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How Do Under-Inflated Tires Affect the Difficulty of Riding a Bike?

TWC PI auto racing nitrogen
Difficulty
Time Required Average (6-10 days)
Prerequisites None
Material Availability This science project requires a bike with training wheels and a special spring scale to measure force. See the Materials and Equipment list for details.
Cost Low ($20 - $50)
Safety Only do this science project in a safe area where there are no cars, like a playground or a sidewalk. Do not do this science project in a street or parking lot. The volunteer riding the bike must wear a bike helmet.

Abstract

If you ride a bike, you probably know that you have to occasionally pump up the tires to keep them fully inflated. Over a long period of time, the tires slowly leak air, so their pressure will decrease. Have you ever noticed that it is actually harder to ride a bike when the tire pressure is too low? This is because the tires are a big factor in the rolling resistance of the bike. In this sports science project, you will measure how tire pressure affects the force required to move a bike. How important is it to keep your tires fully inflated?

Objective

Use a spring scale to measure how tire pressure affects the force required to pull a bike in a straight line.

Credits

Ben Finio, PhD, Science Buddies

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Last edit date: 2014-01-03

Introduction

Riding a bike is good exercise, and can be hard work! When you ride a bike, there are many different forces you must overcome in order to move the bike forward. There is always friction between moving parts of the bike, like the gears, chain, and wheel hubs. If you are pedaling up a hill, you must work against your own weight, plus the weight of the bike. Air resistance—the collective force that results from pushing millions and millions of air molecules out of the way when you move forward—also works to slow you down. Figure 1, below, shows where all these different forces act on a bike.

forces acting on a bike
Figure 1. The different forces a rider must overcome to move a bike forward. (mountain bike image credit: Wikimedia Commons, user Ralf Roletschek)

What about the fourth force shown in Figure 1; what is rolling resistance? At first, you might guess that rolling resistance is just another type of friction, between tires and the ground, but this is actually not true. Bike tires are made of rubber, which is a soft material. When the soft tires come in contact with the hard ground, they deform slightly (Figure 2, below). Some energy is lost when the tire deforms and then bounces back to its original shape. Rolling resistance is the result of this energy loss as a tire continuously rolls along.

viscoelastic deformation of a tire
Figure 2. A tire deforms slightly when it comes into contact with the ground.

Now, can you imagine how tire pressure could affect rolling resistance? In this sports science project, you will do your own experiment to find out! You will use a spring scale, which measures force in newtons (N), to pull a bike along. You will change the tire pressure and measure how this changes the force required to pull the bike. Do you think it will require a larger force to pull a bike with under-inflated tires, or will under-inflated tires not make much difference?

Terms and Concepts

  • Force
  • Friction
  • Weight
  • Air resistance
  • Energy
  • Rolling resistance
  • Tire pressure
  • Spring scale
  • Newtons (N)

Questions

  • What forces must you overcome in order to move forward when you pedal a bike?
  • How do these forces change in different conditions?
    • For example, does air resistance change depending on how fast you are going?
    • Can weight make it harder or easier to ride a bike, depending on whether you are going uphill or downhill?
  • What causes rolling resistance?
  • If a tire deforms more, is more energy lost?
  • Would a fully inflated tire deform more or less than an under-inflated tire?

Bibliography

Learn more about the science of auto racing with this easy-to-read guide:

Materials and Equipment

  • Bike with training wheels
    • Note: Due to the very low speeds when testing this science project, it is very difficult to balance a bike that does not have training wheels. It may be possible to do the science project if you have a third volunteer walking alongside the bike to keep it upright, but it is highly recommended that you use a bike with training wheels.
    • The bike should be relatively clean and well-oiled before you start the experiment. See the Procedure for details.
  • Volunteer to ride the bike
    • Note: Rolling resistance increases as more weight is placed on the bike. The rolling resistance on a bike without a rider is very low, so it is very difficult to measure. So, for this science project, you need a volunteer to sit on the bike while you tow it along with the spring scale. This makes rolling resistance easier to measure.
  • 50 newton spring scale; available from Amazon.com
  • Large zip ties (3); available at a hardware store or from Amazon.com
  • Bike pump with pressure gauge; available at a sporting goods store or from Amazon.com
  • Open, flat area with a hard surface (asphalt or concrete) where you can pull the bike. A playground or long driveway will work well. Do not do this science project in a street or parking lot where there may be cars. It is very important to do the science project on a flat surface and not on a hill.
  • Bicycle helmet for your volunteer
  • Lab notebook

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Experimental Procedure

  1. Make a data table like Table 1, below, in your lab notebook.
    1. Your goal is to measure rolling resistance at 25%, 50%, 75%, and 100% of the recommended tire pressure for your bike. You can find this information written on the side walls of the tires. For example, if your bike tires have a maximum recommended pressure of 40 pounds per square inch (psi), you would test 10, 20, 30, and 40 psi. Once you have this information, calculate and fill in the second column, "Tire Pressure (psi)," accordingly.
Tire Pressure (%) Tire Pressure (psi) Force (newtons)
Trial 1 Trial 2 Trial 3 Average
25      
50      
75      
100      
Table 1. Data table in which to record your results. You will have to fill in the second column based on the recommended pressure for your tires.
  1. Since you will be doing this experiment at very low speeds, the force from air resistance is negligible. Since you will be doing it on flat ground, the force required to pull the bike and rider's weight uphill is zero. Therefore, as long as the bike is clean and well-maintained, you can assume that the force you will be measuring is mostly due to rolling resistance. Make sure your bike is clean and lubricated before you start the experiment—ask an adult if you need help.
  2. Take your bike, bike pump, zip ties, spring scale, volunteer, volunteer's bike helmet, and lab notebook to the area where you will do your testing.
  3. Use the zip ties to hook the spring scale onto the handlebars of the bike, so you can tow the bike along by pulling the spring scale. Figure 3, below, shows one way you can do this. You may need to adjust where you attach the zip ties, depending on the type of bike you have.
spring scale attached to bike with zip ties
Figure 3. How to attach the spring scale to the bike handlebars using zip ties.
  1. Using the pressure gauge on your pump to monitor the pressure, let air out of both of your bike's tires until they are each at 25% pressure. For example, if the recommended pressure for the tires is 40 psi, let air out until they are at 10 psi. This information should already have been calculated and be in your data table like Table 1, above.
  2. Have your volunteer put on his or her helmet and sit on the bike. He or she should not touch the brakes or the pedals. The volunteer should only touch the handlebars lightly enough to steer the bike and make sure it goes straight.
  3. Grip the handle at the top end of the spring scale. Hold the spring scale horizontally (parallel to the ground) with the numbers facing up so you can read them. Make sure the hook at the bottom of the spring scale is attached to both of the zip ties, as shown in Figure 3 above.
  4. Slowly and steadily begin to walk forward and tow the bike along behind you. Do your best to walk at a constant pace. If you speed up or slow down, this will affect the reading on the spring scale.
  5. As you walk, look behind you at the spring scale and take an average reading. The spring scale might bounce around slightly, even if you do your best to walk at a steady pace. For example, if you see the spring scale bouncing between 10 N and 14 N, you would record an average value of 12 N. Record this value in your data table.
  6. Repeat steps 6–9 two more times for this tire pressure, for a total of three trials. Be sure to record all your results in your data table.
  7. Repeat steps 6–10 for each of the remaining tire pressures (50%, 75%, and 100%). Use your bike pump and pressure gauge to increase the pressure in both tires before each new set of trials. Be sure to record all your results in your data table.
  8. For each tire pressure, calculate an average force and enter this value in your data table. For example, if you measured 10 N, 11 N, and 13 N, the average would be (10+11+13)/3 = 11.33 N.
  9. Make a graph with average force (in newtons) on the y-axis (vertical axis) and tire pressure (in psi) on the x-axis (horizontal axis).
  10. Analyze your results. What is the relationship between tire pressure and rolling resistance? How does this compare to your prediction?

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Variations

  • How does weight affect rolling resistance? Repeat the experiment, but have different riders of different weights ride the bike. Does this change the force that you measure?
  • How does speed affect rolling resistance? If you walk faster or run, does the force required to pull the bike increase? Do you think air resistance plays a role at higher speeds?
  • Does it take more force to pull a dirty or rusty bike than a clean, well-maintained bike?
  • How much more force does it take to pull a bike up a hill, as opposed to flat ground?
  • Measure the rolling resistance of different tires or different types of bikes; for example, a road bike versus a mountain bike.

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