Diagnosing and correcting tire and wheel imbalance can cure many of your customer’s complaints of vehicle vibration. But as you may be aware, there are times when you can balance the wheels and find that the vehicle still shakes. Lets take a look at some of the more common driveshaft and engine-related causes of vibration. If wheel or tire runout is within specifications and the vehicle has rear-wheel drive (RWD) or four-wheel drive (4WD), driveshaft vibration may be what’s causing the problem. Driveshaft vibration is rarely encountered in front-wheel-drive (FWD) cars and minivans because the halfshafts turn at the same speed as the wheels, which run at about a third of the speed of the driveshaft in most RWD and 4WD vehicles. So unless a halfshaft is bent or damaged, it’s unlikely that it would be out of balance enough to cause a noticeable vibration. FWD halfshafts also run smoother because the CV joints on the ends of the shafts do not create cyclic vibrations as the operating angle of the joints change. With U-joints, though, changing the operating angle of the joint causes a cyclic change in the rotational speed of the driven shaft. The greater the operating angle, the greater the speed changes in the shaft. The speed of the driveshaft also amplifies U-joint-induced vibration. The maximum acceptable operating angle of a U-joint decreases in direct proportion to shaft speed. At 2,000 rpm, the maximum angle is about 8°, while at 4,000 rpm, it is only about 4°. It’s important that you inspect the driveshaft angle or pinion angle on the rear axle. If someone has modified the suspension to change the vehicle’s ride height, they may have created a U-joint vibration problem. Shimming the rear axle and rear transmission mount to reduce the operating angles of the U-joints may be necessary to reduce or eliminate this kind of vibration. Another source of vibration can be a worn center carrier bearing on a two-piece driveshaft. The bearing should be replaced if it shows any looseness. Alignment of the bearing is also important. If off-center, it can create unequal drive angles and cause vibrations. Driveshaft runout can also cause vibrations. Runout can be checked by positioning a dial indicator near the center of the driveshaft, then rotating the shaft to determine the amount of total run-out. More than .010" of runout can cause trouble. Unbolt the rear U-joint and rotate it 180° in its yoke to see if that eliminates the run-out problem. If it does not, the shaft is bent and needs to be replaced. Driveshaft Imbalance Test Vibrations caused by driveshaft imbalance are more difficult to diagnose. One way to do this is to raise the rear wheels off the ground while supporting the rear axle (don’t let the axle hang otherwise it may create a driveline vibration by increasing the operating angle of the U-joints). 1. Start the engine and run the rear wheels up to speed (no more than 55 mph to be safe). When the vehicle starts to shake, note the speed. 2. Stop the engine, remove the rear wheels and reinstall the lug nuts to hold the brake drums in place. Then repeat the same test to see if the vibration is still present. If the vibration does not return, the problem is not the driveshaft but wheel and tire imbalance. If the vibration is still there, proceed to Step 3. 3. Stop the engine again, remove the brake drums and repeat the test once more. If the vibration is gone, the problem is drum imbalance. If the vibration persists, it’s the driveshaft. Caution: Do not step on the brake pedal while the drums are off, doing so will force the pistons out of the wheel cylinders. Just shut the engine off and let the drivetrain bring itself to a stop. To rebalance the driveshaft, send it to a machine shop that does balancing, or use an electronic on-car balancer to balance it yourself. (See following procedure.) If you’re using an on-car balancer, place the magnetic pickup head just behind the pinion nose under the axle housing and the strobe light under the rear of the driveshaft. Draw a chalk line on the shaft for a reference mark. Then run the shaft up to speed and note the relative position of the mark when it is illuminated by the strobe. The strobe will flash when the heaviest part of the driveshaft is at the six o’clock position (straight down). To correct the imbalance, install worm screw hose clamps on the shaft with the heavy part of the clamps positioned 180° opposite the heavy spot. Then run the shaft back up to speed to see if additional weight is needed to cancel out the vibration. Add more clamps as needed or weld a small chunk of iron to the shaft opposite the heavy spot. Repeat until the vibration is eliminated. If you don’t have an on-car balancer, install a pair of hose clamps on the shaft and make four reference marks 90° apart. Try the clamps at each of the various positions until you find the one that produces the least amount of vibration. Motor Mounts Often-overlooked engine components that may need to be replaced to eliminate vibration are motor mounts. These rubber mounts can deteriorate, collapse and/or separate with age. Fluid-filled "hydraulic" type mounts can often leak, allowing annoying engine vibrations to be transmitted to the chassis. Most mounts are designed so that separation won’t allow the engine to fall out onto the roadway. But a bad mount may cause a myriad of problems - many easily misdiagnosed. Often, bad motor mounts allow the engine to rock and move around, causing noise and interference problems with the throttle, transmission and clutch linkages. For example, a thumping noise when the transmission is put into gear or when the vehicle is accelerating is a classic symptom of a bad mount. Excessive engine rocking also can create exhaust leaks and rattles where the head pipe joins the exhaust manifold. Plus, the donut that seals the exhaust joint can be crushed or broken by the motions of the engine, or the head pipe or pipe flange may crack. Cracked or broken motor mounts can be an annoying source of vibration and noise, typically a clunk or shudder when accelerating hard. A broken or separated mount may even allow an engine-driven fan to scrape the fan shroud or contact the radiator, which also contributes to annoying noise. Because motor mounts maintain engine and driveline alignment in FWD cars and minivans with transverse-mounted engines, it’s important that the mounts be in good condition. The mounts support the engine and transmission or transaxle, and help dampen noise and vibration to isolate the powertrain from the rest of the vehicle. The upper mounts on FWD applications also help control engine rock as the engine applies torque through the driveshafts. While the design of the mount may prevent the engine from literally falling out of the car, it won’t keep the engine from twisting or hopping on its mounts every time the vehicle accelerates or is under load, which can produce thumping and rattling noises. It also can overstress components such as radiator and heater hoses, wiring connectors and the exhaust system. A broken or loose motor mount in an FWD application can be even more serious because it may allow engine movements that interfere with the throttle or shift linkage. If the bad mount is an end mount, it may also contribute to a torque steer condition and cause accelerated wear or separation of the inner CV joints on one or both driveshafts. The noise produced by a separated or broken motor mount often sounds like a bad U-joint or inner CV joint (a clunk when accelerating or placing the transmission or transaxle in gear). So before either of these other components are replaced, the mounts should be checked. Some mounts are "hydroelastic" and have hollow chambers filled with hydraulic fluid to dampen vibrations that would otherwise be transmitted across the mount to the chassis. Motor mounts need to be replaced when they’re loose, broken or collapsed. And, replacement mounts should be the same (fluid-filled hydroelastic or solid rubber) as the original. Caution: Substituting a less expensive solid mount for a fluid-filled mount can increase the transmission of engine noise and vibration to the rest of the chassis. These mounts may save your customer a few bucks, but won’t do the same job as the original. They feel harsher and transmit more noise and vibration to the rest of the vehicle, and ironically, may cause a customer to return with complaints of a harsh rides or vibration. Harmonic Balancer The harmonic balancer, also referred to as a vibration damper, is a device that is connected to the crankshaft in order to reduce the torsional vibration. As the cylinders fire, power is transmitted through the crankshaft. Since the front of the crankshaft takes the brunt of this power, it often moves before the rear of the crankshaft. This causes a twisting motion. As the power is removed from the front, the halfway twisted shaft unwinds and snaps back in the opposite direction. Although this unwinding process is quite small, it can cause "torsional vibration." To eliminate this vibration, a harmonic balancer is attached to the front part of the crankshaft that’s causing the trouble. The balancer is constructed of two pieces connected by rubber plugs, spring loaded friction discs, or both. Therefore, when the power from the cylinder hits the front of the crankshaft, it tries to twist the heavy part of the damper. Instead, it ends up twisting the rubber or discs connecting the two parts of the damper. Since the front of the crank can’t speed up as much with the damper attached, the force is used to twist the rubber and speed up the damper wheel. This helps keep the crankshaft operation calm. According to one parts manufacturer, replacement "harmonic balancers" are quickly becoming a hot item for today’s car owners. The reason is "harmonic vibrations," which can lead to a variety of mechanical failures. Harmonic vibrations are specific and repeated vibration patterns, which pass through an object. In today’s cars, such vibrations result from the combustion of the air-fuel mixture. Each time a cylinder fires, the connecting rod pounds the crankshaft journal as the force turns the crankshaft, causing energy to be dispersed throughout the engine. Multiply this by the number of cylinders (with variations in engine speed) and you have what is commonly called harmonic vibrations. Contributing to this column were Larry Carley and Gary Goms.