Rolling resistance is the resistance of an object (e.g. a ball, tyre or wheel) when it rolls on the surface of another object. It is mainly caused by the plastic deformation of the rolling body and the rolling surface on which it is located or the contact surfaces of other objects under pressure [1][2][3], which means that some of the energy dissipated by the deformation of the rolling body or rolling surface is not converted into kinetic energy after the contact pressure has disappeared. Rolling resistance can be divided into two types: hysteresis losses and plastic deformation of the rolling body or rolling surface (e.g. sand), respectively. If the rolling body slides between the rolling surface and the rolling body, there is also a loss of energy. Some researchers believe that this should be taken into account as part of the rolling resistance, but others believe that it should be called “slip loss” or “slip resistance” [1]. In addition, only the sliding loss is related to friction and the rest is not. Some literature refers to rolling resistance as rolling friction, but “rolling friction” is not the exact name.
Rolling resistance is similar to sliding friction in that it can be expressed as the product of a coefficient and a positive force. The coefficient of rolling resistance will generally be smaller than the coefficient of sliding friction [2].
A carrier with wheels will slowly slow down and eventually stop when there is no power because of rolling resistance (also rolling resistance of the bearings). However, on a railway track, a railway train with steel tyres will travel further without power than a bus of the same weight, with rubber tyres, travelling on the road. The former has a lower rolling resistance coefficient than the latter. Factors that can affect rolling resistance include the degree of deformation of the wheel and the degree of deformation of the road surface. Other influencing factors are wheel diameter [3], load on the wheel, surface adhesion, slippage and the relative micro-slip of the contact surfaces. The losses due to the hysteresis phenomenon are also related to the material properties of the wheel and the road surface. For example, the rolling resistance of a tyre on a tarmac road is less than that of a steel train wheel on a railway track. Sand on a road surface also has a higher rolling resistance than concrete. Rolling resistance is independent of speed.
A characteristic of deformable materials is that the energy of deformation is greater than the energy of recovery. The rubber in a tyre has hysteresis. As the wheel rolls with the weight of the vehicle, the parts of the wheel deform and recover repeatedly, and the hysteresis energy is dissipated in the form of heat. Hysteresis is the main cause of energy loss in rolling resistance and is related to the viscoelasticity of rubber:- National Academy of Sciences[6]
The main principle can be seen in the diagram of a rolling cylinder. If two cylinders of the same size are pressed together by pressure, the contact plane will be flat. In the absence of surface friction, the contact stress is positive (perpendicular to the plane). Consider an object that enters the contact plane from the right, follows the contact path and finally leaves from the left. At first the vertical deformation increases, but the increase is smaller due to hysteresis, which creates additional pressure so that the two surfaces do not interfere with each other, and then the vertical deformation decreases, which is also affected by hysteresis. In this case, the hysteresis reduces the pressure required to separate the two cylinders.
The resulting pressure distribution is asymmetrical, with more on the right. The line of action of the combined pressure in the diagram does not pass through the centre of the cylinder, creating a moment to resist the rolling motion.
Some materials (e.g. rubber) have a higher hysteresis and bounce back more slowly and have a higher rolling resistance than materials with less hysteresis (e.g. stone or silica), while materials with less hysteresis bounce back more quickly and are easier to recover afterwards. Low rolling resistance tyres use silica instead of carbon black in their tread rubber to reduce low frequency hysteresis without affecting wheel traction [7]. Railways may also suffer from hysteresis because of their road structure [8].
Definition
Rolling resistance” is broadly defined as the force per unit weight of a vehicle that is required to keep the vehicle moving at low speed, where wind resistance is omitted and the vehicle’s engine and brakes are not activated. In other words, if no force is exerted to maintain the vehicle at a constant speed,china Track Roller Suppliers the vehicle will idle and slowly stop [9]. This broad definition includes track resistance, the energy dissipated by the road surface and the vehicle due to vibration, and the sliding of the wheels on the road (or track).
However, the broader term ‘rolling resistance’ includes the energy loss due to sliding caused by the moment. When sliding, the tangential speed of the wheel is faster than the speed of the vehicle. Since work is equal to the force multiplied by the speed, a faster wheel speed requires a correspondingly higher power.
The pure “rolling resistance” on a train is the resistance caused by deformation and a little sliding at the point of contact between the wheel and the rail [10]. In the case of rubber tyres, a similar energy loss occurs over the entire wheel, but is still referred to as ‘rolling resistance’. The broader term ‘rolling resistance’ also includes the resistance of bearings, losses due to vibrations in the road surface (and the ground beneath), losses due to vibrations in the vehicle itself, and sliding at the point of contact between the wheel and the road/rail. Textbooks for rail vehicles add up all these losses, but do not call them all together (in the broad sense) “rolling resistance”, as this entry does. Textbooks for rail vehicles also add wind resistance, which is referred to as basic train resistance. [11]
The broad “rolling resistance” can be several times the pure rolling resistance [12], so the definition of “rolling resistance” can vary considerably from one source to another. When a train is in motion, the engine needs to provide energy to overcome the broad term ‘rolling resistance’.
The rolling resistance of a tyre is defined as the force required to move the tyre a unit distance [13]. It is also known as rolling friction. As a vehicle moves forward, it is subjected to forces in the opposite direction of travel, and rolling resistance is one of these forces. The main cause of rolling resistance is the deformation of the tyre as it turns and comes into contact with the ground [14].
In the case of vehicles on motorways, energy is also consumed through the vibrations in the road surface caused by driving, the vibrations of the vehicle itself and the sliding of the tyres. However, apart from the friction of the wheel bearings and the power required for acceleration, almost all the other forces required are pure rolling resistance, probably because the pure rolling resistance of the tyre is several times greater than the other resistance.