SPEED SENSITIVE VIBRATIONS
Nine out of ten 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 drive shaft? 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. Some balancers can also check radial and lateral run out. 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 vibration problem doesn't go away even though the wheels have been rebalanced, 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. Not rotating the tires often enough can also cause unusual wear patterns to develop on the rear tires of many front-wheel drive cars. 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.
DRIVE SHAFT 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 drive shaft. This would be more likely on a rear-wheel drive vehicle than a FWD car. Here's why.
FWD drive shafts turn at the same speed as the wheels. At 55 mph, a typical FWD drive shaft 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 drive shafts are not balanced at the factory (unlike RWD shafts), nor is runout as critical. Maximum runout for a RWD drive shaft is generally .010 inches. For FWD, twice as much runout is considered acceptable.
On some FWD drive shafts, 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 drive shafts 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 drive shaft angle. This may be a problem if somebody has raised or lowered the stock ride height of the vehicle by more than several inches.
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 you hear a whining, squeaking, chirping or grumbling noise that seems to be coming from a wheel, better check the bearings.