14.3. Diagnostics of tires

Uneven and premature wear of tires
Many reasons cause uneven and premature wear. Some of them are given below: non-optimal pressure in tires, uneven rotation, insufficient skills of the driver and the inappropriate angles of installation of wheels.
If repeated adjustment of wheels taking into account wear of tires is carried out, it is necessary to establish a corner of a convergence of wheels so close to zero as far as it technical requirements allow. At detection of wear it is necessary to examine tires at rotation to reveal the following defects:
– distinctions in wear of forward and back tires;
– uneven wear on protector width;
– unequal wear of the left and right tires on forward wheels;
– unequal wear of the left and right tires on back wheels.
It is necessary to execute repeated adjustment if the following defects are found:
– unequal wear of the left and right tires;
– uneven wear on width of a tire any of forward tires;
– a sharp edge ("small beard") on one of the parties of a protector.

Signs of wear of a protector
Signs of wear of a protector are:

Fig. 14.9. Arrangement of indicators (A) of wear of a protector

– emergence of strips 12,7 mm wide, where depth of the drawing of a protector less than 1,6 mm (fig. 14.9);
– if there were 2 or more flutes in three places, it is necessary to replace the tire.

Fig. 14.10. Diagnostics of wear of tires: and – wear of outer sides of a protector; b – unilateral wear of a protector of the tire; with – strong wear in the middle of a tire protector profile

Continuous check of a condition of tires will allow to find in due time intensive wear and to remove its cause that will allow to avoid more serious consequences (fig. 14.10).
Wear of outer side of the tire (the forward tire) occurs when passing turns at high speeds. With such wear it is recommended to rearrange tires from a forward axis on a back axis of the car.
Wear of outer sides of a protector happens at the movement of the car to the lowered pressure in tires.
Unilateral wear of a protector of the tire. The wrong angles of installation of wheels cause not only the increased wear of tires, especially unilateral wear, but also promote decrease in traffic safety.
Strong wear in the middle of a tire protector profile. Happens because of the frequent movement of the car with the maximum speed of the tire therefore from action of centrifugal forces the protector of the tire becomes convex in the center and intensively wears out. This type of wear is most brightly shown on back wheels at elevated pressure in them.

Zavalivaniya, arising when using radial tires

Fig. 14.11. Zavalivaniye of the car as a result of defect of tires

Zavalivaniye is the movement here and there of a back or forward part of the car (fig. 14.11). This phenomenon arises at an uneven arrangement of a metalcord breker in the tire or at excessive side wear of the tire or a wheel. The zavalivaniye is especially noticeable at small speeds from 8 to 48 km/h. It is also shown as a driving roughness at speeds from 80 to 113 km/h.
Trial run allows to find out, behind or in front there is a tire with defect. At defect of the back tire the zavalivaniye occurs in a back part of the car. On a driver's seat there is such feeling as though someone pushes the car sideways.
Zavalivaniye it is especially noticeable at defect of the forward tire.
There is a feeling that a forward part of the car moves alternately forward and back as though the rotation axis was in a driver's seat.
At detection of a small zavalivaniye and at impossibility to define the defective tire execute the following checks.
1. Define in the movement, behind or in front there is a zavalivaniye.
2. To the place of estimated wheels with defect establish the serviceable wheels borrowed from the similar car (if it is not possible to define, in front or behind there is a zavalivaniye, begin with back couple).
3. Repeat trial run. If the movement improved, replace in turn tires and wheels with those that stood originally until the tire with defect is not revealed. If there is no improvement in the movement, it is necessary to replace all four wheels on faultless, and then to serially change them for wheels, "own" before the detection, which caused a zavalivaniye.

Withdrawal of the car as a result of defect of radial tires
Withdrawal is a deviation of the car in the movement from the rectilinear movement without impact on a steering wheel. Usually are the reason of withdrawal:
– defects of a design of tires;
– uneven adjustment of brakes;
– wrong adjustment of angles of installation of wheels.
Shortcomings of technological process of production of tires can be the cause of withdrawal. Defects of placement of a cord in the tire can be an example.
Cord shift from the center can cause emergence of side force at rectilinear movement along the road. The tire slides like a cone.

Back tires cannot be the cause of withdrawal.

Torque on a steering wheel
To the right at some types of front-wheel cars which have length of power shafts unequal it is rather simple to determine torque size by strong pressing an accelerator pedal. The torque arises because the right power shaft is longer than left: distinction in axial coal is connected with it. Cars with an intermediate shaft have power shafts, almost identical on length.
The difference in length of power shafts is the reason of bigger torque on coal of a convergence of the left forward wheel.
The torque appears at accelerations of the motionless car or at low speeds.

1. Paste a small piece of an adhesive tape in the center of a steering wheel.
2. Note on what distance it was required to turn a steering wheel to hold the car for the rectilinear movement at sharp acceleration.
3. Compare the received result to the size received on cars of similar brand. Existence of big torque on a steering wheel can be caused by the following reasons:
– dispersion of parameters of tires and wheels which have the greatest value at emergence of torque on a steering wheel. A little reduced diameter of the tire of a forward right wheel will cause the right torque and withdrawal of the car to the right. Examine tires on forward wheels for the purpose of detection of distinctions in a design, the sizes and firms manufacturers. If tires of similar distinctions have no, trade their places and again test the car on the run;
– a big difference in pressure of the right and left forward tires;
– any side play of the plug of a control lever, cross steering draft, steering transfer leading to rolling-out of a forward wheel, and also emergence of the torque causing a bigger corner of a convergence than on a wheel from the opposite side of the car;
– a side play in suspension bracket elements, also bringing to I will run out when braking;
– the big height of a trim leading to emergence of a corner on the leader driving to a shaft;
– jamming or too dense connection of power shafts. Dense connection or big height of a trim lead to wagging of forward wheels at speeds from 24 to 48 km/h;
– incorrectly established, worn out or weakened fastenings of the engine which cause emergence of an undesirable corner on power shafts.
Conditions under which there can be an effect, similar to torque on a steering wheel:
– wrong adjustment of angles of installation of forward or back wheels;
– distortion of geometry of a frame or its wrong installation;
– destruction of a forward suspension bracket.

Diagnostics of emergence of vibrations

Fig. 14.12. The reasons causing vibration at movement along the flat road: and – ovality of the tire; b – uneven rigidity of the tire on a circle; with – curvature or ovality of a disk of a wheel

The main reasons causing vibration of a wheel at movement of the car along the flat road are (fig. 14.12):
– ovality of the tire;
– uneven rigidity of the tire on a circle;
– curvature or ovality of a disk of a wheel.

Communication of vibration with defects of a wheel, nave of a wheel and axis
Two factors can cause vibration of the tire or a wheel: imbalance or beating.
The vibrations arising at speeds less than 64 km/h are usually caused by a beating. Vibrations at speeds more than 64 km/h at movement along the highway can be caused both by an imbalance, and a beating. Before to start repair, it is necessary to carry out running inspections and careful survey for detection of the following defects:
– considerable beating of the tire and wheel;
– considerable beating of a power shaft;
– deviations from optimum pressure in tires;
– height deviations from an optimum longitudinal inclination;
– the bent disks of wheels;
– the garbage which collected on tires and wheels;
– unreliably tightened nuts or their absence;
– wrong landing of the tire to a rim;
    – damages of tires, such as deformation of tires, peelings or a thickening owing to blows. Small dents on sidewalls of tires do not exert negative impact on quality of the tire and do not influence the movement.
Balancing – the simplest type of repair. If there is a vibration, it is necessary to carry out it first of all. At first it is necessary to make dynamic two-plane balancing with removal of a wheel. It will eliminate an imbalance of the tire and a wheel assembled.
Also final balancing of a wheel on the car is necessary. It reaches balancing of the brake drum, a disk and a decorative cap of a wheel. If balancing does not eliminate vibration at big or small speeds, then it the wheel beating is the most probable cause. The beating can be caused as defects of tires, wheels, and the wrong fastening of a wheel on the car. It is necessary to perform the following operations.
A. Measurements of a beating of a wheel at free rotation without loading are carried out without its removal from the car. The indicator of hour type with a roller on the end is most convenient for such measurement, however it is also possible to use the indicator with a push-button tip. The peripheral beating (here and there) should be measured as it is possible closer to a humeral zone of a protector. The radial beating has to be measured in the center on a longitudinal edge of a protector. Design features of some protectors cause the necessity to apply an adhesive tape. For the best contact of the indicator with a protector it is necessary to wrap up densely its central circle an adhesive tape. At measurement of a radial or axial beating do not pay attention to the sharp fluctuations of an arrow of the indicator caused by side hollows or inflows on a protector.
Write down the maximum indications of the indicator and note the place of the greatest beating. The total value of a beating of the tire and a wheel assembled should not exceed 1,5 mm.
If the beating exceeds 1,5 mm, start implementation of the recommendations described in part B.
B. If at the measurements described in part A, the radial or axial beating exceeds the size of 1,5 mm, it is necessary to install the tire and a wheel assembled on the device for dynamic balancing of wheels and to take repeated measurements of sizes of a beating. Installation of a wheel on the balancing device should be made the central basic opening. Repeating the sequence of the actions described in part A measure the size of a beating of the tire and a wheel and the place of the maximum indication of the indicator. Then measure beating size separately of a wheel. If it exceeds specified, it should be replaced. If the size of a radial or axial beating exceeds 1,27 mm on a tire protector, start implementation of the recommendations provided in part C.
C. If the values of a beating given in part B exceed 1,27 mm, it is necessary to carry out adjustment of the tire and a wheel so that the place of the greatest beating of the tire coincided with the place of the smallest beating on a disk. Then repeatedly pump up the tire and establish a wheel on the balancing device. Take repeated measurements. In many cases such adjustment allows to reduce beating size in assembly up to the size less than 1,27 mm.
D. If beating size on the wheel removed from the car is in admissible limits while measurements on this wheel without removal show the size exceeding this limit, then it is necessary to look for the reason in a way of fastening of a wheel on a nave. Turn wheel assembly on two hairpins and again measure a beating. It is necessary to repeat this operation several times to find the optimum provision of a wheel on a nave.
E. If the beating does not manage to be reduced up to the admissible size, remove a wheel and measure a beating on wheel hairpins by the indicator of hour type. On one hairpin install the indicator on zero. Accurately remove an indicator tip from a hairpin and turn a rim so that the following hairpin appeared opposite to a tip. Measure a beating on all hairpins. The indicator will stop again on zero when it appears opposite to the first hairpin. If the beating exceeds 0,76 mm, the nave and an axial shaft have to be replaced. In case of shift of tires on wheels, and also replacements of tires or wheels wheel assemblies have to be balanced again. Except imbalance or a beating of tires and wheels without loading, fluctuations of rigidity of tires can be the cause of vibrations (a radial beating of tires under loading). However it is possible to measure the size of such beating only by means of the detector of defects of tires or the polishing device of a radial beating of the loaded tires.
The detector of defects of tires represents the drum which is slowly rotating the loaded tire installed on the car. Errors of rotation are the reason of fluctuation of rigidity of the tire and can be measured by such detector.
The polishing device of a radial beating of the loaded tires is more improved. On it the wheel removed from the car assembled is established. The tire slowly rotates under loading and measurements of fluctuations of rigidity are performed. Then "adjustment" of the tire by polishing and removal of a small layer of rubber from outer side of the drawing of a protector in places where rigidity size the greatest is carried out. This device is rather effective at measurements of beats, and also for exact polishing. The detector of defects of tires and the polishing device of a radial beating of the loaded tires – two devices allowing to measure and eliminate fluctuations of rigidity of tires, a beating of tires and wheels assembled. However not always the similar equipment happens well. Besides, both devices have the shortcomings therefore the most widespread way of measurement of not loaded wheel is the way given above with use of the indicator of hour type. Wear of tires is usually connected with existence of a spot of rigidity.
The method of replacement of wheel assemblies is also applied to diagnostics of vibrations. Make replacement of all originally standing assemblies by assemblies who are in advance known that they have no defects and if vibration disappears, start consecutive measurement of assemblies on standing originally. Do it until again vibration does not appear. Thus, it is possible to reveal the tire with the rigidity fluctuations exceeding admissible.
Rigidity fluctuations to a greater or lesser extent depend on the direction of rotation of the tire.