If the car had smaller tires, it would be like a lower gear. If the Corvette had larger tires, it would travel farther with each revolution of the wheel, which would be like a higher gear. This means that for every complete revolution of the wheel, the car travels 82.1 inches. The example Corvette Z06 is equipped with 295/35-18 tires, which have a circumference of 82.1 inches. The car’s tires can almost be thought of as a third type of gearing. The differential ratio multiplies with the transmission ratio, so in 1st gear, the engine makes 10.16 revolutions for every revolution of the wheels. The ratio means that for every 3.42 revolutions of the transmission’s output, the wheels make one revolution. The Corvette above has a differential ratio of 3.42:1. 5th and 6th gears are known as overdrive gears, in which the output of the transmission is revolving faster than the engine. In 4th gear, the gear ratio of 1:1 means that the engine and the transmission’s output are moving at the same speed. In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmission’s output. These components might be separate and connected by a driveshaft, or they might be combined into one unit called a transaxle.Ī 2004 Chevrolet Corvette C5 Z06 with a six-speed manual transmission has the following gear ratios in the transmission: But whichever form of drive is employed, on four-stroke engines the crankshaft/camshaft gear ratio is always 2:1, which means that for every two revolutions of the crankshaft the camshaft will rotate through one revolution.(it is the case because in case of 4 stroke engines the valve cycle is repeated after every two rotations of the flywheel.)Īutomobile drivetrains generally have two or more areas where gearing is used: one in the transmission, which contains a number of different sets of gearing that can be changed to allow a wide range of vehicle speeds, and another at the differential, which contains one additional set of gearing that provides further mechanical advantage at the wheels. A chain, called a timing chain, is used on some automobiles for this purpose, while in others, the camshaft and crankshaft are coupled directly together through meshed gears. From the time the car is driven off the lot, to the time the belt needs replacing thousands of kilometers later, it synchronizes the two shafts exactly. Again, exact accounting of teeth and revolutions can be applied with these machines.Ī belt with teeth, called the timing belt, is used in some internal combustion engines to exactly synchronize the movement of the camshaft with that of the crankshaft, so that the valves open and close at the top of each cylinder at exactly the right time relative to the movement of each cylinder. Special gears called sprockets can be coupled together with chains, as on bicycles and some motorcycles. In other words, the gear ratio is proportional to ratio of the gear diameters and inversely proportional to the ratio of gear speeds.īelts can have teeth in them also and be coupled to gear-like pulleys. Since the diameter is equal to twice the radius Since the number of teeth is also proportional to the circumference of the gear wheel (the bigger the wheel the more teeth it has) the gear ratio can also be expressed as the relationship between the circumferences of both wheels (where d is the diameter of the smaller wheel and D is the diameter of the larger wheel): Since the intermediate (idler) gear contacts directly both the smaller and the larger gear it can be removed from the calculation, also giving a ratio of 42/13 = 3.23. Suppose the largest gear in the picture has 42 teeth, the gear ratio between the second and third gear is thus 21/42, or 1/2, and for every revolution of the smallest gear, the largest gear turns 0.62/2, or 0.31 revolution, a total reduction of about 1:3.23. In practical terms, the idler gear turns more slowly. It also means that for every one revolution of the pinion, the idler gear has made 1/1.62, or 0.62, revolutions. The ratio means that the pinion gear must make 1.62 revolutions to turn the idler gear 1 revolution. In the picture to the right, the smaller gear (known as the pinion) has 13 teeth, while the second, larger gear (known as the idler gear) has 21 teeth.
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