EVERYBODY HAS a good idea of what vibration is — a shaking or trembling that can be heard or felt.
The problem comes in determining how to deal with it.
Mitch Weller, product manager of heavy-duty equipment for Hunter Engineering, recommends the following steps for finding the vibration source:
- Verify the problem
Obtain a complete description of the problem. Road test the vehicle to confirm the problem. Use a specific procedure to duplicate the vibration complaint. Record abnormal noises and vibrations. Note if the vibration is related to the vehicle/wheel speed (the vibration occurs at a given mph range), engine RPM (the problem occurs at a given RPM range), torque (driveshaft angles during acceleration), or jounce (vibration starts after a bump or dip).
- Isolate the problem
Determine if it's wheel-speed or engine-RPM-related. Do a visual inspection and get vehicle history information.
- Repair the problem
Do not “shotgun” the problem with parts or assume the last repair is the cause of the problem. Keep in mind the problem may have more than one source. Justify any repair through proper diagnostics.
If it is related to the wheels and tires, meticulous care needs to be taken, according to Dave Scribner, Hunter Engineering's product manager for wheel balancers.
He says technicians using an off-truck balancer should be aware there is “blinding” going on. The balancer can display either an “actual” or “blinded and rounded” amount of imbalance, according to Hunter Engineering's instructions on vibration-control systems. “Blind” is a tolerance or amount of imbalance required before an imbalance amount is displayed. “Round” allows the balancer to display weight imbalance to a desired increment.
“Zero weight means absolutely nothing,” Scribner says, “because I can change the distances and see the weight again. That's the problem with looking at just weight. If you're looking at the forces, then you can just cancel or treat them differently. But right now the balancer only sees weight.
“So if they're using it in the round-off mode, two-plane balancing, they should always check to make sure that the residual static is as low as possible. And that's a very simple thing to do if they're aware of it. There's a button or something to look at on the screen, but most people ignore it. That's the problem — it takes manual steps right now to do something about residual static. Sometimes you have to do something about it, and sometimes it's OK.
“It depends on where the wheel weights are placed on the rim edges in relation to each other in the phase angle, from zero to 6 o'clock and anywhere in between. The combination of the two of those dictate whether there's residual couple or residual static. If you had a wheel weight on the inboard flange at 12 and wheel weight on the outboard flange at 3, you'd have a combination of static and couple remaining.”
Scribner — who wrote a paper, “A New Tire/Wheel Balancing Methodology Based Upon Absolute Force Calculations,” that was presented at the International Tire Exhibition & Conference last year — says that on heavy-duty trucks, static balancing is virtually all that's required.
“A dynamic balance with wheels spinning is OK,” he says. “But putting one weight on — preferably in the middle of the wheel — or not increasing the twisting moment but doing a static balance, is all you need on these trucks. No one is doing dynamic balancing on heavy-duty trucks in the factories. There may be an audit and they're checking the twisting moment.
“A good static balance is all you need, providing the wheel and hub aren't severely bent to where there's excessive run-out. A really high wobble can cause dynamic imbalance.”
He says SmartWeight, Hunter Engineering's new product, is a perfect static balance that has not been done before on most balancers that just read weight.
SmartWeight computes correction weights by measuring and evaluating the “absolute” or pure static (shake) and couple (shimmy) forces that cause vibration. Unlike traditional balancing, which judges balance condition based on correction weight values, SmartWeight balancing uses the actual static and couple forces, which directly addresses the source of vibration problems, resulting in the best possible balance.
He says the efficiency of SmartWeight balancing comes from not only the direct relationship to actual imbalance forces, but also from the application of independent tolerances placed on static and couple forces.
“SmartWeight balancing evaluates static and couple forces independently with separate imbalance tolerances,” says Scribner, who adds that SmartWeight is being used now on some trucks up to 22,500 lb and will be completely rolled out later in the year. “Each tolerance limit is set based on the amount of force needed to induce a noticeable vibration on a given wheel assembly. SmartWeight balancing puts the appropriate emphasis on the appropriate force and calls for correction weights only when a force exceeds the limit that will cause a noticeable vibration. It also recognizes that not all wheel assemblies are the same size and automatically adjusts the tolerances to more accurately correct imbalance.”
He says many people have preconceived misunderstandings about the difference between a force and weight.
“If the force is constant, the wheel weight size changes depending on its location,” he says. “And so people tend to think of wheel imbalance in terms of wheel weight. It's not. It's a force.”
He says the vehicle's eccentricity, or “out of round”, can be a combination of the tire, wheel, the way the tire is mounted on the wheel, and the way the entire assembly is centered on the truck.
“That is far more critical than even balance,” he says. “That's what we're going to be doing on the SmartWeight. It's not just a truck balancer. We've done enough work with Michelin and other companies to know that with large, heavy, rotating masses, the biggest issue is not imbalance. It's the eccentricity under load as you're rolling down the road. If that thing isn't round, your trucks are going to go off the road. You see it in farm equipment and heavy-duty fire engines they use in airports that use huge tires that run at high speeds. If those tire and wheel assemblies aren't round, they'll lose control of the truck. The farm tractor comes off the road at 45 mph. There's no suspension.
“So they should pay a lot of attention to the fact that when that wheel and tire are mounted on a truck, it should be as true as possible. You can see the things that cause big issues. If you have a truck jacked up, take a tire hammer and stick it on the ground next to the tire tread and just rotate the wheel and see how much wobble there is. You can stand the hammer up and check side-to-side wobble or compare it against the pavement. If you see a large radial or lateral wobble, that's bad news. And it could be a safety issue, too.”
Rounding and blinding
He says that virtually all computer off-truck balancers use a weight rounding and a blinding.
“The rounding function would be if you could purchase wheel weights in one-ounce or two-ounce increments — that incremental size,” he says. “A blind on a balancer is usually set somewhere in the range of the round-off, but it might be set a lot higher. On a passenger-car or light-truck balancer, the rounding and blinding are both around one-quarter ounce. On truck balancing, blinding might be set much higher than the round-off. The round-off may still be in quarter-ounce increments, but the blinding may actually be much higher. Most dynamic balancers split the wheel and tire in half, if you look at it from the tread section. They have an inboard and outboard half, and it treats them with a fixed blinding per plane.
“So let's say the round-off is one-ounce increments on a truck balancer. You may have a blinding function set that numbs the machine so it ignores everything less than four ounces per plane. So even though it's reading in one-ounce increments, you can leave a residual up to eight ounces of imbalance in the tire and wheel because of the blinding function. If the rounding and blinding were both one-ounce increments, the way current balancers work, it would chase weights all the time. It would act like it never repeated, because the one-ounce increment is such a small percentage of the mass of the wheel.”
He gives a real-life example of a common truck balancer that has a rounding of four ounces or fewer per plane.
“Let's say you have a weight on the inboard flange at 12 and another weight on the outboard flange at 6: You have what we call a residual up to four ounces of couple imbalance,” he says. “They're separated by a distance. That's four ounces of couple, which equals about eight ounces of weight. That's a twisting moment, and it's completely insignificant. You'd never feel it. That type of weight would have no effect on the twisting moment of wheel.
“However, if you're doing dynamic balance on a truck balancer and actually making a pure static correction or close to a pure static correction, that's what's wrong with the wheel. It had two forces: a twisting moment, and up-and-down moment. If it happens to have a higher up-and-down moment — which it will, because truck wheels have a huge static imbalance; that's the biggest issue — you are going to end up in many cases putting two weights across from each other instead of having 180 out. Now you can end up with almost eight ounces of static blinded imbalance. That is a problem. That's enough to cause a vibration on a heavy-truck wheel. This happens randomly when they're doing a two-plane correction. If you're dynamic balancing on an off-truck balancer, check and make sure that the residual static you're leaving behind is not excessive. It should be below the smallest increment in the weight tray.
“People are fooled by that. Some balance manufacturers have warnings and reminders of residual static, but most popular ones don't unless the guy turns it over to static and looks to see what's left over. You can leave a significant amount of imbalance on the wheel that will contribute to vibration and tire wear.
“That's what we're going to be doing differently when we come out with the truck mode for heavy-duty trucks with SmartWeight. That issue that's so common won't exist because you're canceling the static force and checking the twisting moment. The quality of static balance that takes place with SmartWeight will be far greater and it'll happen automatically. We're looking at each force individually. Truck balancers now only display the weight. They don't show the forces that are being measured and treat them separately. You end up with static air and wasted wheel weight.”
This story was based on a presentation given at the Truck Frame & Axle Repair Association (TARA) meeting in Rochester, New York, October 25-28.
From Hunter Engineering:
Amplitude (Magnitude): The amount of force or the intensity of the vibration.
Back Coning: When the wheel requires the cone to center the wheel on the balancer's shaft from the backside, primarily due to the chamfer of the wheel. Also referred to as Back-Cone Mounting.
Backspacing: The distance measured from the mounting face to the back edge of the wheel.
BDC: The abbreviation for bottom dead center; also referred to as 6 o'clock.
Bead seating: The process of seating the tire to the rim bead seats. Bead seating preferably occurs just after the tire and rim have been assembled, but may gradually change and optimize over a longer period. The position of the bead may optimize or always remain seated improperly, unless the tire is demounted, lubricated, and remounted. However, the load force and its relatively short duration will not necessarily solve defective mounting of the tire bead seat to the rim seat.
Bolt Pattern Circle: The diameter of an imaginary circle drawn through the center of each lughole, and virtually always on the same centerline as the hub bore of the wheel.
Computerized Vibration Analyzer: A device used to determine the frequency of the vibration by isolating the vibrations with the greatest magnitude.
Cycle: One complete disturbance.
Dampen: To decrease the magnitude of a vibration or sound.
Dampers: Used to reduce the magnitude of a given vibration. Rubber is commonly used to isolate and dampen vibrations.
Dynamic Balance: A procedure that balances the wheel assembly by applying correction weights in two planes so that up and down imbalance and wobble imbalance are eliminated.
Electro-Mechanical Ear: A device used much like a doctor's stethoscope and is for noise diagnosis problems only.
Force Matching: A method of aligning the high spot of the tire's radial force variation with the low spot of rim runout to decrease rolling vibration in the wheel assembly.
Forced Vibration: Vibrates when energy is applied.
Free Vibration: Continues to vibrate after the outside energy stops.
Frequency: The number of disturbances that occur per unit of time.
Front Coning: When the wheel requires the cone to center the wheel on the balancer's shaft from the front. Also referred to as Front-Cone Mounting.
Harmonic: A vibration that is identified by the number of occurrences per revolution. For example, a 1st harmonic vibration has a once per revolution vibration component.
Hertz: A unit of frequency: one disturbance per second.
Hub Centric: The wheel is centered using the center hole of the wheel.
Lateral Runout: The amount of side-to-side movement as the tire/wheel assembly rotates.
Lug Centric: The wheel is centered using the lugholes rather than the wheel center hole.
Magnitude (Amplitude): The amount of force or the intensity of the vibration.
Natural Frequency: The point at which an object will vibrate the easiest.
Order: The number of disturbances per cycle (rotation). For example, a 1st order vibration occurs once per cycle, and a 2nd order vibration occurs twice per cycle.
Phase: The position of a vibration cycle relative to another vibration cycle in the same time reference.
Phasing: The cycle pattern of two or more vibrations that overlap and combine to increase the overall magnitude.
Pressure Ring: The accessory used to prevent the wing nut from contacting the wheel when on the balancer shaft.
Radial Force Variation (RFV): A term describing a measurement of the tire uniformity, under load, measuring the variation of the load acting toward the tire center.
Radial Runout: A condition where the tire and wheel assembly is slightly out of round forcing the spindle to move up and down as the vehicle rolls along a smooth surface.
Reed Tachometer: A mechanical device that uses reeds to indicate the frequency and magnitude of the vibration.
Resonance: The point where a vibrating component's frequency matches the natural frequency of another component.
Responding Component: The noticeable component that is vibrating.
Servo-Stop: The ability to locate varying positions of the tire/wheel assemblies and hold the position in place while correctional weights or OE-Matching marks are applied.
Source Component: A component causing another object to vibrate, such as a tire/wheel assembly.
Static Balance: A procedure that balances the wheel assembly using only a single weight plane.
TDC: An abbreviation for top dead center. Also referred to as 12 o'clock.
Torque Sensitive Vibration: The vibration occurs when accelerating, decelerating, or applying the throttle.
Transference Path: The object(s) that transfer the frequency.
Vibration: A shaking or trembling, which may be heard or felt.
Wheel Diameter: Dimension measured on the inside of the rim at the bead seats.
Wheel Offset: The measured distance between the mounting face of the wheel and the centerline of the rim.
Wheel Width: Dimension measured on the inside of the rim between the bead seats.