To ensure that the car runs safely on the road, the tire tread must have sufficient friction (adhesion), usually expressed as an adhesion factor. The adhesion coefficient Υ is the ratio of the maximum value Σmax of the tangential stress of the road surface to the tire and the normal stress PG of the road surface: Υ=ΣmaxPG(1) The friction between the tire and the road surface From the aspect of tribology, the tire tread and the road surface are one Two objects that are in contact with each other are also called a pair of friction pairs. In this pair of friction pairs, due to the manufacturing error of the tread and the requirements of the material and construction of the road surface, there are a large number of tiny irregularities on the contact surfaces thereof. These raised parts are called micro-convex bodies; the recessed parts are called valleys (peaks and valleys). Due to the uneven height of the bumps on the road surface. When the vehicle is in a normal load, the tread and the road are in contact with only a few small areas in the contact area (called contact spots). The sum of these spots is called the profile contact area AG, and the vertical (normal) load on the contact area of ​​the profile is called the profile normal contact stress PG. The friction between the tread and the road surface is due to the friction between the tread and the road surface, and the tire will inevitably generate friction when it moves. Friction can be understood as the resistance of the road surface to the relative movement of the tread, which is opposite to the direction of movement of the tire. The friction between the tread and the road surface is a solid external friction. The size of the external friction is determined by the displacement of the tire along the tangential direction. Therefore, to calculate the adhesion coefficient, it is necessary to first determine the frictional force T between the tread and the road surface. Analysis shows that the friction between the tread and the road surface consists of two parts: one is determined by the roughness of the road surface, the type of road surface, pollution (oil pollution), humidity, tire materials and patterns, etc. These are called adhesion components TM; the other part is This is the resistance when the road's convex body is pressed into the rubber on the tread surface to cause deformation during tire movement, and it is called deformation component TB. It should be noted that these two components are interrelated. When calculating the total friction, as an approximate calculation, you can use both arithmetic sums: T = TM + TB (2) 212 factors affect the friction in order to improve the friction between the tire and the road (adhesion) to prevent it from slipping on the road. Treads are designed with various patterns. The actual test shows that the tread pattern plays an important role in the working performance of the tire. The tread pattern is mainly used to remove the moisture in the rain and snow during the actual contact area between the tread and the road surface, and increase the adhesion component in the total friction force. Since the tread pattern protrudes from the tread, it will cause the actual contact stress to be significantly larger than the contact stress when the tread pattern is not counted. In addition, the tire structure, tread pattern shape, vehicle load, wheel driving torque and braking torque, tire pressure, will affect the size and distribution of contact stress on the tread. Force analysis of tires and road surface 1 The force analysis of the tire is a schematic diagram of the force on the driving wheel when the vehicle is traveling at speed v. Because the tire itself is also filled with compressed air, it has a certain internal pressure. In this way, it will deform under the influence of internal, external forces and torque. For a specific car, its vehicle load, tire structure parameters, and tire pressure are all fixed values. At this time, the amount of adhesion depends on the friction between the tread and the road surface, and the friction depends on the roughness of the road surface. The roughness depends on the road surface appearance. Pavement Topography Pavement topography is the geometric shape of the pavement that remains after the pavement construction. It is usually characterized by the number of micro-convex bodies per unit area, and the geometry and height distribution of the micro-convex bodies. Cross section outlines for cement concrete and asphalt pavements. From these profiles, it can be seen that the shape of the road surface is similar to the spherical section. These spherical segments are located on a certain reference line and are distributed along the height direction. It is a geometrical parameter assessment diagram of the longitudinal profile of the road surface convexity. H is the height of the microbump; L is the sampling length of the road surface. Figure 2 The surface profile of cement concrete and asphalt pavement Rmax is the maximum contour peak-to-valley distance of the microbump; Rp is the maximum height of the contour; Ra is the contour of the microbump The arithmetic mean of the valley distance; tp is the length of the microbump contour support line (thick black line on the datum line) Explanation: The arithmetic average, which is the number of micro convexes and divided by the number of micro convexities Evaluation of Geometric Parameters of Longitudinal Sections of Microbumps According to the literature, the road surface specifications in China have proposed roughness standards for roads at all levels—characterized by the TD value of the road surface. The standard stipulates that expressways and first-class highways: asphalt pavement, TD≥0155mm; cement concrete pavement, TD≥018mm; other grades of cement concrete pavement, TD≥016mm. Because the micro-convex approximation is similar to the spherical truncated body, the actual TD value only reflects the arithmetic average Ra of the peak-to-valley distance of the micro-protrusion contour, and does not reflect the basic morphology of the entire micro-convex body. We used to illustrate this issue. A set of schematic diagrams of microbumps with equal roughness TD values. As can be seen from the figure, although the three kinds of micro convex bodies (a), (b), and (c) have the same height, the parameters such as the radius of curvature r at the top of the peak and the size tp of the support surface are not equal, making micro The contact stress between the convex body and the tread and the depth of the rubber layer pressed into the tread can not be equal. This will cause the friction between the road surface and the tread to be very different. If these conditions are not taken into account, there will be a large difference between the theoretically calculated adhesion coefficient and the measured value. For this reason, the geometrical parameters Rmax, r, v, b, which reflect the profile of the road surface's micro-convexity, have been introduced as evaluation parameters for the roughness of the road surface. The v, b, and values ​​of these evaluation parameters can be calculated according to equation (9), and the other parameters of the convexity involved in the formula are calculated. Fig. 4 Outline of the longitudinal profile of the pavement asperity v=2tmRpRa-1b=tmRmaxRpv=Rmaxrb1v (9) where: tm is the arithmetic mean of the length tp of the microbump longitudinal section support line; r is the average radius of curvature of the top of the microbump. (r = rHrV, rH, rV are the lateral and longitudinal mean curvature radii at the top of the asperity). The calculation of the adhesion coefficient can be seen from equation (2) that the adhesion force (friction force) is the sum of the frictional adhesion component and the deformation component. Among them, the frictional adhesion component depends on the interaction between the tread and the pavement material molecules in the actual contact zone; the frictional deformation component depends on the hysteresis produced when the road surface embossment is pressed into the tread surface to deform it. loss. When the car is driving on a highway, if the driver finds that the tire slipping occurs, in order to ensure the safety of the vehicle, the method of increasing the tire pressure or the vehicle load may be used to increase the adhesion coefficient between the tire and the road surface, but when adjusting the two indexes, It must control its adjustment. Otherwise, it will not only increase the adhesion coefficient between the tire and the road surface, but also make it fall, which will make it easier for the tire to slip on the road surface. In addition, if the driver needs to brake, especially a vehicle that does not have an ABS device, the brake should not be stepped to death, so as to avoid the accident that the adhesion coefficient decreases due to the pure sliding of the tire when the tire is locked. Steel Welded Gas Cylinder,Liquid Welded Steel Cylinder,Lpg Gas Cylinder For Haiti Market,Gas Cylinders For Restaurants Jiangsu Minnuo Group Co., Ltd , https://www.minnuo-cylinder.com