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Home Chassis Suspension Vibration Diagnosis

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The song “Good Vibrations” by the Beach Boys may have been a big hit in its day, but there’s no such thing as good vibrations or noise when it comes to a customer’s vehicle. Vibrations are annoying and may be a warning signal that something is amiss. So don’t ignore such symptoms. Investigate any unusual noises or vibrations immediately to determine the cause.

Most vibrations and noises can be classified according to when they occur:

  • Engine speed sensitive — The vibration/noise is directly related to engine speed and nothing else (not vehicle speed or driving conditions). The symptom appears or changes in direct proportion to engine rpm.

  • Vehicle speed sensitive — The vibration/noise only occurs at certain speeds or changes in direct proportion to vehicle speed.

  • Acceleration/deceleration sensitive — The vibration/noise is only noticed when vehicle speed changes (as when starting out from a stop, passing, slowing down or coasting).

  • Random — The vibration/noise comes and goes with no obvious connection to engine rpm, vehicle speed or driving conditions.

DIAGNOSIS
When a customer comes to you with a vibration or noise complaint, the first diagnostic step is to get as accurate a description as possible of what’s happening from your customer. That may not be easy because the typical motorist may describe a vibration as anything from a thumping noise to a shudder, rattle or shake. The point is, don’t get bogged down in the semantics of describing the symptom. Focus on the driving conditions that accompany the vibration or sound.

Does it only occur under certain driving conditions? If so, when? Does it change with vehicle speed? When is the problem most noticeable? When did the customer first notice the problem? Is it a noise (sound only), vibration or both? Where does the noise or vibration seem to be coming from? Can they feel it in the steering wheel, seat or both?”

Next, take the vehicle for a test drive if possible, and try to duplicate the operating conditions that create the vibration or noise. This will confirm the problem and hopefully give you a clearer picture of what exactly is happening.

  • A vibration that appears above a certain vehicle speed should point you in the direction of the wheels, driveline and suspension. Because the list of potential causes is a long one, don’t jump to conclusions until you’ve performed some preliminary checks.

  • A vibration or noise that only occurs when accelerating or coasting typically indicates a problem in the driveline or chassis.

  • A vibration or a noise that depends on engine rpm alone should tell you to look in the engine compartment.

  • Random noises and vibrations that come and go are often a clue that something is loose or broken and is rattling around.

On FWD vehicles, it is necessary to consider all possibilities – click here for details

SPEED SENSITIVE
Nine out of 10 times, speed-sensitive vibrations are due to an out-of-balance wheel. But what about the tenth time when balancing doesn’t cure the shakes? Is it a bent rim, an out-of-round rim or tire, an off-center wheel or hub, or a bent or imbalanced driveshaft? Sometimes the problem is worn shocks or a loose part in the suspension or steering linkage. Most tire/wheel imbalance problems will make themselves evident at speeds above 45 to 50 mph. A back-and-forth shimmy in the steering wheel means one or both front wheels are dynamically imbalanced, or a rim is bent. A simple bubble balancer can’t address either of these.

An off-car electronic spin balancer will check both static and dynamic balance of the tire and wheel assembly. You can also use the machine to check radial and lateral runout if your balancer has these capabilities. But off-car balancers only do the tire and wheel assembly.

They do not take into account brake rotors or drums that may be out of balance. So when a customer complains about a persistent vibration problem even though you’ve rebalanced the wheels two or three times, it should tell you the balance problem is probably on the vehicle. First, try re-indexing the rotor one or two lug positions on the hub to see if that reduces the imbalance. If there is no change, the rotor may need to be balanced or replaced.

Or, use an on-car balancer to balance the wheel on the car. Unfortunately, a conventional on-car spin balancer can’t be used on most vehicles with FWD or fulltime all-wheel drive (AWD) because spinning the wheel with the suspension unsupported risks damaging the CV joint.

Also, if a FWD or AWD vehicle has any type of limited-slip differential (including the viscous clutch type), you can’t spin just one wheel. For these kind of balance problems, you may need an off-car balancer that can also detect force variations in the tire. Variations in the stiffness of the sidewall, particularly in low profile tires, can have the same effect as excessive radial runout as the tire rolls down the highway. Match mounting the tire on the rim with the stiffest point on the tire over the lowest point on the rim may reduce the effect of force variation. If not, the tire may have to be replaced.

Vibrations caused by out-of-round tires or wheels, bent rims or excessive hub runout can be diagnosed on the vehicle with a dial indicator, or on a wheel balancer that has this capability. Minor runout problems can be corrected by tire matching (lining up the high and low spots on the wheel and tire to minimize runout) or tire truing (shaving the tire to make it rounder, another job that requires special equipment).

As a rule, most hubs with sealed wheel bearing assemblies should have less than .002 inches of runout. More than that may cause a vibration and/or tell you the bearing is failing. A sawtooth or heel-and-toe wear pattern on the tires can also produce vibrations and noise that may be speed sensitive. This type of wear is common on the rear tires of some front-wheel-drive cars, and may be caused by a toe-out condition on one or both rear wheels.

Run your hand around the rear tires to feel for roughness. If the tread blocks are worn unevenly, you’ll probably feel ridges one way, but not the other. The fix here is to replace the worn tires and realign the rear wheels.

DRIVESHAFT VIBRATIONS
If the wheels are in balance, the rims are not bent and the tires are round — and the car still shakes —the imbalance may be in the driveshaft. This would be more likely on a rear-wheel drive vehicle than a FWD car. Here’s why.

FWD driveshafts turn at the same speed as the wheels. At 55 mph, a typical FWD driveshaft may only be turning at 800 rpm — which isn’t fast enough to cause a vibration unless the shaft is bent or severely out of balance. Because of this, most FWD driveshafts are not balanced at the factory (unlike RWD shafts), nor is runout as critical. Maximum runout for a RWD driveshaft is generally .010 inches. For FWD, twice as much runout is considered acceptable. On some FWD driveshafts, a “vibration damper” weight is used to control torsional vibrations. If the weight has been removed or lost, it may cause cyclic vibrations to occur at certain speeds.

Worn U-joints on RWD driveshafts can cause vibrations at any speed as can an incorrectly installed U-joint. A worn U-joint (or inner CV joint in FWD applications) will usually “clunk” when the transmission is put into gear or when changing speed abruptly. A bad U-joint may also emit a cyclic chirp when starting out from a dead stop.

Cyclic vibrations can also be caused by excessive driveshaft angle. This may be a problem if somebody has raised or lowered the stock ride height of the vehicle by more than several inches. It’s a common problem on 4×4 trucks that have been raised excessively with a lift kit. When a U-joint runs at an angle of more than a few degrees, the geometry of the joint induces cyclic vibrations in the shaft. The greater the angle, the stronger the intensity of the vibrations.

With FWD, bad CV joints usually don’t vibrate, but they do click. The best way to check for worn outer joints is to turn the steering wheel to one side, then put the car in reverse and accelerate backwards. Running the joint in the opposite direction to which it normally turns exaggerates any wear that might be present.

Bad wheel bearings will usually make themselves evident before they fail. If the customer complains of a whining, squeaking, chirping or grumbling noise that seems to be coming from a wheel, better check the bearings.

RESONANCE
One of the most annoying sources of noise and vibration, and one that is sometimes confused with wheel vibration, is that caused by resonant vibration in the exhaust system. At certain speeds, the natural oscillations that occur in the exhaust may “stack up” causing the exhaust system to buzz or resonate. The problem may be due to a change in the exhaust system (replacing the muffler or tailpipe with one that differs from the original equipment design), a bent or mislocated pipe (which touches the chassis) or a broken or bent pipe hanger.

“Tuning” weights are also attached to the transmission tailshaft, differential or rear axle mounts on some vehicles to dampen chassis and drivetrain vibrations A spring has a natural frequency at which it wants to oscillate. If that frequency happens to fall within the range of normal driving, the factory engineers (if they catch it) may add a little weight to the suspension component to alter its frequency. If the original weights have been removed or are missing, an annoying vibration or shake may result at certain speeds or when driving on rough roads.

ACCELERATION/COAST VIBRATIONS & NOISE
Clunks, groans and squeaks that only occur during acceleration, deceleration, sudden speed changes or when encountering bumps usually point to loose, worn or deteriorated suspension components. Check the control arm bushings, ball joints, sway bar mounts and shocks. If a customer says his car shakes or shudders after hitting a bump or on rough roads, it usually indicates worn shocks. On cars or trucks equipped with a hydraulic steering stabilizer, a bad stabilizer can cause the same kind of symptom.

Excessive backlash in the differential gears, or worn bearings or gears in the transmission or transaxle will typically make a whining noise that’s loudest during deceleration, but rarely cause a vibration. If you detect audible gear noise, check the lube levels in the differential, transmission or transaxle.

A shudder that comes and goes with changing vehicle speed can sometimes be traced to a lockup torque converter that is engaging and disengaging. Such problems may be caused by a bad vehicle speed sensor, MAP sensor, engine computer or TCC solenoid.

RPM REVERBERATIONS
A vibration that comes and goes with changing engine speed can be due to a broken or collapsed motor mount, loose or broken engine accessories, or sometimes by loose or worn drive belts. Vibrations can be transmitted to the chassis or body by metal-to-metal contact at the mount, by an exhaust pipe or manifold making contact with the chassis, steering linkage or suspension, or by an engine-driven accessory (A/C compressor, alternator, etc.) making contact.

To rule out a broken motor mount, check for signs of physical contact (scuff marks, shiny spots, etc.) between the engine, transmission, transaxle or exhaust system and the rest of the vehicle. The integrity of the motor mounts can be checked by prying up on the engine and transmission or transaxle.

On rear-wheel-drive vehicles, the rubber insulator that holds the left motor mount together is often loose because torque twists the engine away from the mount and eventually rips it apart. Additional symptoms that would point to this problem often include a clunk or shudder during hard acceleration or when the transmission is put into gear.

On front-wheel-drive applications with transverse-mounted engines, the upper “torque strap” or motor brace is often the one that suffers the most as a result of engine torque. The combination of heat, vibration and oil contamination can cause the rubber bushings to deteriorate. The result can be an engine that bucks, shakes and rattles when accelerating or shifting. Transverse-mounted engines roll quite a bit during hard acceleration, so bad motor mounts are fairly common.

If a bolt works loose on an automatic transmission torque converter, it can create an imbalance that increases in intensity with higher rpm. To check for this condition, drop the inspection plate on the bellhousing and crank the engine while watching for missing or loose bolts.

RANDOM NOISES & VIBRATIONS
Loose shock absorbers, spring mounts, ball joints, stabilizer bars, control arm bushings, poorly hung exhaust systems, etc. can all make clunks or rattles when starting from a dead stop, when braking or when cornering, or when traveling over rough roads. If there is also looseness in the steering, the tie rod ends, idler arm or steering rack mounts may be bad. In any event, get the vehicle up in the air and thoroughly inspect the steering and suspension. An often overlooked cause of suspension noise may be a loose spare tire or other items in the trunk area.

Exhaust rattles and/or vibrations may also be transmitted through the chassis and feel like something else. To rule out this possibility, inspect the exhaust with the engine running and the vehicle on a lift.

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Larry Carley

Larry Carley has more than 30 years of experience in the automotive aftermarket, including experience as an ASE-certified technician, and has won numerous awards for his articles. He has written 12 automotive-related books and developed automotive training software, available at www.carleysoftware.com.
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