“The gear ratio of a gear train, also known as its speed ratio, is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. The gear ratio can be calculated directly from the numbers of teeth on the gears in the gear train.” https://en.wikipedia.org/wiki/Gear_ratio
It’s usually an input:output thing. A ratio of 2:1, or 2, means that the input shaft (engine) makes two revolutions for every revolution of the output shaft. As a result, the output shaft spins at half the speed of the engine, but with twice the torque. A ratio of 3:1 or 3 means the output shaft spins once for every three revolutions of the engine, and thus it spins at one third of the speed but with triple the torque. A ratio of 10:4, which can be rewritten as 2.5:1 or 2.5 means the engine spins 10 times for every four revolutions of the output, or 2.5 times for every revolution of the output; torque is multiplied by 2.5 and rotational speed is divided by 2.5. These are known as reduction ratios, because the speed of the output is reduced to make more torque. Power is still the same, because the increased torque and decreased speed cancel each other out perfectly (in theory; there will always be some losses from friction, etc.)
On the other hand are overdrive ratios. These are when the output shaft spins faster than the input, but with less torque. A 1:2 ratio, which can also be written as 0.5:1 or simply 0.5, means that the input shaft spins half a revolution for each revolution of the output shaft. As a result, speed is doubled and torque is halved. A 0.1 ratio (this is extreme, this kind of overdrive is never used in a car transmission) means the input spins 0.1 of a revolution for every output revolution; speed is multiplied by 10 (or divided by 0.1, depending on how you look at it) and torque is divided by 10/multiplied by 0.1.
Typically ratios are written as 4, 2.3, 1.2, 0.8, etc. This number basically means that torque in the output is multiplied by that number and speed in the output is divided by this number.
The ratios in the transmission are not the only ones, the differential gear usually has a reduction on it too. Depending on how high the engine revs, the diameter of the tyres, the ratios in the transmission and what kind of top speed/acceleration is desired, the differential ratio can be anywhere between 2 and 6. To find out how fast the wheels spin, simply multiply the two ratios.
For example, a C5 Corvette has a first gear ratio of 2.97 in first gear and a differential ratio of 3.42. This means that the torque is multiplied by 2.97 after the transmission, and multiplied again by 3.42 after the differential, so the wheels spin with (2.97*3.42) ~10.2 times the amount of torque the engine makes, at a rate of (1/10.2) ~0.098 times the speed of the engine.
Hope that helps, I have a bad habit of writing really long comments :P
Haha its ok thanks it really helped and was really imformative.
Comments
T1/N1=T2/N2
by witchcraft
“The gear ratio of a gear train, also known as its speed ratio, is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. The gear ratio can be calculated directly from the numbers of teeth on the gears in the gear train.”
https://en.wikipedia.org/wiki/Gear_ratio
It’s usually an input:output thing. A ratio of 2:1, or 2, means that the input shaft (engine) makes two revolutions for every revolution of the output shaft. As a result, the output shaft spins at half the speed of the engine, but with twice the torque. A ratio of 3:1 or 3 means the output shaft spins once for every three revolutions of the engine, and thus it spins at one third of the speed but with triple the torque. A ratio of 10:4, which can be rewritten as 2.5:1 or 2.5 means the engine spins 10 times for every four revolutions of the output, or 2.5 times for every revolution of the output; torque is multiplied by 2.5 and rotational speed is divided by 2.5. These are known as reduction ratios, because the speed of the output is reduced to make more torque. Power is still the same, because the increased torque and decreased speed cancel each other out perfectly (in theory; there will always be some losses from friction, etc.)
On the other hand are overdrive ratios. These are when the output shaft spins faster than the input, but with less torque. A 1:2 ratio, which can also be written as 0.5:1 or simply 0.5, means that the input shaft spins half a revolution for each revolution of the output shaft. As a result, speed is doubled and torque is halved. A 0.1 ratio (this is extreme, this kind of overdrive is never used in a car transmission) means the input spins 0.1 of a revolution for every output revolution; speed is multiplied by 10 (or divided by 0.1, depending on how you look at it) and torque is divided by 10/multiplied by 0.1.
Typically ratios are written as 4, 2.3, 1.2, 0.8, etc. This number basically means that torque in the output is multiplied by that number and speed in the output is divided by this number.
The ratios in the transmission are not the only ones, the differential gear usually has a reduction on it too. Depending on how high the engine revs, the diameter of the tyres, the ratios in the transmission and what kind of top speed/acceleration is desired, the differential ratio can be anywhere between 2 and 6. To find out how fast the wheels spin, simply multiply the two ratios.
For example, a C5 Corvette has a first gear ratio of 2.97 in first gear and a differential ratio of 3.42. This means that the torque is multiplied by 2.97 after the transmission, and multiplied again by 3.42 after the differential, so the wheels spin with (2.97*3.42) ~10.2 times the amount of torque the engine makes, at a rate of (1/10.2) ~0.098 times the speed of the engine.
Hope that helps, I have a bad habit of writing really long comments :P
Haha its ok thanks it really helped and was really imformative.