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Amontons' Law

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Credit: Ros_K Photographie
Source: http://www.flickr.com/photos/ros-k/5748024681/
License: CC BY-NC 3.0

Don’t try this at home! You probably don’t do tricks like this on your bike. People who do such tricks know the importance of good bike maintenance. One of the most important steps in keeping a bike safe to ride is maintaining the proper tire pressure.

Why It Matters

  • Small changes in tire pressure can affect how a bike handles and how fast it can go.
  • The greater the pressure in a tire is, the harder the tire is and the less rolling friction there will be to slow it down. However, if tire pressure is too high, it increases the risk of a blowout.
  • Tire pressure that is too low can also be a problem. Low pressure may increase the risk of flat tires and damage to tire rims.
  • Tire pressure is affected by temperature. According to Amontons’ law, higher temperature results in higher air pressure. For example, a tire may start out at the right pressure on a cool morning but be over-inflated later in the day when it is warmer.
  • The bottom line is that tire pressure should be checked often. You can see how it’s done by watching this video: 

http://www.monkeysee.com/video_clips/801-bike-care-checking-your-tire-pressure

Can You Apply It?

Learn more about bike tire pressure and temperatureat the link below. Then answer the questions that follow.

  1. How often should you check the pressure in your bike tires?
  2. How can you find out the correct pressure for a bike tire? What units are generally used to measure bike tire pressure?
  3. Assume that a bike tire is inflated to 120 psi. For each 10-degree F change in temperature, how much does pressure change in the tire?
  4. What amount of temperature change do you need to watch out for when riding a bike? What change in tire pressure does this cause?
  5. Assume that the initial tire pressure (P_1) is 120 psi and the initial temperature (T_1) is 65°F (525 Kelvin, or absolute temperature). Also assume that the final temperature (T_2) is 90°F (570 Kelvin). Show how to use Amontons’ law to calculate the final pressure (P_2).

Image Attributions

  1. [1]^ Credit: Ros_K Photographie; Source: http://www.flickr.com/photos/ros-k/5748024681/; License: CC BY-NC 3.0

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