Science of Cycling: Gear Ratios + Activity

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Drives & Gears Page: 2 of 3

Penny Farthing & French Twist

Paul explained that the method for counting teeth was only one way of demonstrating gear ratios. In England, the gear ratios are converted into the diameter of the large front wheel of a high-wheeled bicycle, called the "Penny Farthing." Paul's earlier example used a 2-to-1 ratio. To convert this, the diameter of the rear wheel would be multiplied by two. A gear with a 2-to-1 ratio and a 27-inch-diameter rear wheel would be considered a 54" gear.

In England, the Penny Farthing bicycle is still used today to measure the gear ratios of safety bicycles.

In France they use the metric system. They take the circumference of the wheel in meters and multiply it by the gear ratio. Again using Paul's example, given a 2-to-1 ratio and a tire with a circumference of 1.5 meters, the result would be 3 meters. (Unlike the English system, this method tells you how far you have traveled, in this case 3 meters.)

Chain Drive Activity

The chain drive is what connects the pedals to the rear wheel. It allows the power you apply to the pedals to be transferred to the rear wheel, moving the bicycle forward.

What You Need:
You can explore the concept of the chain drive quite simply. You'll need the following items:

Thread spools: a pair of the same size, and one of a larger or smaller size
A flat wooden board
Nails
Rubber bands

To Do:
Mount the two spools onto the board with the nails, far enough apart that the rubber band will have to stretch to connect them, and loosely enough so that the spools can turn easily. Then connect the two spools with the rubber band.

A variety of spool sizes makes this activity more interesting.

Notice the red triangles which are drawn on the tops of the spools. The marks make it easier to follow the movement of each of the "gears."

Explore how turning one spool now causes the other spool to turn. Do they turn at the same rate? In the same direction?

Other possible explorations:

1) Mark a point on both spools and rotate them. Try using one small and one large spool. How does the rotation of one relate to the other?

2) Put one twist into the rubber band, so that it forms a figure-8 between the spools. Does this affect the rate of rotation? Does it affect the direction of rotation?

3) Look at a bicycle's chain and gears. How is the spool contraption similar to a bicycle? How is it different?

Drives & Gears Page: 2 of 3
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