Vibrations are caused by a sequence of pulses. In the case of speed-related vibrations, these pulses are generated by an imbalance in some element rotating with the wheel, which is transmitted to the corresponding wheel suspension.
The frequency of these pulses changes with speed.
They are usually damped by a specially selected design of the suspension elements and supports, except for vibrations of a strictly defined frequency, called "natural oscillations of the suspension," which depend on its longitudinal stiffness.
The moments of occurrence of natural oscillations of the front and rear suspensions usually differ significantly and can be expressed in terms of speed, since the "rolling radius" of the wheels does not change.
The number of pulses per wheel revolution is the ratio between the moment of occurrence of natural oscillations of the corresponding suspension, expressed in terms of speed, and the speed at the moment of vibration occurrence (see the table of natural oscillations of the front and rear suspensions of the vehicles covered by this Technical Note).
There are two types imbalance.
- – Unbalance: mass imbalance = one pulse per wheel revolution.
- – Radial runout: geometric deformation = one or more pulses per wheel revolution.
Vibrations occur in the following components and assemblies:
- – Rotating elements (tires, wheel rims, wheel trims, brake mechanisms, wheel drive shafts, hollow axle gear of an open-type differential of a manual transmission);
- – Damping elements (shock absorbers, mounts and/or damping elements of the suspension, subframe, rack and pinion steering gear, 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 then through the steering knuckle (the center of rotation).
Their "path" (or "transmission function") then depends on the vibration energy, the nature of the connections between the parts (tightness of fasteners, condition of vibration-damping elements, etc.), their rigidity, or on various possible components of the impulses causing vibrations.
In some cases, vibration in the steering wheel occurs due to the destruction of engine mounts.
The rubber elements of the mounts fail after prolonged use.
Vibration from the rotating elements of the engine is strongly felt on the steering wheel.
It can be assumed that in most cases of vibration transmission to the steering wheel, they originate from the front suspension and are transmitted through the steering.
However, this is not an absolutely universal rule.
Steering wheel vibrations can be caused by several factors Very specific defects of the rear tires.
Vibrations are caused by a sequence of pulses (impacts) coming from elements rotating with the wheel.
Vibrations are caused by two types of factors:
- – mass imbalance;
- – one pulse per wheel revolution.
The front and rear suspensions have characteristics that are used for diagnostics.
They dampen vibrations, except for those that have a "natural oscillation" frequency.
The moment of occurrence of the suspension's natural oscillations depends on its longitudinal rigidity.
The frequency of oscillations is determined by the vehicle's speed.
The moments of occurrence of the front and rear suspensions' natural oscillations differ from each other.
Vibration diagnostics allow us to determine which suspension (front or rear) they are coming from, and, in Features, determine the number of pulses per wheel revolution.
This initial diagnostic data serves as a guide for selecting the necessary technical intervention.
Therefore, it is important to obtain this data from the owner or perform a road test.
The amount of imbalance required to create noticeable vibrations is approximately:
- – 20 g for wheels (tires on a 15-inch rim);
- – 100 g for a brake disc or drum;
- – 500 g for a wheel hub;
- – 1.16 kg for a wheel drive shaft.
Some types of engine-induced vibrations can be mistaken for speed-related vibrations.
In this case, the vibration is most likely related to engine speed or load.
Make sure This can only be determined by reproducing the fault with appropriate testing.
Special cases of vibration
A possible cause of vibration is a tire design defect.
Such a defect is not externally visible. To detect it, it is necessary to perform an appropriate test.
Note:
If there is a tire design defect, heavy balancing weights (50 g or more on one side of the wheel rim) are usually installed on the wheel. "Hardening" of the suspension shock absorbers.
There are cases of Vibrations may only occur on particularly smooth roads (motorways or new expressways).
Friction in the shock absorbers combined with a stiff suspension can cause a bounce similar to vibration.
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 suspension.
After installing new parts or on a new vehicle, this phenomenon gradually disappears as the parts "break in."
Excessive tire pressure.
Overinflated tires cause a bounce phenomenon that can be mistaken for vibration.
Troubleshooting Algorithms
Check:
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).
Reproduce the vibrations, making sure they are related to the driving speed (reproduce the vibrations in several gears of the transmission).
Record the speed range in which the vibrations occur.
Check the effect of braking on vibrations:
– In the range of speeds where the vibrations occur, lightly brake the car (until the brake pads touch the disc) and determine the presence or absence of a change in vibrations while maintaining the driving speed.
Check the effect of changing engine torque (driving "tight" when engaging lower gears):
– In the range of speeds where the vibrations occur, change the engine load (to create conditions of driving "tight", then when shifting to lower gears) and determine the effect of this change on Vibration.
Determine which suspension is causing the vibration and the number of pulses per wheel revolution using the table of natural oscillation rates for suspensions.
Test 2: Road test to determine the natural oscillation rate of the suspensions.
This test involves determining the natural oscillation rate of the suspensions by artificially creating an imbalance (producing one pulse per wheel revolution) on one of the front wheels, then on one of the rear wheels.
The best solution is to use another car for this test (the same model, but not necessarily with the same engine).
Install an additional 50g balancing weight on one of the front wheels and mark its position (the position of the weight on the wheel rim is not important).
Leave the existing weights or weight in place. Perform a road test and record the speed range at which vibrations occur.
The result corresponds to the natural oscillation rate of the front suspension of the vehicle being tested.
Remove the additional weight from the front wheel and place it on one of the rear wheels.
Repeat the road test and record the speed range at which vibrations occur.
The result corresponds to the natural oscillation rate of the rear suspension of the vehicle being tested.
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 wheel revolution.
Test 3: Road test to determine the presence of tire design defects.
A possible cause of vibrations is a defect in the internal design of the tires.
Such a defect is not visible from the outside.
To detect it, swap the front and rear wheels (left wheels on the left, right wheels on the right) (right), having first recorded the vibration velocity range during Test 1, and repeat Test 1.
If, during this new test, the vibration velocity range changes and matches the natural vibration velocity of the other suspension, the defect is related to the tires or wheel balancing.
If a tire design defect is present, heavy balancing weights (50 g or more on one side of the wheel rim) are typically installed on the wheel.
The reliability of Test 3 results is based on the difference between the natural vibration velocities of the front and rear suspensions.