General information
Vibrations are caused by a sequence of pulses. In the case of vibrations related to the speed of movement, these impulses are generated by an imbalance of some element rotating with the wheel, which is transmitted to the corresponding wheel suspension. The frequency of these impulses changes with the speed of movement. Usually they are extinguished due to a specially selected design of suspension elements and supports, in addition to vibrations of a strictly defined frequency «natural vibrations of the suspension», which depend on its longitudinal rigidity. The moments of occurrence of natural vibrations of the front and rear suspensions are usually very different and can be expressed in terms of the speed of movement, since «rolling radius» wheels do not change.
The number of impulses per revolution of the wheel is the ratio between the moment of occurrence of natural oscillations of the corresponding suspension, expressed in terms of the speed of movement, and the speed of movement at the moment of occurrence of vibrations (see table of natural oscillations of the front and rear suspensions of the vehicles covered by this Technical Note).
There are two types of imbalance.
- Unbalance: mass unbalance = one impulse per wheel revolution.
- Radial runout: geometric deformation = one or more pulses per revolution of the wheel.
Vibrations occur on the following nodes and parts:
- rotating elements (tires, wheel rims, decorative wheel caps, brake mechanisms, wheel drive shafts, hollow side gear of the open differential type manual gearbox);
- damping elements (shock absorbers, mountings and/or damping elements for suspension, subframe, rack and pinion steering, steering column, seats, etc.).
Determining the source of vibrations
Determining the source of vibrations is also an element of diagnostics, but it is not systematized. Vibrations are transmitted through the suspension and further through the steering knuckle (center of rotation). Then them «path» (or «transfer function») depends on the energy of vibrations, the nature of the bonds between the parts (tightening of fasteners, condition of vibration damping elements, etc.), their stiffness, or from various possible components of the impulses that cause vibrations.
It can be assumed that in most cases of transmission of vibrations to the steering wheel, they come from the front suspension and are transmitted through the steering. But this is not an absolutely universal rule. Steering wheel vibrations can be caused by some very specific rear wheel tire defects.
Vibrations are caused by a sequence of pulses (blows), coming from elements rotating with the wheel. Vibrations are caused by two types of causes:
- weight imbalance;
- one pulse per wheel revolution.
The front and rear suspensions have characteristics on the basis of which diagnostics are carried out.
They dampen vibrations, except for those that have a frequency «natural vibrations».
The moment of occurrence of natural vibrations of the suspension depends on its longitudinal stiffness. The oscillation frequency is determined by the speed of the vehicle. The moments of occurrence of natural oscillations of the front and rear suspensions differ from each other.
Vibration diagnostics allows you to determine which suspension (front or back) they come out, and, in particular, to determine the number of impulses per revolution of the wheel. These initial diagnostic data serve as a guide to the selection of the necessary technical intervention. Therefore, it is important to obtain this data from the owner or to carry out a road test.
The amount of unbalance needed to create perceptible vibrations is about:
- 20g for wheels (tires on 15" rim);
- 100 g for brake disc or drum;
- 500 g for the wheel hub;
- 1.16 kg for the wheel drive shaft.
Attention! Some types of vibrations caused by the operation of the engine can be mistaken for vibrations associated with the speed of movement. In this case, the appearance of vibration is most likely associated with engine speed or load. This can only be verified by reproducing the fault with an appropriate check.
Special cases of vibration
A possible cause of vibrations is a defect in the design of the tires. Such a defect is outwardly invisible. To identify it, you need to perform an appropriate check.
Note. In the presence of a defect in the design of the tire, balancing weights of a large mass are usually installed on the wheel (50 g or more on one side of the wheel rim).
«hardening» suspension shock absorbers
There are cases of «vibrations» only on particularly smooth roads (on freeways or new express lanes). Friction in the shock absorbers, combined with a stiff suspension, can cause vibration-like bounce. This phenomenon stops under normal driving conditions (during a normal overtaking maneuver, on a rougher road surface, etc.).
Check the tightening torques of the threaded connections and the condition of the hangers. After installing new parts or on a new vehicle, this phenomenon gradually disappears as «lapping» details.
Excessive tire pressure
When tires are overinflated, a rebound phenomenon occurs, which can be mistaken for vibrations.
Troubleshooting Algorithms
Check 1: road test to determine the nature of the vibrations and their sources.
Mark the position of the tire on the wheel rim (for less visibility, the mark is applied to the metal on the inside of the wheel)
Play vibrations, make sure they are related to the speed of movement (reproduce vibrations in several gearbox gears).
Record the speed range in which the vibrations occur.
Check the effect of braking on vibrations:
- Slow down the car slightly in the area of vibrations (until the brake pads touch the disc) and determine the presence or absence of a change in vibrations while maintaining the speed of movement.
Check the effect of changing engine torque (movement «tightness» when engaging low gears):
- change the motor load in the area of vibration occurrence rates (to create traffic conditions «tightness», then when switching to lower gears) and determine the effect of this change on the vibrations.
Determine from which of the suspensions the vibration comes and the number of impulses per revolution of the wheel according to the table of the rates of occurrence of natural vibrations of the suspensions.
Check 2: road test to determine the rate of occurrence of natural oscillations of the suspensions.
This test is to determine the rate of occurrence of natural oscillations of the suspensions by artificially creating an imbalance (receiving one impulse per wheel revolution) on one of the front, then on one of the rear wheels.
The best solution is to use another vehicle for this test (the same model, but not necessarily the same engine).
Install an additional 50 g balance weight on one of the front wheels and mark its position (the position of the weight on the circumference of the wheel disk does not matter). Leave the old weights or the weight in place.
Carry out a road test and record the speed range in which the vibrations appear. The result corresponds to the rate of occurrence of natural oscillations of the front suspension of the tested vehicle.
Remove the extra weight from the front wheel and install it on one of the rear wheels.
Carry out a road test again and note down the speed interval in which the vibrations appear. The result corresponds to the rate of occurrence of natural oscillations of the rear suspension of the tested vehicle.
Compare the result of test 1 with test 2 and determine which suspension is the source of the vibrations and the number of pulses per revolution of the wheel.
Check 3: road test to determine if there are structural defects in the tires.
A possible cause of vibrations is a defect in the internal structure of the tires. Such a defect is outwardly invisible. To identify it, rearrange the front and rear wheels (put left wheels on the left, right wheels on the right), having previously recorded the vibration velocity interval during test 1, and perform test 1 again. If, during this new test, the vibration occurrence velocity interval has changed and began to correspond to the natural oscillation velocity of another suspension (see table 4.2 «The rate of occurrence of natural oscillations of the suspensions»), then the defect is related to tires or wheel balancing.
Note. In the presence of a defect in the design of the tire, balancing weights of a large mass are usually installed on the wheel (50 g or more on one side of the wheel rim).
Note. The reliability of test 3 results is based on the difference between the natural oscillation speeds of the front and rear suspensions.
Pic. 4.8. Vibrations of the front or rear suspension at one impulse per wheel revolution
Table 4.1. Possible causes of imbalance
Table 4.2. The rate of occurrence of natural oscillations of the suspensions
* Megane II 5- and 3-door hatchback, sedan, station wagon and convertible.