific road speed? Is it accompanied by noise? Where does the noise come from?

BAD VIBES

There are three types of driveline vibration you should know about. The first is critical speed. Critical speed vibration happens when the drive shaft rotates too quickly for its length, diameter, and material construction. As it spins faster and faster, the shaft starts to bow out like a jump rope.

Eventually, it blows apart and leaves a mess on the highway (and, if you’re lucky, no injuries). The best way to prevent a critical-speed failure is to spec the right driveshaft for the job based on your gross weight rating, net engine torque, transmission, and axles. Your salesperson will have the tools to help you make the right choice for the speed at which the drive shaft will operate.

The second type of vibration is transverse vibration. A drive shaft is balanced to close tolerances and has thin, metal weights welded on to make sure it spins straight and true. If you lose a balance weight, or a lump of road grime gets stuck to the shaft, the faster it turns and the more out of balance the shaft will be. So if the vibration starts at low speeds and gets worse as you accelerate, you probably need to have your driveshaft balanced by a qualified shop.

The third type of vibration is torsional vibration. Unless the driveline is perfectly straight, each U-joint has to speed up and slow down twice during each revolution. A good rule of thumb is the six-to-one-and-a-half rule: the driveline working angle should not exceed six degrees, and the U-joints at either end of the shaft should be different by no more than one-and-a-half degrees. When the angles on each end are equal within one-and a-half degrees, the output and input yokes are operating at a uniform speed. They cancel out the torsional vibration that is generated at the drive end of the driveshaft.

Excessive driveline angles create a harsh vibration throughout the driveline, which can lead to premature U-joint life, transmission gear failure, synchronizer damage, and potential damage to the differential. With a U-joint operating at a 30-degree angle and rotating at a constant speed of 1,000 rpm, the driveshaft actually speeds up to 1,300 rpm and slows down to 700 rpm. If the other end of the driveshaft also has a 30-degree angle, it slows down to 700 rpm at the same time that the other end speeds up to 1,300 rpm.

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