NVH revisited: Correcting concerns relating to vibrational issues
Noise, vibration and harshness (NVH) issues are certainly not limited to wheels, tires, brakes and steering/suspension areas. Engine faults (misfires, rotating assembly balance, etc.), worn or damaged engine mounts and driveline issues (driveshaft, transmission and drive axle) are all potential contributors to NVH concerns. In this article we’ll discuss a variety of NVH issues along with tips to avoid and/or cure these ills, with an emphasis on chassis-related areas.
Wheel shimmy (steering wheel right/left oscillation) can be caused by a number of variables. Follow an organized step-by-step inspection:
1. Inspect the tires for excessive and uneven wear, and inspect for tire structural damage (badly deformed sidewall, etc.). Poorly constructed tires or damaged tires where plies are misaligned or separating can cause a shimmy/vibration. 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.
2. Inspect and adjust tire inflation pressure. Low or uneven pressures on the same axle can cause pulls or wander.
3. Check for lateral runout. Using a dial indicator at the wheel rim, slowly rotate the wheel (on the vehicle) a full 360 degrees, noting maximum lateral runout. If excessive lateral runout is found, don’t automatically blame the wheel, since runout may be the result of hub or rotor runout and/or a stack-up of tolerances between the hub, rotor hat and wheel.
4. Before removing the wheel/tire assembly, with the suspension unloaded, check for wheel bearing looseness by rocking the tire inward/outward at the 12- and 6-o’clock positions.
5. Remove the wheel/tire assembly and inspect the wheel fasteners (studs and nuts or bolts, depending on design). Damaged threads can be an indication of abuse/over-tightening which can contribute to wheel/hub distortion.
6. Inspect the mating faces at the rear of the wheel, rotor hat and hub face. A buildup of corrosion can cause a lateral runout condition. Clean all surfaces.
7. Check for lateral runout at the hub itself.
8. Inspect the wheel rims for damage (bent rims, signs of pothole/impact damage).
9. Check/adjust each tire/wheel assembly for dynamic balance.
10. Inspect the entire steering and suspension system. Looseness (caused by wear, damage or improper installation) can contribute to a shimmy, in addition to wander and directional pull.
11. On the alignment rack, measure all angles, paying particular attention to caster angles. An excessively positive caster (caster angle beyond factory specification for the specific vehicle) can promote a shimmy condition.
12. During reinstallation of wheels to the vehicle, always use a torque wrench and follow both the specified torque value and tightening pattern.
Centering of the wheel, both on the balancer and the vehicle hub is critical. If incorrectly mounted on the balancer, a precision balance won’t be possible. By the same token, a wheel that is correctly centered (and balanced) on a dynamic balancing machine won’t be able to duplicate the balance (and road force variation variable) if incorrectly centered onto the vehicle hub.
Most wheels are mounted to a balancing machine with a tapered cone that centers the wheel onto the balancer’s shaft.
Cones are available with different tapers. Cones that feature a low taper are preferable to better center the wheel. Back-cone mounting is preferred (where the cone mounts from the backside of the wheel). However, some wheel designs will not center using back-cone mounting. In these cases, front-cone mounting is required, where the cone centers the wheel from the outer side of the wheel.
When using the back-cone method, once the clamping cup and wing nut secures the wheel, depress the foot pedal to hold the spindle in place, and slowly roll the wheel towards you during initial tightening of the wing nut.
This helps the wheel to roll up the taper of the cone, instead of trying to force the wheel to slide onto the cone.
When using the front-cone method, pull the top of the wheel/tire away from the hub face while tightening the wing nut. This will help to overcome the force of gravity and improve wheel centering on the machine.
• Verify wheel centering on the vehicle.
• During tire mounting/inflation, if inflation limit is exceeded, deflate, loosen tire bead and re-inflate.
• Match-mount tire to wheel using balance/high spot dots where available.
• Verify wheel centering on balancer before spinning.
• Always use two-plane dynamic balancing, even for hidden weights.
• Adjust tire inflation per vehicle specifications.
• Verify wheel centering when installed to the vehicle.
HUB-CENTRIC AND LUG-CENTRIC WHEELS
The terms hub-centric and lug-centric refers to the manner in which the wheels center onto the hubs.
If the wheel relies on centering at the hub, it’s hub-centric. If the wheel relies on centering by the fastener locations, it’s lug-centric.
If the wheel is not centered onto the hub, radial runout will occur, resulting in a vibration/bounce. Some aftermarket custom wheels that are designed for hub-centric centering may be made with a large center hole that will accommodate the largest hub in a certain vehicle application.
In order to adapt the wheel (and retain centering), hub-centric rings are required (for example to reduce the hub hole from 88 mm to 72 mm, etc., in order to center onto another vehicle make/model. If the hub rings are not installed, and the wheel center hole is larger than needed for the hub in question, you can’t necessarily rely on the fastener locations to properly center the wheel on the hub.
Multi-pattern wheels are available in the aftermarket, designed to provide a fitment to more than one bolt circle dimension. While some of these wheels feature multiple bolt holes (for instance, 10 holes where the application is intended for a five-bolt hub), some multi-pattern wheels feature oblong lug holes, designed to accommodate more than one bolt circle.
Instead of the holes being round, they’re oval shaped, allowing mating to likely two bolt circle diameters.
In theory these wheels should be hub-centric so that the wheels are properly centered to the hubs. These wheels are not very common today, but if you encounter them, extra attention to wheel centering is a must.
After mounting to the hubs, check for radial runout (visually at first, then with a dial indicator). Inexpensive aftermarket wheels may feature slightly oversized bolt holes and oversized center holes, which can easily create a radial runout condition, resulting in a “thumping” vibration.
Check hub-to-wheel centering, to verify that the clearance is even and within the target value of 0.004 inch (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.
While we’re considering wheel centering, it’s important to note that if the wheel is centered improperly on the balancer, and installed properly centered on the hub, or if properly centered on the balancer but improperly centered on the vehicle hub, a vibrational problem is bound to occur.
If the wheel does not provide a precise “slip fit” onto the hub (being centered by the hub/hub-centric), extra care needs to be taken to center the wheel during installation.
This situation can be common for some budget level aftermarket wheels.
WHEEL BEARINGS/HUB BEARINGS
Loose/worn or improperly installed front wheel bearings/hub-bearing assemblies can contribute to shimmy/wander, and will make proper wheel alignment impossible. Always check wheel bearing condition prior to any alignment work, and be sure to measure hub flanges for lateral runout. Note that some bargain-basement-priced hub assemblies may feature short-lived bearings and may feature excessive lateral runout. High-quality, precision-made hub assemblies produced by reputable makers offer higher quality bearings and tighter runout tolerances to avoid NVH issues.
When the piston(s) in a disc brake caliper are forced outward against brake pad backing plates, the frictional heat generated naturally transfers into the caliper and the hydraulic fluid, elevating fluid temperature. In addition, if the pads are not perfectly aligned to the rotor disc, a harmonic vibration generates which results in a pad “squeal” and potentially a pedal bounce. A trick to address both of these issues is available in the form of Nucap US Inc.’s “piston cushions,” lightweight and thin discs that snap into the pistons, creating an insulating effect between piston and pad. The cushions are constructed using a 0.015 inch-thick steel core, layered on both sides with 0.005 inch nitrile rubber. The cushions serve to absorb vibration, which reduces or eliminates annoying brake noise, and acts as a thermal barrier to reduce heat transfer to the brake fluid. In addition, the cushions provide protection to the caliper pistons in terms of reduced wear and heat transfer.
Installation is simple. The correct shim size is selected from the kit to accommodate the piston’s inside diameter and wall thickness. A provided lubricant is brushed onto the rear side (piston side) of the cushion. The lubricant is applied simply to allow the nitrile rubber facing to move a bit to prevent it from sticking to the piston. The cushion is then inserted into the piston via locating tangs. The floating design of the cushion, in addition to the vibration absorbing characteristics, absorbs pad vibration and reduce heat transfer.
CORRECTING FOR DISC BRAKE LATERAL RUNOUT
If the brake rotor-to-hub interface is not perfectly aligned, deviation in surface thickness of the hub and/or rotor hat will result in a lateral runout condition of the brake disc. This lateral runout will cause the brake pads to bounce against the brake disc when brakes are applied. If runout is severe enough, the pads can bounce on/off of the rotors during driving, even without applying the brakes. The resulting vibrations produce annoying brake squeal due to the harmonics generated as the pads bounce/chatter against the disc.
NOTE: Since runout stack-up can occur between the hub and rotor, first match-mark one stud location on the rotor and its stud. Perform a runout check with a dial indicator. Then re-locate the rotor to the hub in the next clockwise position and measure again, etc. in order to obtain the optimum location of the rotor to the hub with the least amount of lateral runout. If further correction is needed, a tapered alignment shim may be installed to remove any remaining lateral runout. Lateral runout of the brake rotor can result in not only a vibrational issue, but can lead to premature brake pad wear as well.
Corrective tapered shims are available to address this issue, potentially eliminating the need to re-machine the rotor. Shims offered by Nucap are a good example (213 SKUs available). Available in different bolt patterns and shim thickness to correct up to as much as 0.009 inch lateral runout, the shim is simply placed between the hub and rotor. Using a dial indicator, slowly rotate the rotor on the hub, noting (and marking) the low point of lateral runout. The shim is then installed with the thickest section aligned with the low point of measured runout (180 degrees from the high point of lateral runout).
For example, if the rotor was measured with a high point lateral runout of 0.006 inch, the low point will be located 180 degrees from the high point. A tapered-thickness shim with the thickest section of 0.024 inch on one side (the side with the “V” notch) and 0.027 inch on the opposite side is placed onto the hub prior to remounting the rotor. The 0.003 inch difference in shim taper makes the initial 0.006 inch runout disappear. The side of the shim that features the “V” notch is the thinnest side of the shim, making the thin/thick sides readily identifiable.
TIRE RADIAL FORCE VARIATION
In addition to dynamic balance of a tire/wheel assembly, we need to consider tire radial force variation (RFV) when diagnosing tire-related vibrations that occur at varying speeds and conditions. Ideally, the wheel should be 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).
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.
Radial force is determined by measuring loaded radial runout. According to Hunter Engineering Corp.’s research, on an average passenger car tire/wheel assembly, one thousandth of an inch (0.001 inch) of loaded radial runout is equivalent to approximately one pound of road force. For example, a measured 0.030 inch of loaded radial runout (about 30 pounds of road force) will cause the same amount of vibration as 1.5 ounces of wheel imbalance at 50 mph, which is five times greater than the .30 ounce imbalance limit.
Tire makers usually supply a red dot on the tire sidewall, which indicates the tire’s maximum RFV. When mounting the tire, this red dot should be aligned to a white dot on the wheel (if the wheel is so marked). The white dot on the wheel indicates the wheel rim’s minimum radial runout point (on a steel wheel, a dimple may be featured that indicates the wheel’s low point). When using alloy wheels that don’t feature a white dot, the tire’s red dot should be aligned with the valve stem, as this should be the minimum radial runout point.
A yellow dot may be found on the tire sidewall which indicates the lightest point of the tire (in terms of weight from a balance consideration). If a yellow dot is found, this should be aligned to the wheel’s valve stem, which should be the wheel’s heavy spot in terms of balance.
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.
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.
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 dynamic 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 hub-centric (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 lug-centric (as we mentioned earlier, 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 hub-centric wheels) or the wheel studs (for lug-centric wheels).
HIGH TECH TIRE ANALYZER
Here’s an interesting way to generate additional wheel alignment, tire and steering/suspension components. Tread Spec (see www.treadspec.com) offers a drive-over laser scanning system that reads tire tread and collects several hundred data points. The technician can refer to the data to talk to the customer about the measured tread wear in terms of tread depth and uneven wear, which can naturally lead to the need for tire replacement, wheel alignment, and steering and suspension components sales.
While any experienced technician may be able to interpret tire wear visually (hey, you have excessive inside shoulder wear, so you’re likely running too much negative camber, etc.), this system provides documented data that substantiates the technician’s diagnostic opinion.
Two different systems are available, including a service bay system and a service drive system. In each case, the tires pass over laser scanners housed in low profile ramps. The bay system allows a technician to drive the front axle onto the scanner platform and stop, allowing a scan of the tires. A technician-operated bar code scanner is included that allows scanning the vehicle VIN for record keeping purposes. The service drive system can be located at the entrance of the bay area, where the customer drives over the ramps as the vehicle enters the service area.
The basic difference between the two systems is that the technician-operated drive-over system features two lasers, where the customer drive-over system features eight lasers to capture all four tires at the same time.
Data that is collected by the system is sent to the manufacturer’s cloud-based collection point and is then available (in real time or after-the-fact time) on the shop’s desktop computer.
While such a system may not be mandatory, it certainly can help to boost shop sales by increasing parts and service tickets, since the “high tech” data collected by the tire scan provides documented proof of tire tread condition, and immediately allows the technician to begin a discussion of tread wear and the potential causes and remedies. ●