PARTNER'S NAME

EXPERIMENT 10.1

Equipment:
• 1 multi-speed bicycle
• 1 tape measure or ruler

This experiment is designed to test what we learned about gear ratios, using a bicycle. Place the bicycle upside down on a table so that the rear wheel and pedals are free to spin.

Using the tape measure or ruler, measure the radius of the gear on wheel. This is the distance from the center of the gear to where the chain sits on the teeth of the gear. Make sure you’re measuring the gear that has the chain on it. You may not be able to easily get the ruler close enough to make an accurate measurement, but do the best you can. Write this radius down on Data Table 1. Now measure the radius of the gear on the pedals. Write this radius on Data Table 1 as well.

We’re ready to start pedaling. One person will turn the pedal while a second person counts how many times the back wheel rotates. Use a reflector on the wheel or the air valve sticking out of the wheel rim as a reference to easily mark how many revolutions the wheel makes. Many bicycles are designed so that the back wheel will continue rotating even after you stop pedaling. To prevent this from happening, the second person should keep their hand resting on the back wheel so that the wheel stops as soon as the pedals stop. Have the first person turn the pedal one full time around. The second person will count how many times the wheel turned, and write that on Data Table 1. Do your best in estimating fractions of a revolution. Counting the number of spokes can be a useful guide. Next, have the first person turn the pedals 5 times around while the second person counts the wheel rotations. Write this on Data Table 1 as well.

For our second trial, change gears so that the chain is on different gears with different radii (plural of radius). Measure the radii of the new gears being used and repeat the wheel revolution measurements. Change the gears once again and complete a third trial. Write all your data down on Data Table 1.

Data Table 1

Time for some calculations! One way to find the gear ratio is from the ratio of radii, so we can simply find the ratio of the gear radii.

Gear Ratio = Wheel gear radius ÷ Pedal gear radius

Write the gear ratio for each trial down in Data Table 2. To find the ratio of pedal revolutions, divide the number of wheel revolutions by the number of pedal revolutions. To find the average ratio of revolutions, add the ratio of revolutions for 1 pedal revolution to the ratio of revolutions for 5 pedal revolutions, then divide this sum by 2. Write this average down in Data Table 2.

Data Table 2

Gear ratio is the reciprocal of the ratio of revolutions. Calculate the reciprocal of the average ratio of revolutions by taking one divided by the average. According to what we know about gear ratios, this reciprocal should be about equal to the gear ratio. If it’s off by a bit, think about sources of error in the experiment that could have caused the numbers to be a little different.