Gears and Gear Ratios

Gears are wheels with teeth that mesh together. This can happen either directly (the wheels touch each other) or indirectly (a chain might be used to make one gear turn the other). Sometimes a screw (worm gear) or a shaft with teeth (a rack) acts in place of one of the wheels. In your car, a gear is a combination of gear wheels that are designed to produce certain speeds. The gear ratio is dependent on the size of the two gear wheels working together. The larger wheel will always rotate more slowly than the smaller wheel (the smaller wheel has to turn faster to keep up with the larger wheel). This means that the greater the size difference between the two gear wheels, the greater the variance in speed and force between the two wheels. For example, if the smaller gear turns three times for every one turn of the larger wheel, the gear ratio would be 3 to 1 (3:1). This fundamental principle is at work in the transmission in our autos. The mechanical elements and complexity of manual transmissions and the even more complex automatic transmissions are just a more sophisticated version of a bicycle's derailleur gears. The purpose of all three types of transmissions is to use the relatively stable power output from our engines, or bodies, to the varying power needed to begin and sustain motion. If you have ever tried to pedal a multi-speed bike uphill in the "wrong" gear, you have felt this principle at work. Our legs tire quickly because too much is demanded from them. We can either stop and walk, or stop and fall over. A better answer is to shift to an easier (lower) gear. "Downshifting" the gear allows the power from our bodies to be leveraged, or multiplied; the pedals will turn faster with less effort. What becomes confusing at this point is that shifting downward INCREASES the gear ratio. It's easier to go uphill at a 10 to 1 (10:1) ratio than it is with a five to one (5:1) ratio. With a 2 to 1 (2:1) gear ratio, the engine turns twice to turn the wheels once. Gear ratios are determined by the number of teeth on them. A gear with 20 teeth meshing with a gear with 60 teeth would yield a 3:1 ratio. By moving the shift lever, we have increased the number of pedal revolutions needed to turn the rear wheel, but decreased the amount of effort required to turn the wheel. Both the amount of power and the RPM's (revolutions per minute) needed to turn our bicycle or car wheels are controlled by gears or sprockets of varying sizes.