Vibration Troubleshooting

There are many rotating items in a car's drive train which can cause vibrations. Each part has its own range of natural frequencies. If a natural frequency is excited ,the part causes the greatest disturbance as it vibrates. Engine vibrations occur at the highest frequency, many hundreds of vibrations per second, and cause a buzzing or humming sound. These vibrations can be felt in the floor or dashboard. Tire and wheel related vibrations occur at a much lower frequency, typically 10 to 15 times a second. This results in the "shake" most of us have experienced. Even a small amount of wheel imbalance can translate into vibration problems.


Bobby Henry is qualified to diagnose
problems with tires as well as auto
and light truck repairs.

Tire-induced vehicle shake is most often caused by:
(1) a tire poorly seated on the rim;
(2) a heavy spot in the tire,
(3) a stiff spot in the tire, or
(4) an out-of-round tire.
The most critical speed for tire-induced shake is from 55 to 65 mph. At this speed, a tire rotates at 10 to 15 times per second. This is the frequency that corresponds to the natural frequency of a car suspension for virtually all cars, making shake a potential problem for all drivers.
A heavy spot in a tire or tire/rim combination causes a radial force which bounces the tire up and down (static imbalance) once per tire revolution. This force increases as the cars speed increases. It is only at higher speeds, faster than 40 mph, that this weight induced bounce becomes noticeable. An out of round tire or wheel causes a once per tire revolution up and down force that is independent of speed If the vibration is not evident at low speeds but begins at higher speeds, imbalance is the likely cause. If a bounce is felt at very low speeds, the likely cause is an out of round tire or wheel. Sometimes a low speed bounce is caused by "flat spotting". Make sure the tire is warmed up from several miles of driving to eliminate flat spotting as a source of low speed bounce.
Eddie Richardson is an excellent diagnostic
technician for auto and light truck repairs.

While the up and down shake or bounce results from a heavy spot in the tread, the side-to-side shake or wobble (dynamic imbalance) can result from poor bead seating or a heavy spot in a sidewall. If both front tires are affected, the vibration will be added together when both tires are in phase, and may go away completely after a corner, which causes the outside tire to roll more than the inside tire. This tire repositioning can cause the vibrations to cancel each other. Soon, however, the tires will be in phase again, and the vibrations will recur. Proper balancing can correct vibrations resulting from heavy spots in tires.

An out-of-round (run-out) tire/wheel, or a stiff spot in a tire can cause vibration even in a perfectly balanced tire. In these situations, forces create vibrations that are present regardless of vehicle speed. Wheels don't have force variation, but they all have some run-out. Likewise, every tire has some force variation. Each of these variations, by itself, would not cause a problem but there will be a vibration problem if the stiff spot or high point of a tire happens to be placed at the high spot on the rim during mounting. It is easy to avoid this condition. New tires and rims are marked with dots or similar devices. When these dots are matched, the high spot on the tire is lined up with the low spot on the wheel, and vibration is minimized (see Match Mounting).

 

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Red Dot - To be used in Match Mounting

If wheels and tires are not marked, and the vehicle exhibits runout vibration, the tire should be rotated one half turn (180 degrees) on the wheel. This will generally reduce the vibration.

Worn shocks or suspension will allow a minor vibration to magnify until it is felt as a major one.

Harmonic Marking/Match Mounting

For the smoothest ride and least vibration, the tire must be carefully matched to the wheel. Even though new wheels are marked, when mounting new tires on a used wheel, the original marks may not be found. Even if they were found, the wheel may have shifted around and must be re-measured.

Rim runout can be measured using a dial indicator. Measure and mark the lowest spot of the inside flange and the outside flange. Midway between these two marks is the average low spot of the rim. Mark this spot on the valve stem side of the rim.

Tire runout is not so simply measured. Tire runout includes both measurable runout, as well as the stiff spot in the sidewall. Interaction between the visible runout and the stiff spot make it impossible to find a tire's true high spot virtually without specialized equipment. This specialized equipment, loads a tire is against a road wheel which simulates a highway surface. The tire is rotated against the revolving road wheel, and the radial force variation of the tire acting against the road wheel is measured. This force variation is analyzed and the high spot of the first harmonic is marked on the tire.

Now that both the rim and tire have marks, it is easy to mount the tire so that the harmonic mark on the tire is lined up with the average low spot of the rim. This is called match mounting and cancels the tire runout and wheel runout, providing the smoothest possible ride.

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