Taşıt lastiklerinin dinamik karakteristiklerinin ölçülmesi
Experimental analysis of dynamic characteristics of pneumatic tires
Yazar: ÖZGEN AKALIN
Danışman: DOÇ. DR. AHMET GÜNEY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü
Derece: Yüksek Lisans, 1992
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Diyagonal hareket = Diagonal motion ; Lastik basıncı = Tire pressure ; Motorlu taşıtlar = Motor vehicles ; Otomobil lastiği = Automobile tire ; Tekerlek yükü = Wheel load

Özet:
Bu çalışmada taşıt lastiklerinin dinamik karakteristik lerini etkileyen faktörleri saptamak, sürekli ve yavaş değişen diyagonal hareket açılarında, dönen tamburda de neysel olarak bu faktörlerin etkinliğini araştırmak amaçlandı. Deneylerde Lassa GT 70 175/70 SR 13 tipi çelik kuşaklı radyal lastiğin Carl-Schenk tekerlek test cihazında di yagonal hareket karakterleri grafikler halinde çıkarıl dı. Tekerleğin dinamik performansını etkileyen faktörler diğer test koşulları sabit tutularak ayrı ayrı incelendi, Sonuç olarak tekerlek yükü ve lastik basıncının diyago nal hareket karakteristikleri üzerinde önemli etkisi ol duğu, değişen diyagonal hareket açılarında yan kuvvet ve geri çevirme momentinde tekerlek yükü ve frekansa bağlı olan bir geri kalma oluştuğu saptandı. Çevresel kuvvet lerin diyagonal hareketteki etkileri gözlendi. Düşük frekanslarda hızın önemli bir etken olmadığı görüldü.

Abstract:
The changes that have taken place over the years in the roads and in the power and speed of the average car have created a need to crystalize our understanding of stability. Finally, the increase in hazards attending the elevated speeds makes it more important than ever. As one would expect, stability is intimately related to the tire. The tire, when steered, develops the forces that guide the vehicle. When not steered, the tire forces hold the car steady on its path. If a steered tire is run long enough to attain a condition of equilibrium, the value of this force is determined by the size of the "slip angle". This is the angle between the plane of the tire and direction of travel along the road. For normal maneuvers on the road, only small slip angles are encountered. In this limited range of angles, the lateral force and aligning moment are almost exactly proportional to slip angle. The ratio of the cornering force to slip angle is termed the cornering power. This ratio, the cornering power, has generally been taken to indicate the degree of stability of the tire. The magnitude of the slip angle depends on many factors of which the most important are the magnitude of the side force and vertical load on the wheel, the inflation pressure and the construction of the tire itself. The speed at which the wheel rolls, however, has little effect on the value of the slip angle. However, the side force does not act in the vertical plane containing the axis of the whell but in a parallel plane lying slightly behind the wheel axis. That distance is called the pneumatic trail and the couple Fyng (side force x pneumatic trail) called the aligning moment. Tire functions play a predominant role in the overall dynamics of the vehicle. Linearized tire characteristics are used to study stability and responses to small steering inputs. The non-linear shape of tire characteristics may cause considerable changes in handling properties at different levels of lateral acceleration. A vehicle negotiating a stable turn may become unstable as a result of the action of a sufficiently large disturbance impulse. VITotal performance of a tire can be divided into two major categories as structural performance and mechanical performance. The structural performance of a tire is the primary determinant of its reliabilitiy and that the mechanical performance of the tire relates primarily to the concept of function. The performance vector of a tire is dependent upon three calsses of factors. Tire design implies a host of geometric, material, and fabrication variables. The operational history of a tire is an important determinant of the performance vector, since material and geometric changes occuring with use influence both the reliability of the tire and its mechanical characteristics. Finally operating-state variables will be influential in controlling the performance of a pneumatic tire. The motions of a tire-vehicle system accur both within the plane of symmetry (e.g., ride, braking, etc.) and outside the plane of symmetry (e.g., the response to steering, wind gusts, etc.). When these motion consist of small perturbations about an equilibrium condition established in straight-line motion over a perfectly smooth surface, the tire can be represented as a linear force-producing element. Studies have shown that the directional response of an automobile, at levels of lateral acceleration less than 0.3 g, is dependent only on the slope of tire side force versus and the static value of vertical load. As the severity of the maneuver is increased, the tire is required to generate forces that are not linearly related to the pertinent kinematic variables. Frictional conditions prevailing at the tire- road interface become a determining factor resulting in a strong interaction between the side and fore and aft compenents of shear force produced by combined side and longitudinal slip. The prediction of vehicle behavior at or near the friction limit (for a range of speed and surface conditions) therefore demands an extensive amount of tire-mechanics data covering a wide range of kinematic variables and interfacial friction conditions. It should be noted that all the phenomena discussed above are steady-state processes, following conditions exist: (1) constant vertical load, (2) a fixed value of slip angle, and (3) interface conditions that are constant with time. These steady-state conditions normally do not prevail. In the real world, the tire- wheel assemly undergoes a dynamic variation in vertical loading and vertical deflection, a result caused by irregularities in the road profile. The functional performance of the pneumatic tire can be broken down into four categories. These are dynamic variables, interface contamination, road properties and tire properties. The quantities categorized as dynamical variables are system variables that vary continuously as vxithe system moves in time. Speed, normal load, slip vector, vertical tilt angle, traction and braking force are dynamical variables. Interface contamination indicates the condition of the road. The contaminants faund on the road are generally oil, mud, ice, snow or water. Road properties contain macro and micro geometry, temparature and material properties of the road. Tire geometry, inflation pressure, carcass construction and material properties are classified as tire properties. The only factor in this category which is a test variable is inflation pressure. The influence of water contamination on mechanical tire performance depends on just a single contaminant property, the quantity, or depth, of undisturbed water. However, the study of water contamination effects is complicated tremendously by the strong interction of other dynamical and non dynamical interface factors, most prominently speed and tire and road surface geometry. The tire-road interface variables categorizied as road properties present the most difficulty in definition and control. Bulk friction is a consequence of the viscoelastic character of rubber. It arises as a result of the damping, or hysteresis, losses associated with the deformation of such materials. Thus it is also refferred to as the hysteresis, of deformation, friction component. Bulk friction is a primary function of interface microgeometry, since the nature of the rubber and even shapes of the proximate surface asperities. Temparature is also an important factor, because the damping loss rates of viscoelastic material are highly temparature dependent. Adhesion is also strongly affected by interface microgeometry, which controls the from of the actual contact area, and by temparature, which modifies the atomic state of the interactants. The distance traveled and the way the tire is steered during this distance, effect the cornering force and aligning moment. One of the many approaches for determining this is through the study of the frequency response. In this method, the tire is steered through an angle that changes sinussoidally in time. The force developed by the tire also varies sinusoidally and has the same frequency as the motion of the whell. However, the size of the sinusoidal force and its time lag behind steering depends in general on the frequency at which the whell is moved. This approach can be checked in two other ways. In one method, the tire is quicly steered from one previously fixed position to another fixed position and the reaction is noted. In the other, the tire is steered so that the slip angle increases in proportion to the passage of time. These three experimental procedures lead to a single description of vxxithe dynamic behavior for the cornering force. The consecuences with regard to the side forces generated by time-varying side-slipping or steering are known to be the following: (a) A lag or delay exists between side force and steer angle, which delay increases with the rate of steering (b) This delay decreases with increasing forward velocity of the tire. Time varying normal load has also effects on dynamic characteristics of pneumatic tires. It may be observed that sinusoidal variation of normal load produecs a non sinusoidal variaton of side force. The average side force produced by varying normal load is always less than the side force corresponding to the average load. As the normal load increases from a minimum value, the side force lags behind the applied vertical load. The relatively large lateral deflection of the tire carcass under these condition may cause large sliding velocities in the contact region when the load is increasing. These large sliding velocities would result in a lower friction coefficient between tire and drum, thereby reducing the average side force produced by the tire. The plane of the whell is not always vertical but in practice vehicle wheels frequently are inclined. This tilt of the wheel is called camber and the angle is called the camber angle. Cambering the wheel has an effect on the steering force the tire will develop under given conditions of slip angle, wheel load etc., and also effects the aligning moment. The side force will be greater with positive camber and smaller with negative camber. Both traction and braking moments produce reduction in the side force at a constant slip angle, The contact surface moves laterally as a result of the lateral motion. Thus, braking force reduces the aligning moment and traction force has the opposite effect. The purpose of this study is to investigate this collected factors, effect the dynamical performance of pneumatic tires, in labratory conditions. 175/70 SR 13 stell-braced radial-ply tire has been tested on rolling drum and lateral force and moment characteristics are measured at very low excitation frequencies. Test Equipment: Laboratory equipment for measuring the mechanical characteristics of pneumatic tires can be placed in one of two categories. The first, a rolling drum, is the device most commonly employed. The second category is the slow-speed, flat-bed (or flat-plank) device. Experiments have been carried out with Carl-Schenk wheel xxExperiments have been carried out with Carl-Schenk wheel testing equipment-rolling drum. Drum diameter 2 m Drum width 0.9 m Drum speed 300 km/h Force and moment magnitudes are obtained as electrical signals by five component measuring hub. Measuring hub ranges are as follows: Wheel load Traction force Side force Aligning moment Overturning moment An axle and braking system have been constructed in order to observe the effects of braking forces on the dynamic characteristics of pneumatic tires. A hydroulic disc brake system and servo mechanism-vacuum pump have been used. Brake linings are cooled by an electrical fan. Camber angle can be fixed by a screw and nut mechanism. Slip angle values are obtained by using a linear potensiometer maunted in the axis of pivoting. The tire is steered up to maximum steer angles of + 10 deg by an electrical actuator and frequency can be adjusted from the control panel. The steer angle can be fixed during the test sequence. An electric tachometer is used to record the drum speed. The recording equipment consists of a X-Y plotter, used to obtain steady-state plots of side force, wheel load, traction force, aligning moment, overturning moment and steer angle. Each factor that effects the dynamical performance of the pneumatic tire is investigated seperately when the other test conditions are held constant. The effects of normal load, inflation pressure and rolling speed on the lateral force and moment characteristics are tested. The effects of braking force are examined in two different ways. In first method slip angles are kept constant and characteristics are recorded depending on the braking force. In the other method the braking force is held constant and changes are observed in slow varying slip angles. The tyre has been tested at a very low excitation frequency such as 0.01 Hz. Thus, the results obtained are very close to the statistical measurements. In addition, the tire is tested also in reasonably high excitation frequencies. In frequency response, the effects of time varying slip angle are examined. The results showed that lateral force and aligningmoment do not rise proportionaly with the slip angle, but appears to a lag behind. Frequency response measurements showed that the delays depend on the frequency. In addition, we have noted that the apperent delays increase with the normal load. According to these test results, normal load has very large effects on lateral force and aligning moment. The increase in the normal load causes considerable rise in the lateral force and aligning moment. The results obtained at different speed levels showed that despite some minor effects, the speed effect was small. Although registering some different vibrations, there was no significant change in the lateral force and aligning moment at different speeds. The measurements at different inflation pressures showed that inflation pressure has important effects on lateral force and aligning moment. Especially, in increased inflation pressures, we observed very small aligning moment values. Finally, we noted that braking force causes very large effects on force and moment characteristics. The lateral force decreases with the increasing braking force. In this study, first four chapter gives theoretical knowledge about the force and moment generation properties and parameters that effect the characteristics of pneumatic tires. In the following chapter, the testing equipment has been introduced. The test results have been illustrated. Finally the results obtained have been discussed. It can be seen that the results, are agree with previous studies. xx