Tire Friction - USC Viterbi School of Engineering



Engineering Safety: New Technology to Prevent Automotive CollisionsRichard ChanAbstract: This article illuminate the fundamental concept of Electrical Stability System existed in modern cars. Electrical Stability System is a great engineering accomplishment to helps prevent automotive accidents. It uses wheel-speed sensors, steering-angle sensors and yaw rate sensors to determine most current information of the vehicle to determine if any adjustment is needed to keep the car in the road. Advanced knowledge of friction, braking, aerodynamic, lift force and down force were explained to assist the understanding of ESC. Biography: The author is a junior student majoring electrical engineering at University of Southern California. He has a great passion for cars and intend to become a world-changing automobile engineer. Richard Chanchanrich@usc.eduEngineering Safety: New Technology to Prevent Automotive CollisionsHave you ever seen the orange triangle sign flashing rapidly on your dashboard while you were driving in the rain? You might have wondered what it means and why it quickly disappeared. You continue driving without giving much attention to what just happened. However, it cannot be ignored. What just happened is actually a great engineering achievement that helped prevent an accident. That flashing light warns the driver the electric stability control, or ESC, system was activated because it detects one or more wheels are losing traction that might lead the car into possible collision. Nevertheless, ESC system is fully automatic and the warning light simply informs the driver that the safety system is intervening to help the wheels regain their tractions and maintain the direction where the driver is going. ESC is a safety system that helps prevent the car from sliding by analyzing the information collected from wheel-speed sensors, steering-angle sensors and yaw rate sensors deployed all around the vehicle. This new technology had greatly improved the safety for all travelers. Statistics show that two-wheel drive vehicles equipped with traction control are twenty-five percent less likely to be involved in accidents, and all-wheel drive vehicles equipped the traction control reduces the numbers of accidents by a staggering fifty percent [1]. Without traction control, like all vehicles manufactured before 1971, drivers are have higher chance to lose control when their cars started sliding. Consequently, the cars could hit ANYTHING that were in their way. A sliding car is similar to walking into a frozen lake causing your shoes to lose traction. You will start sliding in one direction until your shoes regain their traction. Now imagine you are sliding towards a wall at high speed. Isn’t that dangerous? Luckily for us, all the cars manufactured in the last decade were equipped with the new and improved ESC system.To properly design effective safety systems, engineers are required to intelligently examine the effects of traction between tires and roads, aerodynamic forces on the car body, and brake forces to reduce the spinning rates of wheels. Tire FrictionWhen designing a stability system, the knowledge of frictional forces is important as it is closely connected to driving. Frictional forces exerted by wheels onto the ground enable cars to move in the direction same as the direction of frictional forces [Figure 1]. As you can see in the picture, the engine rotates the wheel by supplying clockwise torque to the wheel. Spinning the wheel clockwise produce a forward frictional force to the ground to resist the clockwise torque. Then, tires roll forward as the friction resists the torque supplied by the engine. The traction for wheels is determined by how much frictional forces tires exert on the ground. The sizes, materials and treads of tires determined their tire-road friction coefficient [4]. The tire-road friction coefficient is a measurement of how much friction a tire provides. The higher the coefficient means better friction. For example, the coefficient of a tire rotating on dry roads is about 0.7. When the same tire rolls to wet roads it is reduced to about 0.4 [3]. To measure the tire-road coefficient, engineers implant sensor inside the tire. Then, the traction control system monitor the coefficient of each tire as the car travel to different surfaces. If the coefficient suddenly drops, the traction control system keeps the number at an optimum level by applying brake forces or cutting engine power to the wheel that lost traction (spinning freely). This way, it help the tire regain its traction and the car continues to go in the direction desired.AerodynamicsThe study of aerodynamics is another factor that has a significant role in the engineering of cars. Due to the increases in the speed of vehicles, the aerodynamic designs of a car became extremely important as it has considerable effects on both fuel economy and traction of our cars. We will focus on the traction provided by aerodynamic design. What aerodynamic design has to do with the safety for our cars? At lower speed (less than 20mph) aerodynamics might be insignificant, but as the technology continues to improve, our car can now travel faster than ever. This moves engineers focus toward aerodynamics because its impacts on cars grows exponentially with the increase in speed. Life ForceWhile driving fast on the freeway, a great quantity of air will be passing through your car, or, more accurately, a great quantity of air will be split by your car. The air that is being split separates into two major layers. One layer goes over the top of your car and the other goes under. The curvature of the top of the car increase the speed of the air since it need to travel more distances as the air at the bottom will be traveling slower due to the shorter distances. The faster the air travels creates lower pressure and vice versa. Lift forces are formed because air tend to go from high pressure area to low pressure area. The lift forces push the car upward, which is the reason your car is not stable on the freeway [5]. Down ForceThere are two ways to remedy the upward lift force. The first way, and also the most popular way, is to create down forces, like all aerodynamic engineers did to their racecars [Figure 3]. They installed big rear wings that have shapes like panel tilting downward facing the direction of travel. The wings create huge amounts of down forces to keep the tires maintaining good traction with the ground when the car is traveling at higher speed. With the down force provided, the drivers will have much better control over their cars as it overcome lift forces. Air SplitAnother way to increase down forces is to eliminate, or minimize, the air flowing through the bottom of the car with air splitter installed at the front-lower portion of the car [Figure 2]. This technique ‘split’ the air to the side instead of going under. Although it will not split 100% of the air, it still effectively reduces the upward force exerted by air pressure under the car. Therefore, the car will gain more traction and able to ‘grab’ firmly to the ground.Electronic BrakingEngineers also precisely calculated the amount of brake forces each wheel needs in order to prevent the car from losing traction. Every vehicle is produced with a similar braking system, as it is the most common way to stop a car. All electrical systems have more control over the brakes then the driver has because the driver can only engage in four wheels braking while the electrical systems can apply brake to each wheel individually. Thus, with well-calculated braking forces distribution, the electrical stability system can effectively reduce the chances that our cars lose control by synchronizing wheel speeds. Manual BrakingStepping on the brake pedal will engage the braking system but what mechanical reaction is taking place? [Figure 4] Pressing brake pedal sends pressures to both master cylinders and the air distributor [6]. The air distributor amplifies pressure in the master cylinder as we depress the brake pedal. As shown in Figure 4, the pressures push brake fluids down the pipe toward each of the four wheels. However, for demonstration purpose, figure 4 only shows detailed graph for two front brake fluids. The brake fluids push brake pads on to the rotors. Rotors are the big disc shape metal that are inside the rims. As the brake pads are pushed against the rotor, friction forces are produced and it is pointing toward the other direction that the wheels are spinning. The frictional forces turn into heat and eventually dissipated by the rotor. The wheel spinning rate is then reduced. Electric Stability ControlElectric stability system control all these traction variables to maintain the direction of the vehicle. Two most common and dangerous way to lose traction is speeding through corners that will result in over-steering and under-steering. Over-steering happens in rear wheel drive cars when the rear of the car slides out toward the outside of the corner that often result in a spin [7]. Under-steering is the exact opposite of over steering. In this case, a front wheel drive car’s front wheels slide toward the outside of the corner and the car moves to the outer corner. Consequently, it might collide with other vehicles on the lanes. ESC is able to detect these situations and adjust the brake as necessary to maintain tractions. This ensures minimal over-steering or under-steering. How did ESC system know the car loses traction? ESC system used three types of sensors to determine if the car is under-steering or over-steering, and they are wheel-speed sensors, steering-angle sensors and rotational-speed sensor. Wheel-Speed SensorMost passenger cars utilizes two types of sensors: active wheel speed sensors or passive wheel-speed sensors. Active sensor requires an input voltage (a battery) as the passive sensor can work without an input voltage. Thus, passive sensor only works when the vehicle is traveling at higher than five mile per hour. On the other hand, active sensor, which requires extra power source, works even when car is not moving at all [8]. Active wheel-speed sensors, or also known as the Hall Effect sensors, and passive wheel-speed sensors are located near the surface of the tooth like metal disc that is attached to each wheel [Figure 5]. As each metal tooth pass through these sensors, they pick up a signal. Active sensor amplifies these signals before transmitting the information while passive sensor transmit the value it read, big or small, directly to the ESC without amplifying them. Then, electric stability system analyze the rate each sensors are collecting these signals and make appropriate adjustment to the brake force needed to keep the car balanced.Direction SensorA steering-angle sensors is located in the steering column (directly connect to the steering wheel) and there will be more than one steering-angle sensor in the steering column [9]. Each steering-angle sensor sends signals, which usually are numbers between 0 and 5, depending on the direction of the steering wheel, to the central computer and the data is analyze to determine the accurate position of steering wheel. Steering-angle sensors is used with the rotational-speed sensor, which also called the yaw rate sensor, to determine whether the vehicle is traveling in the direction desired. In [figure 6], the yaw rate sensor measures the vehicle’s rotational rate with respect to the vertical axis of the chassis [10]. It records the degree of tiling and sends the data to the central computer. ESC then take those number and apply brake forces to cancel out the tilting of the car so the car maintain its stability. Final ProcessingWith the information provided by these sensors, ESC knows exactly which tire is losing traction and how much adjustment needed to be made for the tire. All the data are actually first stored and collected in the central computer (ECU). ECU is like the brain of the car. It send out processed information to ESC module. ESC take those information and goes through complex algorithms before instructing ECU the proper amount of braking that it should applies to each wheel.Works Cited Page[1] S. Luntz. Electronic stability control reduces car crashes.?Australasian Science?pp. 9. 2008. Available: .[2] J. V. Kosgolla. Numerical simulation of sliding friction and wet traction force on a smooth tire sliding on a random rough pavement.?ProQuest Dissertations and Theses?pp. 111. 2012. Available: .[3] R Nave. Friction and Automobile Tires [Online]. Available: [4] G. Erdogan. New sensors and estimation systems for the measurement of tire-road friction coefficient and tire slip variables.?ProQuest Dissertations and Theses?pp. 110. 2009. Available: .[5] B. Parker, Isaac Newton School of Driving: Physics and Your Car. Baltimore: Johns Hopkins University Press, 2003. 259.[6] M. Marinescu and R. Vilau. GENERALIZED MODEL OF THE PRESSURE EVOLUTION WITHIN THE BRAKING SYSTEM OF A VEHICLE.?Annals of the Faculty of Engineering Hunedoara?11(3),?pp. 87-92. 2013. Available: .[7] C. Csere. (2012, Dec). The Physics Of: Oversteer [Online]. Available: [8] J. Gordon. ACTIVE WHEEL SPEED SENSORS.?Motor Age?127(7),?pp. 12-13,16. 2008. Available: .[9] Hans-J?rg Bullinger, “Magneto-electronic,” in Technology Guide: Principles - Applications – Trends. New York: Springer, 2009, pp. 88-91.[10] T. Chess. (2011, Jan 27). Understanding ‘Yaw Rate’ and the ‘Steering Angle Sensor’ [Online]. Available: 1, from 2 3 4 5 6 ................
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