If there is anything that frustrates a tire tech, it’s the matter of resolving noise, vibration and harshness issues.
The first hurdle is figuring out what the customer is hearing or feeling compared to what the tech hears or feels on a test drive.
No two people are the same, so “persistent” NVH problems are common. And, more often than we care to think, tires are the first thing drivers blame when an NVH problem arises. The real trouble begins when a customer returns a vehicle to your dealership. You thought you had the problem licked, and now you have to search to locate and fix the problem again.
Many drivers who want a luxury ride like that supplied by a high-end car such as a Mercedes, BMW or Lexus believe their less-expensive car should deliver the same kind of ride. To them, NVH in a pricey car should equal the NVH of a Kia. Not true.
Most, if not all, high-priced cars have a well-tuned suspension and automotive polyurethane foam in the hollow sections of the car. No wonder they’re so quiet. This type of foam absorbs sound and vibration waves that travel up and down in the steel members of the car. There are several types of automotive polyurethane foam ranging from rigid to flexible to structural foam. In some cases, these products have been used by tuners to make cars handle better without adding much weight.
Simplifying all of this just a bit, NVH really is a measure of vehicle feel – a combined measure of the noise level, ride and vibration of a car during driving. While it is a quantitative measure, NVH includes subjective measures such as any road or vehicle noise perceived, any vibration that can be felt by the customer, and the harshness of the ride during abrupt transitions in motion, like in the case of potholes.
Vehicle manufacturers put a lot of heat on their quality assurance people to make certain there won’t be any NVH issues when the vehicle is sold. Strenuous testing is completed before the vehicle leaves the plant. Even so, NVH remains one of the most prominent customer complaints.
Because many issues manifest themselves in the form of NVH, quality assurance people do most of their work with microphones and accelerometers. The next most critical activity is panel tightening rigidity testing, which is measuring force vs. displacement of the body metal using a wide variety of tools, including a displacement transducer and a force gauge.
Vibration testing is a search for the source of shake or vibration. Engineers measure the frequency of the vibration and look for components that rotate at the same frequency as that particular vehicle’s speed to determine the source of the vibration – like a tire, transmission or engine.
Vibrations run under 5,000 hz or less, but the frequency range for sound is from nearly zero to 20khz. Sound is measured inside and outside the vehicle and signals feed directly to a device called a wavebook so they can be analyzed with a laptop and specific software.
Silencing the Problem
As you must have already surmised, NVH performance relates to the vibrational response of the vehicle to driving conditions – stiffness of the vehicle structure, the mass of the vehicle and damping effects of the vehicle suspension all come into play during your investigation.
It is paramount that tires and wheels be carefully inspected for obvious damage or signs of trouble. Start by looking for tread or sidewall defects such as cuts, bulges, rips, bruises, scrapes, missing chunks of tread, separations and cracks.
Unfortunately, the kinds of tire defects that can cause steering pull or vibration may not be visible to the naked eye. These are often inside the tire or are otherwise difficult to see, such as off-center belts, mispositioned belt splices, damaged belts or cords, or beads in different planes causing variations in sidewall stiffness. Yes, these things do happen.
A tire with an off-center belt will lean to one side because the location of the belt shifts more weight to one side of the tread than the other. This condition is known as conicity and occurs because the tire behaves as if it were cone-shaped. A cone always rolls in a circular path toward the pointed end. Therefore, a conicity pull will always be directional, either to the left or right, and can be reversed by switching the front tires from side to side or by reversing the offending tire on the wheel.
Though vibrations are often caused by an out of balance wheel and tire assembly, it’s important to note that vibrations can also be caused by excessive radial (vertical) or lateral (sideways) runout in the tire, wheel or hub. Loose, worn or damaged wheel bearings, as well as certain kinds of tread wear, can also cause vibrations.
When troubleshooting the cause of a vibration problem, begin by inspecting the tires and wheels. Look for evidence of missing weights, mud or dirt packed in the back of the wheel or debris embedded in the tread that create an imbalance. Also, rotate and wiggle each wheel by hand to check for excess suspension play or noise from the wheel bearings.
Keep in mind that alloy wheels on small cars with MacPherson strut suspensions are more sensitive to both kinds of runout than steel wheels on larger rear-wheel drive vehicles. If either kind of runout exceeds specification, the wheel should be replaced.
As you work your way toward an NVH solution, don’t forget about muffler hangers and dampers that work in the smallest, hottest places. NVH can begin and end right here, even though there are lots of other places that will require your close scrutiny. Automotive engine mounts can often lead to the solution. Check them out and consult the vehicle builder for proper specifications.
Suspension parts that are painfully subjected to road salt and other extreme environmental hazards must also be inspected. Don’t forget to inspect any turbocharger or intercooler hoses. In fact, include all belts and hoses in your NVH investigation. Often it’s the simplest part that turns out to be the culprit.
Here’s a quick checklist of automotive parts you should check to get to the root of the NVH complaint, which may likely have nothing to do with the tires:
• Engine, engine mounts and
• Mounting system
• Intake and exhaust systems
• Flywheel and torque converter
• Driveshafts, CV joints and
• Body structure at mounting
interfaces, body acoustics and local
and suspected areas of resonances
such as brakes
• Damper and bushings
• Mounting spring damper
• Brake, steering and wheel turning
• Driving dynamics control systems
• Seat track and motor noise
• Window regulator noise
• Door slam, turn signal sound
Finally, remember that quiet is the key to middle America’s most basic necessity. People want a high-speed relaxation cabin to compensate for stressful work environments. If you can solve their NVH problem, you’ve just won a new customer.