- obvious signs of beating tires and wheels;
- obvious signs of beating drive shafts;
- the air pressure in the tires should be normal;
- violation of the normal height of the static position of the body;
- wheel rim deformations;
- coatings on tires and wheels,
- loose or missing balancing weights or fastening nuts;
- abnormal or severe wheel wear;
- correct fit of tires on wheel rims;
- tire defects, such as delamination, bulging from impacts on road bumps. Small dents on the sidewalls of a tire are not a defect and do not affect the performance of the tire or the smooth running of the vehicle.
Wheel balancing is the easiest operation to eliminate undercarriage vibration at high speed, so it should be done first. It is necessary to dismantle the wheels and balance them dynamically on the stand. This eliminates all wheel imbalances.
After mounting the wheels on the car, final balancing can be performed. This operation allows you to eliminate the imbalance of the wheels assembled with brake discs, drums and decorative caps.
If wheel balancing fails to eliminate vibration at high speed or vibration appears at low speed, wheel wobble is a possible cause. Wheel runout can be caused by tire runout, wheel rim runout, or improper mounting of tire to rim. In this case, do the following.
Three suspicions of the presence of a runout should measure the amount of runout of a suspended wheel without dismantling it from the car. For measurement, it is recommended to use a dial indicator with a probe. Axial runout should be measured along the sidewall of the tire as close to the shoulder as possible. Radial runout is measured at the center tread of the tread. In some cases (with a strongly dissected pattern of the central treadmill) it may be necessary to tightly wrap the protector with adhesive tape for better contact with the indicator probe.
When measuring axial or radial runout, one should not take into account significant and short-term deviations of the indicator needle, which are due to local irregularities and the tread pattern. Write down the resulting runout value and note the position of the point on the tire with the maximum deviation. The value of the runout of the wheel installed on the car should not exceed 1.5 mm. If the beat is out of range, perform the following operation.
If, when measuring on a car, an axial or radial runout value of more than 1.5 mm is obtained, the wheel should be removed, installed on a balancing stand and the runout value should be checked again. The centering of the wheel on the stand is carried out along the central hole. Measure and record the wheel runout values following the previous point and note the position of the points with slight deviations. Then measure the wheel rim runout. If the runout values obtained exceed the allowable limit, it is necessary to replace the wheel If the radial or axial runout of the wheel assembly with the tire (measured over the tread) exceeds 1.27 mm, proceed to the next step.
If the wheel radial or axial runout exceeds 1.27 mm, remount the tire by mounting it on the wheel in such a position that the point of greatest radial deviation coincides with the point of least radial deviation of the wheel. Inflate the tire, place the wheel on a balancer and re-measure the radial and axial runout. Mark the position of the points with the largest deviations. Such a rearrangement of the tire often allows you to reduce the amount of runout to a value not exceeding 1.27 mm.
If the runout of the wheel dismantled from the car is normal, and the runout measurements directly on the car give unacceptably large runout values, the possible cause is the misalignment of the wheel relative to the hub. Offset the wheel circumferentially by two hub studs, secure the wheel, and re-measure runout. Repeating a similar operation several times, find the best wheel position.
If it was not possible to reduce the runout of the wheel to an acceptable level, the wheel should be dismantled and the runout of the wheel hub should be measured. To do this, set the indicator to zero when the probe touches the hub. Then carefully turn the hub. Measure and record the amount of runout. If the runout exceeds 0.76 mm, the hub must be replaced. After remounting the tire, which is associated with its replacement or a change in the circumferential position, it is necessary to balance the wheel assembly. In addition to imbalance or wheel runout, an additional cause of chassis vibration may be uneven tire radial stiffness in different sections (tire runout under load). However, it is possible to check the radial stiffness of a tire only on a special tire tester.
The tire tester is a support drum on which the car wheel rests. During the tire test, the drum rotates slowly. The vibration of the radial stiffness around the circumference of the tire causes vertical movements of the wheel axle, which can be measured.
More advanced equipment makes it possible not only to control the change in the radial stiffness of the wheel dismantled from the car, but also to partially correct tire heterogeneity by removing a small layer of rubber from the tread in the most rigid sections of the tire. To control the radial stiffness, as in the first case, the rolling of a loaded wheel on a rotating drum is used. A tire tester and similar equipment provide the ability to simultaneously monitor the measurement of the radial runout stiffness of a tire and a wheel. However, such stands are not always available, therefore, the above-described method of monitoring the runout of a free wheel using an indicator is widely used in practice. Free wheel runout is usually related to the location of the hardest section of the tire.
An additional method can also be used to diagnose the causes of undercarriage vibration. The method consists in completely replacing the wheels of the car with known-good wheels and testing the car on the go. If the vibration has disappeared, gradually replace all the wheels of the car one at a time, returning the old ones to their place, and each time conducting road tests. This helps to identify a non-uniform wheel leading to vibration of the undercarriage. The vibration of the radial stiffness of a tire depends on the direction of rotation of the wheel.