Vibration at low speed (up to 65 km/h) usually caused by beating. Causes of vibration at high speed (over 65 km/h) there may be both beating and imbalance. Before carrying out any repair work, it is necessary to perform a road test of the car and carefully inspect the chassis parts and components. Please note the following:
- obvious signs of tire and wheel runout;
- obvious signs of drive shaft beating;
- the air pressure in the tires must be normal;
- violation of the normal height of the static position of the body;
- wheel rim deformations;
- deposits on tires and wheels;
- loose or missing balancing weights or mounting nuts;
- abnormal or severe wheel wear;
- correct fit of tires on wheel rims;
- tire defects such as distortion of shape, delamination, bulging from impacts with road irregularities. Small dents on the sidewalls of the tire are not a defect and do not affect the performance properties of the tire or the smoothness of the vehicle.
Wheel balancing is the simplest operation to eliminate chassis vibration at high speeds, so it should be performed first. It is necessary to dismantle the wheels and balance them dynamically on a stand. This allows you to eliminate all wheel balancing violations.
After the wheels have been mounted on the vehicle, 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 occurs at low speed, wheel runout may be the cause. Wheel runout can be caused by tire runout, wheel rim runout, or improper tire/rim mounting. In this case, you need to do the following:
If you suspect that there is a runout, you should measure the runout value of the suspended wheel without removing it from the vehicle. For measurements, it is recommended to use a pointer indicator with a probe. Axial runout should be measured along the sidewall of the tire as close to the shoulder area as possible. Radial runout is measured along the central tread groove. In some cases (with a highly dissected pattern of the central running track) it may be necessary to wrap the protector tightly with adhesive tape for better contact with the indicator probe. When measuring axial or radial runout, significant and short-term deviations of the indicator arrow, which are caused by local irregularities and the tread pattern, should not be taken into account. Record the resulting runout value and mark the position of the point on the tire with the maximum deviation. The runout value of the wheel installed on the vehicle should not exceed 1.5 mm. If the runout is beyond the permissible limit, the following operation should be performed.
If, when measuring on a vehicle, the axial or radial runout value obtained is more than 1.5 mm, the wheel should be removed, installed on a balancing stand and the runout value checked again. The wheel is centered on the stand using the central hole. Measure and record the wheel runout values following the recommendations in the previous paragraph, and mark the position of the points with small deviations. Then measure the wheel rim runout. If the obtained runout values exceed the permissible limit, the wheel must be replaced. If the radial or axial runout of the wheel and tire assembly (measured by the tread) exceeds 1.27 mm, proceed to the next operation.
If the radial or axial runout of the wheel exceeds 1.27 mm, the tire should be remounted by installing 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 balancing stand and re-measure the radial and axial runout. Mark the position of the points with the greatest deviations. This type of tire repositioning often reduces the runout to a value not exceeding 1.27 mm.
If the runout of a wheel removed from a vehicle is normal, but runout measurements directly on the vehicle yield unacceptably high runout values, a possible cause is a violation of the wheel centering relative to the hub. Move the wheel circumferentially by two hub studs, secure the wheel and re-measure the runout. Repeat the same operation several times to find the best position for the wheel.
If it is not possible to reduce the wheel runout to an acceptable level, the wheel should be removed and the wheel hub runout measured. To do this, set the indicator to zero when touching the hub with the feeler gauge. Then carefully turn the hub. Measure and record the runout value. If the runout exceeds 0.76 mm, the hub must be replaced. After remounting a tire, which involves replacing it or changing its circumferential position, it is necessary to balance the wheel assembly.
In addition to wheel imbalance or beating, another cause of chassis vibration may be uneven radial stiffness of the tire in different sections (tire runout under load). However, the radial hardness of a tire can only be checked using a special tire tester.
The tire tester is a support drum on which the car wheel rests. During the tire testing process, the drum rotates slowly. Radial stiffness vibration around the circumference of the tire causes vertical movements of the wheel axis that can be measured.
More advanced equipment allows not only to control the change in radial rigidity of a wheel removed from a vehicle, but also to partially correct the non-uniformity of the tire by removing a small layer of rubber from the tread in the most rigid sections of the tire. To control radial rigidity, as in the first case, the rolling of a loaded wheel on a rotating drum is used. The tire tester and similar equipment provide the ability to simultaneously monitor the measurement of radial stiffness, tire and wheel runout. However, such stands are not always available, so the above-described method of monitoring the runout of a free wheel using an indicator is widely used in practice. Typically, the runout of a freewheel is related to the location of the stiffest section of the tire.
An additional method can also be used to diagnose the causes of chassis vibration. The method involves completely replacing the car's wheels with known good wheels and testing the car on the road. If the vibration has disappeared, gradually replace all the wheels of the car one by one, returning the old ones to their place, and each time conducting road tests. This helps to identify the non-uniform wheel causing the chassis vibration. The vibration of the radial stiffness of the tire depends on the direction of rotation of the wheel.