Low speed vibration (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 road test the vehicle and carefully inspect the chassis components and assemblies. Please note the following:
- obvious signs of tire and wheel runout;
- obvious signs of drive shaft runout;
- 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 rims;
- tire defects such as distortion of shape, delamination, bulging from impacts with road imperfections. Small dents on the sidewalls of the tire are not a defect and do not affect the performance of the tire or the smoothness of the vehicle's ride.
Balancing the chassis is the simplest operation to eliminate vibration of the chassis at high speed, 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 imbalances of the copes.
After the wheels are mounted on the vehicle, final balancing can be performed. This operation allows you to eliminate the imbalance of the wheels assembled with brake discs 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 the presence of a runout, you should measure the runout of the suspended wheel without removing it from the vehicle. For measurements, 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 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 pressure tape to ensure 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 wheel runout value when installed on a vehicle should not exceed 1.5 mm. If the runout exceeds the permissible limit, perform the following procedure.
- 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 note the positions of the points with the greatest deviations. Then measure the wheel disc runout. If the obtained runout values exceed the permissible limit, the wheel disc must be replaced. If the radial or axial runout of the wheel and tire assembly (measured by the tread) exceeds 1.5 mm, you should proceed to the next operation.
- If the radial or axial runout of the wheel exceeds 1.5 mm, the tire should be remounted, installing it on the wheel in such a position that the point with the greatest radial deviation coincides with the point of the smallest 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 1.5 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 misalignment of the wheel 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 imbalance or wobble of the wheels, another cause of vibration of the chassis may be uneven radial stiffness of the tire in different sections (tire runout under load). However, the radial stiffness 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. Vibration of the radial stiffness around the tire circumference causes vertical displacements of the wheel axis, which can be measured.
More advanced equipment allows not only to monitor changes in the radial stiffness of a wheel removed from a vehicle, but also to partially correct the tire's unevenness 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 conducting road tests each time. This helps to identify the uneven wheel causing the chassis vibration. The vibration of the radial stiffness of the tire depends on the direction of rotation of the wheel.