How Gear Ratios Work

The key to gear ratios lies in two physical pieces-an input gear and an output gear. A gear is typically a round, flat piece of steel, wood or plastic with teeth around the outer edge, making it look like a drawing of the sun. Gears only work in pairs or more, as a single gear would be useless and wouldn’t technically be a gear.

Physics dictates that speed is traded for strength and vice-versa. It’s easier to turn something larger because the fulcrum of leverage is farther from the center pivot point. What is traded off is speed with the larger gear because it takes longer to go around the full circle. The complete opposite is true with smaller gears.

On a ten-speed bicycle or a mountain bike there are two gear sets. One set is mounted to the frame-the larger set (usually 2 or 3) at the pedals-and a set of sequentially smaller ones are mounted at the center of the rear “drive” tire. When a bicycle is in the first gear both front and rear gears are large. The pedals are easy to rotate, but the bike doesn’t move very fast. As the gear selection at the rear gets smaller in size, the pedals become harder to rotate, but the bicycle moves along much faster with fewer rotations of the front gear. All of this accounts for the gear ratio.

All teeth have to be the same size in order to “mesh”, or fit together, thus making the gear with a higher tooth count larger. The gear ratio is defined by the count of teeth on one gear as compared to the count of teeth on the second. If there are 18 teeth on the first gear and 36 teeth on the second, it would be called a 36 to 18 (36:18) gear set. The numbers are divided to get 2:1, meaning that the smaller gear rotates twice for every time the large gear rotates once.

There are two types of gear sets-hunting and non-hunting. In a hunting gear set, a single tooth will match up with an individual tooth on the other gear, either on every turn (1:1) or every other turn (2:1). The teeth ‘hunt’ for each other. This is typical of a timing gear set in an engine because the crankshaft and camshaft have to line up perfectly on every other rotation, otherwise the valves wouldn’t open at the proper time and the pistons would strike them. This would result in one or more components bending or breaking.

Non-hunting gears mesh at a variable rate-only contacting the same tooth randomly. Their only job is to multiply the force of input to affect output in order to speed something up, or to make it easier to turn. These make up the ‘in between’ gear ratios like 3.25:1 – 4.85:1 and so on. Non-hunting gears are used when the work required doesn’t need to have a precision match-up. If a tire doesn’t match the stripe on the road, it doesn’t matter. There’s no harm done.

They also provide variety to the output needed. Cars that are built for highway driving have a larger span of gears for greater gas mileage, but can’t haul a lot of weight. A heavy-duty work truck has a smaller span so that it can move at a decent rate with a heavy load, but it gets much worse gas mileage.

Race cars use a variety of these gears in order to get up to speed quickly, but they can’t afford to top out at a 45 miles per hour. The difference between 3rd and 4th gear could be as much as 3 gear selections on a mountain bike, which offers them room in the transmission. Race cars can have even larger gears at the high rates of speed, so they can reach 200 miles per hour and never need an 8-speed gear box.

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