Driving a vehicle, any vehicle, with even the smallest vibration can be annoying beyond description. As bad as it might be for this to happen in a personal car or truck, pity the unfortunate firefighter or emergency responder on the way to a fire, accident or other emergency call with a seat, steering wheel or whole truck that seems to be shaking itself apart.

Many vehicle components can be the source of a vibration, but in this article we will concentrate on those vibrations that come from the tire/wheel/hub assembly at the axle ends. Other sources of vibration could be the rotating components of the driveline, starting with the engine and proceeding through the clutch, transmission, driveshaft and transaxle.

If a vibration is coming from the tires, wheels or hubs of the vehicle, the reason is either a high run-out/out-of round condition or an out-of-balance condition.

The issue of balance correction will be addressed later. At this point, we will assume the balance of the tires is fine (balance correction has been confirmed on a dynamic balancer) and the vibration issue still is present. The problem then must be run-out.

Run-Out

Run-out can be of two varieties: lateral or radial. Lateral run-out is the side-to-side movement of the tire/wheel assembly as it rotates on the axle. Lateral run-out is not typically critical to most vibration problems.

The big concern is usually radial run-out, which is simply an up-and-down movement of the tire/wheel assembly. It’s often presumed that the tire is out-of-round.

It is not too difficult to envision how an out-of-round situation can create a bumpy ride. A perfectly round disk with an axle strategically placed in the center circumference will roll smoothly. But, place the axle somewhere off the center of the circle or distort the circle slightly, and you can imagine the once-per-revolution bump that will result.

It is easy to blame the tire for a vibration, particularly when a new tire has been mounted on a vehicle that ran smoothly down the road before the new tire was installed. However, you must remember that the tire is just one of the components attached to the end of the axle and may or may not be the culprit.

It is nearly impossible for the radial run-out of a tire/wheel assembly attached to the end of an axle to be absolutely, perfectly round. Tires are molded from rubber, fabric and steel. Steel wheels are produced by a stamping process and there must be some tolerance in the fit of the wheel onto the hub or it would be nearly impossible for tire mounting. Sometimes, the hub may not be perfectly round, either.

Fortunately, a little bit of out-of-round can be tolerated. Over the years, we have found that some particularly sensitive vehicles can begin to experience a noticeable vibration with as little as 0.060 of an inch assembly run-out; some really sensitive vehicles (like motor homes) can experience vibrations with as little as 0.045 of an inch of assembly run-out.

You can measure run-out precisely with a dial indicator and with a specially made shoe or ski that allows you to spin the tire and document the difference between the high point and low point of the rotating assembly. Or, as many departments don’t own a dial indicator, you can spin the tire with a stationary object placed close by, and visually observe how much run-out is present. An experienced technician soon will develop a “calibrated eye” and be able to identify a tire/wheel assembly that could be cause for a complaint by a driver.

Has the Tire Been Mounted Properly?

If a vibration is being investigated on a vehicle that has just had one or more of its tires replaced, the best place to start is in the mounting itself. If, for example, the technician did not properly lubricate the bead of the tire before inflating, the bead may not have had a chance to seat properly on the flange of the wheel or rim.

All tires have a ring molded into the lower bead area that will give you a pretty good visual indication of proper mounting. By looking closely at the distance between this ring and the edge of the wheel, all the way around the tire, you can tell pretty quickly if the bead has been properly seated. If you don’t feel confident with your naked eye, you can measure the distance between the molded ring on the tire and the rim flange in four places, 90° apart, around the tire. The maximum difference in these measurements around the tire should be less than 1.5 mm (1/16 of an inch).

As you are looking to see whether the tires were properly mounted, you also should be looking for signs of damage to either the tire or the rim. A heavy impact with a curb or other solid object can bend a wheel or severely damage a tire. Either of these situations could be the source of the vibration.

If there is evidence of heavy impact on either the tire, the wheel or both, replace both immediately. Scrap any tire or wheel with obvious impact damage.

Needless to say, if you find that a tire has not been properly mounted, deflate the tire, break the bead from the rim on both sides, lube properly and re-inflate. If close inspection indicates that the tire now is mounted properly, you probably have solved your vibration problem.

However, if the tires are properly mounted on the rims or wheels and no damage is found, and there is still an unacceptable amount of run-out, we must look a bit deeper.

Match Mounting

Getting steer axle tires to roll true is especially important because the driver will feel imbalance or out-of-round through the steering wheel.

One way to reduce the run-out of a tire/wheel assembly is to “match mount” steer tires. Most tire manufacturers place a dot on tires (especially steer axle tires) that indicates where the “high point” is located. In the case of Goodyear tires, this dot is red. Other manufacturers may mark with different colored dots, so check with your dealer to find out exactly what to look for on each brand of tires you use. Technically, this marking indicates the “high point of radial first harmonic” on the tire, but to those less technically inclined, it’s the high point of the tire.

Steel wheel manufacturers mark their wheels with a dimple to indicate the “low point” of the wheel. Due to the way aluminum wheels are made (turned from an aluminum billet), they have very little or no run-out.

SIDEBAR: IMPACT Makes an Impact

All marked tires can be “match mounted” to marked steel wheels. Of course, this process works best when new tires are mounted on new wheels. If the high point of a new tire is matched with the low point of a new wheel, the resulting assembly is more “round” than any random positioning of the tire to the wheel. If the match-mounted tire/wheel assembly still has a combined run-out that is over a specific limit, either the tire or the wheel — or both — are out of tolerance. To get a vehicle back on the road, replace it with a match-mounted tire/wheel within the specific tolerance.

And, don’t forget, as the vehicle rolls down the road, everyday occurrences such as locking the brakes or an impact might change the high and low points of the tire and the wheel from when they were new. So, if the vibration returns, you may need to reset your tires.

Now, if we have tire/wheel assemblies that have acceptable run-out and are properly balance-corrected and we still have a vibration issue, what can we do next? There is one more procedure that might reduce or eliminate the vibration.

There are two ways that wheels are centered on the axles. Stud-piloting requires chamfered bolt holes in the wheel and ball seat (tapered) lug nuts. Simply put, tightening the lug nuts centers the wheel on the hub of the axle.

Hub-piloting is exactly as the name implies. The center hole in the wheel contacts a metal projection on the hub called a tang. The tight fit between the wheel and the tang on the hub centers the wheel on the hub. This is by far the most popular wheel-piloting method.

Stacking and Unstacking

If the kids are playing in the living room and the noise level builds to the point that we can’t stand it anymore, they have reached our tolerance level. In an engineering sense, tolerance means that there is a certain amount of imperfection we are willing to accept because, in most cases, things cannot be made perfectly. In the case of attaching a wheel to a hub, with either the hub-piloted or stud-piloted centering method, a perfectly centered result is impossible due to the tolerance required to make sure that everything fits together smoothly.

Strange as it may sound, we can combine tires and wheels that are not perfectly round. But a vibration may be caused by the fact that the high point of the tire/wheel assembly is “stacked” with an imperfectly centered wheel on a hub. Independently, neither imperfection would cause a problem, but if both are aligned at the point of rotation, the effects are compounded.

For example, a tire and wheel with a combined run-out of 0.035 inches that happens to line up with a wheel that is 0.020 inch from the center of the hub would result in a 0.055 inch total run-out. A very sensitive vehicle that cannot tolerate a run-out of more than 0.045 inches might have a ride issue with this combination.

So, if we measure the run-out (using the dial indicator mentioned earlier) and we mark the high point of radial run-out on the tire (with a crayon or silver pen), we can loosen the lug nuts of the hub-piloted system, spin the tire/wheel until the high point is at the top and let gravity do its job. Tighten the lug nuts with the high point of run-out at the 12 o’clock position.

In this example, the overall run-out would have been measured at 0.055 inches. You would not know, or even need to know, the fact that this number is composed of the sum of 0.035 and 0.020 inches. But, by using the method above, you have dropped the run-out to 0.015 inches.

Other Considerations

It was mentioned earlier that aluminum wheels, because of the way they are produced, have little or no run-out. A good trick is to always mount tires that are marked for high point with the high point dot aligned with the valve stem. Instruct technicians to fit wheels onto vehicles with the valve at the 12 o’clock position (high point up) and tighten the lug nuts. The combination of gravity and tolerances automatically will reduce your run-out.

Steer tires tend to send their vibrations through the steering wheel, but drive tires prefer the driver’s seat. For vibration attributed to out-of-round drive axle tires on a truck or van with dual tires, you can use the fact that you have two tires on the end of the drive axle to an advantage. Find the high point of run-out on each tire, then install the tires on the drive axle with the high points 180° apart. This essentially will create a situation where the forces that create vibration cancel each other and a smoother ride will result. Position the red dots at the 12 o’clock and 6 o’clock positions.

There are several ways to balance tire/wheel assemblies. Many companies market materials that can be placed in the tires or devices that attach to the wheel, and balance correction happens as the vehicle travels down the road.

By placing a tire/wheel assembly on a static balancer, balance correction can be achieved, but only in one plane. On a static balancer (sometimes called a bubble balancer), the tire and wheel are like a child’s seesaw and a balance weight can be attached to make the assembly level.

A dynamic balancer, if properly calibrated and correctly used, can tell the operator how to balance the tire in two planes. The use of lead weights for balance correction also is a somewhat traditional approach; however, because of the health issues surrounding the use of lead, it is probably a method that will disappear in the future. Other materials can be substituted for lead, but this usually means a heavier weight will be required.

Tim Miller is marketing communications manager for Goodyear Tire & Rubber's commercial tire systems.

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