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Explanation of rolling friction and how it affects automobile tires - Succeed in Physical Science. Also refer to physics, coefficient, deformation, train, truck, car, wheels, rubber, forces, velocity, time, pressure, temperature, Ron Kurtus, School for Champions. Copyright © Restrictions Rolling Friction and Automobile Tiresby Ron Kurtus (revised 3 November 2005) When an automobile coasts along the road, the resistive force of rolling friction on tires slows down the motion. The rolling friction of the tire is slightly affected by the static friction of the rubber on the pavement. The adhesion effect of the rubber adds a little more to the rolling friction. But the major contribution to the rolling friction is the deformation of the tire while rolling. The coefficient of friction for the automobile tire can be determined experimentally, but it only applies to the specific configuration. Questions you may have include:
This lesson will answer those questions. There is a mini-quiz near the end of the lesson. Useful tools: Metric-English Conversion | Scientific Calculator.
Time = 4 min. 47 sec. Friction effectsRolling friction for a hard wheel on a hard surface is quite small and is a combination of contributions of static friction and friction from molecular adhesion. For example, the coefficient of rolling friction for a train wheel on a steel rail is only 0.001. That is less than the coefficient of sliding friction on ice. But an automobile tire is made of rubber and is filled with air. It deforms under the weight of the car, and that deformation contributes greatly to the rolling friction. The result is that the coefficient of rolling friction is about 15 times as great. A typical automobile tire has an average coefficient of rolling friction of μR = 0.015. Rolling friction equation for tiresYou can apply the standard friction equation for rolling wheels to try to determine the value of rolling friction. That equation is FR = μRW where:
This equation is not as straightforward as with sliding friction for hard surfaces, since μR varies with the radius, width, treads, amount of inflation and temperature of the tire, as well as the type of rubber and the value of W. The surface roughness of the pavement is also a factor. What this means is that μR can vary considerably depending on the experimental conditions. Measuring coefficient of rolling frictionOne way to measure the tire's coefficient of rolling friction is to roll the tire at a given velocity and then measure how long it takes to come to a stop. The equation used is μR= vg/t where:
Problem of tire by itselfThe problem is that since you are just rolling the tire by itself, the deformation effects from the weight of the car are not included. Thus the coefficient would not be accurate. Problem of other frictionOn the other hand, if you started a car rolling at velocity v, the tires would be deformed as in actual use. The problem now would be that there would be an added friction from the wheels turning on their axles that would also slow down the car. Thus the coefficient again would not be accurate. Separate measurements neededThe only way around that problem would be to take a separate measurement of the friction of the axles and their bearings and then subtract that from the previous measurement. My head hurts from all this! But I guess you have to go through all this trouble if you want to find an accurate coefficient of rolling friction for a tire. But then, if you would change the air pressure, you'd have to repeat the test. SummaryWhen an automobile coasts along the road, the resistive force of rolling friction on tires slows down the motion. The rolling friction of the tire is slightly affected by the static friction of the rubber on the pavement and the adhesion effect of the rubber. The major contribution to the rolling friction is the deformation of the tire while rolling. The coefficient of friction for the automobile tire can be determined experimentally, but it only applies to the specific configuration. Learn by being observant ResourcesThe following resources provide information on this subject: WebsitesRolling and Sliding Friction in a Car - Alaska Science Forum Experiment with toy cars: Measure Rolling Friction Coefficient How Tires Work - How Stuff Works Tyre Rolling Resistance Data - Extensive comparison of different types of tires Rolling Resistance Equations - Part of a government vehicle simulation project BooksThe following books are available from Amazon.com. BasicComplete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) $18.95 What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) $4.95 AdvancedFriction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) $89.95 Tribology: Friction and Wear of Engineering Materials by I. M. Hutchings; CRC Press (1992) $68.88 MiscellaneousMini-quiz to check your understanding1. Does the tire's coefficient of rolling friction change with the weight of the car? 2. Why is the temperature of the tire a factor in friction? 3. Is the rolling friction of a tire the only thing that slows down a car? If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again. What do you think?Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. We will try to get back to you as soon as possible. Share linkFeel free to establish a link from your website to pages in this site. 
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