Noise/vibration/harshness: Chasing the irritant gremlins
Part one of two
A host of various noise and vibration issues normally exist during the operation of any vehicle. However, when levels of these factors become noticeable to the customer, he or she may perceive any annoying noises and/or vibrations as problems. NVH (noise, vibration and harshness) is the commonly used term used when discussing these conditions.
In some cases, any noise or harshness complaints may stem from the vehicle owner’s perception, or their expectations of how the vehicle should behave.
The NVH condition that is a concern does not need to be the strongest vibration or the loudest noise. It could be one that has recently developed and was not previously present.
As an example, tire tread noise from a vehicle equipped with large tread block tires might be acceptable to the owner of a 4x4 who recognizes this as a natural by-product, while others may find this as totally unacceptable. A complaint on the same vehicle could be much more subtle, caused by a driveline problem.
Because we sense vibration and sound using different senses, we tend to discuss them separately. However, vibration and sound are essentially identical.
A sound is a vibration (pressure fluctuation) of the air. Vibrations and sounds are both expressed as waves per second called Hertz (Hz).
Vibrations that are felt are under 200 Hz. Vibrations between 20 Hz – 20,000 Hz are audible by humans. Vibrations over 20,000 Hz are ultrasonic and are not audible by humans.
Vehicles offer three major sources of potential vibration. These include the engine and engine accessories, driveline and wheels and tires. Each of these sources usually rotate at different speeds or frequencies. A component generating a vibration can be associated with one of the source groups if the frequency of the vibration can be determined.
For example, a light truck equipped with an automatic transmission and 31 inch tires, traveling at 50 mph, in overdrive will have an engine speed of 2,050 rpm/34.1 Hz; a driveline speed of 48.5 Hz; and a wheel speed of 10 Hz.
Body shake/steering flutter/shimmy
Body shake, steering flutter and steering shimmy complaints all involve pinpoint diagnosis of the same components. The condition of the component is what determines which of the symptoms occur. The wheels and tires offer a good starting point, especially if the NVH analyzer identifies this area as the generating force of the vibration.
Tire and wheel inspection includes checking all four tires regarding manufacturer, size and specifications. Proper tire pressure is also an important item to confirm. Look for damage, deformation and wear. The technician should also rotate the tire and wheel assembly, inspecting both the sidewall and tread to look for obvious conditions caused by road damage, flat spots or runout. It’s also vital to inspect and feel the tread for unusual wear patterns.
Check the tire and wheel for proper bead seating along the entire bead circumference on both sides. An improperly seated bead will create a radial runout condition.
Check hub-to-wheel centering, to verify that the clearance is even and within the target value of 0.004 in. (0.1 mm) maximum. If the clearance is out of spec, rotate the wheel (wheel clock position relative to the hub). If the clearance is still out of specification, check the hub for runout to determine if the condition is in the hub or the wheel.
Beating or phasing occurs when two similar vibrations or sounds with slightly different frequencies exist in the same area or vehicle. Over a period of time, the phase of the two waves will change due to the slight difference in frequencies. At times, the two higher points overlap and create an even higher peak which raises the level or amplitude. Also, at times the two low points overlap to make an even lower point which lowers the level or amplitude.
This change in intensity or amplitude occurs in a repetitive manner at a constant vehicle speed as the phase of the wave changes over time. The resulting wave creates a sound called “beating,” which is associated with a vehicle having more than one tire out of balance. Even though the vehicle may feature tires of the same brand and size designation, tires are not always the same diameter during vehicle movement (due to variations such as inflation pressure and dynamic load), and will rotate at slightly different speeds (Hz). This condition can be corrected by eliminating either one of the vibrations. If one tire is balanced, then the beating noise will be eliminated, leaving the constant vibration from the remaining out of balance tire. Correcting the second tire will return the vehicle to its original condition and ensure customer satisfaction.
A single vibrating force may generate more than one vibration. For example, an out of balance tire can develop multiple vibrations due to the distortion of the tire as it rotates. This is a characteristic of radial tires. The tire is no longer round, and bumps rise on the tire, causing additional vibrations. The distortion of the tire is caused simply by centrifugal force as the tire rotates. Centrifugal force is similar to swinging a yo-yo in a circle. The faster you swing it, the more the pull. This pulling force is what causes the tire to change shape.
As the tire rotates, the heavy spot on the tire causes an up and down motion as it contacts the road. This will induce a vibration into the suspension and steering system, which will be felt by the driver. The centrifugal force of the rotating heavy spot also contributes to the up and down movement.
The vibration caused by the heavy spot is a “first order” vibration. It occurs once every revolution of the tire. A first order vibration can be the largest amplitude vibration of the vibrations caused by the imbalance. Due to centrifugal force and the heavy spot, the tire changes shape raising additional high spots on the tire. As these spots contact the road, they also cause an up and down motion that is induced into the suspension and steering systems.
This second vibration is caused by a second bump in the tire as a result of the change in shape. It is usually smaller in amplitude than the first order vibration. This is called the “second order,” or second component vibration.
Because there are two vibrations in one rotation of the tire, the second order vibration will be approximately twice the frequency of the first order and a spike on a frequency analyzer will appear at that frequency.
The third vibration is caused by a third bump as a result of the change in shape. It is generally smaller in amplitude than the second order vibration, though there are some applications and speeds where it may be greater in amplitude than a first order vibration. This vibration is called the “third order” or tertiary component vibration. It will appear as a spike on a frequency analyzer at three times the first order vibration, due to the three vibrations per revolution of the tire.
Tire RFV (radial force variation)
Tire radial force variation is a key consideration when diagnosing tire-related vibrations that occur at varying speeds and conditions. All OEM wheel assemblies are phase matched to align the tire’s point of maximum RFV with the wheel’s point of minimal radial runout (high point of tire to low point of wheel).
Maximum RFV is generally indicated by a red dot on the tire which should be aligned with the white dot on the wheel (or to a dimple on a steel wheel). The red dot on the tire indicates the tire’s maximum RFV, while the white dot on the wheel indicates the wheel rim’s minimum radial runout point (a dimple on a steel wheel indicates the wheel’s low point). When using alloy wheels, the tire’s red dot should be aligned with the valve stem, as this should be the minimum radial runout point. A third dot, yellow in color, is also featured on the tire sidewall, and indicates the lightest point of the tire (in terms of weight from a balance consideration). A tire’s yellow dot should be aligned with the wheel’s valve stem, which should be the wheel’s heavy spot.
Even if dynamic wheel balance (from a standpoint of weight) is correct, misalignment of the red and white dots will likely result in a vibration complaint. On OEM tires and wheels, always align the red tire dot and the white wheel dot (or valve hole, in the case of alloy wheels) when mounting. If the tire features a red dot and a yellow dot, the red dot is more critical and should be aligned with the wheel’s low point (dimple or valve stem).
Red dot on tire: Align to the wheel’s low-point dimple (steel wheel) or to the valve stem (alloy wheel); or to a white dot on the wheel if the wheel features a white dot.
Yellow dot on tire: Align to the wheel’s valve stem.
Both red and yellow dots on tire: The red dot takes precedence. Align the red dot to the wheel dimple or valve stem.
Radial force variation is a term that relates to a tire-sourced out-of-round/vibration that occurs, and masks itself as an imbalance vibration, only under dynamic conditions...when the wheel and tire package rolls in a loaded state. It must be noted that the term “radial” refers to forces applied at the radius of the tire, not to the type of tire construction. Radial force vibration could potentially occur with any type of tire, regardless of its construction (radial, bias ply, etc.). In other words, a radial force variation may prove to be the cause of a vibration that won’t reveal itself during a static or dynamic balance job, or by checking the mounted tire for runout in an unloaded state.
When a customer complains of a “tire vibration,” although the root cause may simply involve a static imbalance, other factors may be at play, including a static radial runout of the wheel and/or tire, a suspension/chassis problem, or a dynamic-only runout condition, known as radial force variation of the tire.
If static imbalance is the culprit, this is easily cured by balancing the tire/wheel assembly. If runout is the cause, this can be cured by replacing the faulty wheel or tire; or possibly by match-mounting the tire/wheel package. However, when that approach does not fix the problem, the technician must begin a diagnostic approach in order to locate the cause.
Inspect for radial runout
When a “mystery” vibration enters the shop, approach the problem in a systematic manner to eliminate possible variables. Naturally, check the tire/wheel assembly for balance on your shop’s balancer. If static balance is verified, begin measuring for excessive runout. First check hub runout in order to identify or eliminate the hub as the possible root cause of the problem. With the wheel/tire removed from the vehicle, check the runout of the hub. This can be tricky because of clearance at the wheel studs, but can be accomplished with enough patience.
If the wheel is hubcentric (where the wheel relies on hub centering at the wheel hub hole to the hub protrusion), you’ll want to check the runout of the hub itself, at the contact area for the wheel’s center hole. If the hub center protrudes far enough from the wheel studs, mount the dial indicator so that the plunger contacts the hub surface. In some cases, it may be necessary to remove the wheel studs to gain access to the hub contact area. Pre-load the plunger slightly and zero the dial. Rotate the hub slowly, watching for runout on the gauge.
If the wheel is lugcentric (where the wheel-to-hub centering relies on the location of the wheel fastener holes to the hub’s studs only), you can mount the dial indicator so that the plunger is about .040 -inch away from the outer edge of the wheel stud pattern diameter. Using a feeler gauge, check for changes in the gap between the dial indicator plunger and the outer edge of the wheel studs as you slowly rotate the hub 360 degrees. Granted, this can be a time-consuming and nit-picky job, but this will either recognize the wheel-to-hub mating as the culprit, or eliminate this variable from your diagnosis.
NOTE: Do not use the outer edge of the brake rotor as your measurement point when trying to check hub runout. You must take this measurement at the centering area that the wheel uses, whether this is the hub (for hubcentric wheels) or the wheel studs (for lugcentric wheels).
Checking wheel runout
In order to isolate wheel runout on its own, dismount the wheel from the tire and thoroughly clean the bead seat area. Mount the wheel onto a rotating fixture with acceptable runout (the vehicle hub or on your shop’s balancer, as long as your chosen fixture is verified for acceptable runout). Mount the dial indicator to allow the plunger tip to contact the wheel’s bead seat area, perpendicular to the seat surface. Don’t try measuring runout at the outer edge of the rim, as this may have no bearing on runout — it’s important to check for runout at the area where the tire bead seats, and where the wheel has an effect on the mounted radius shape of the tire.
Preload the plunger slightly and zero the dial gauge. Rotate the wheel a full 360-degrees while monitoring the dial indicator gauge. Again, whenever measuring runout of any rotating part, use of a roller-tipped indicator is always preferred, to avoid snags and bumps that can be encountered with a non-roller tip plunger. If runout is beyond acceptable tolerance, the wheel may require replacement. However, once you find the low spot of the wheel’s runout, mark this and remount the tire, matching the high spot of the tire to the low spot on the wheel. Balance the assembly and road test. If the problem vibration was indeed a case of wheel and tire combined runout, match-mounting may solve the problem.
Understanding the severity of loaded tire runout
As a general rule-of-thumb, a minimum range of between .3 to .5 ounce (7 to 14 grams) of imbalance is usually enough for the average motorist to notice an imbalance-induced vibration. If a vehicle is sensitive enough to exhibit noticeable vibration at only .3 to .5 ounces of imbalance, that same amount of vibration may be present with as little as 10 to 15 pounds of radial force variation, which (although hard to believe) can be caused by as little as .010-inch to .015-inch of loaded radial runout. Using this as an example, it’s easy to see how loaded runout can dramatically affect vibration. In other words, a little bit of “loaded” tire runout variance can make a big difference in terms of operating smoothness or harshness.
Don’t automatically blame the tires
While a customer may be quick to place any vibrational problem with the tires, or your handling of those tires, we need to remember that many other variables are potentially involved, any one of which could be the root cause of the vibration. In addition to those mentioned earlier, consider the following as possible problem areas, none of which directly involve the tires.
* Worn or loose wheel/hub bearings.
* Loose wheel fasteners (check the torque!).
* Wheel not mated squarely to the hub face (due to debris on the mating surfaces).
* Distorted or out-of-balance CV shaft (maybe the rubber damper fell off)
* Worn, loose or too-soft bushings at critical control arm locations (torque rod in the case of a single-pivot lower arm setup), control arm pivots, etc.
* Out-of-balance primary driveshaft.
* Flat-spotted tire treads (the result of previous severe brake lockup).
* Mud/dirt collected in the inboard wheel cavity.
* Broken weld joints in a unit-body chassis (it’s a stretch, I know, but if the vehicle was equipped with stiff, short sidewall tires and stiff springs/shocks, the unit-body seams may have been fatigued. This condition may or may not help to create a vibration, and could also contribute to wander and reduction of steering response).
* A too-large tire setup (in terms of mass) that is not designed for use on the specific vehicle.
* Hubcentric wheel mated to a lugcentric hub flange or lugcentric wheel mated to a hubcentric hub. If not centered properly, this would create an eccentric out-of-round condition.
* Severely worn dampers (shocks/struts).
These are but a few examples of conditions that may cause or contribute to a problem vibration. The morale: don’t automatically assume that the tires are at fault until you have completed a thorough diagnosis. ●
See Part 2 in our March/April 2013 issue for information on types of vibrations and causes and cures for harshness issues.