- Understanding the Concept of Gear Ratio
Understanding the concept of the gear ratio is easy if you understand the concept of the circumference of a circle. Keep in mind that the circumference of a circle is equal to the diameter of the circle multiplied by Pi (Pi is equal to 3.14159...). Therefore, if you have a circle or a gear with a diameter of 1 inch, the circumference of that circle is 3.14159 inches.
The following figure shows how the circumference of a circle with a diameter of 1.27 inches is equal to a linear distance of 4 inches:
View picture on the actual link here
Let's say that you have another circle whose diameter is 0.635 inches (1.27 inches / 2), and you roll it in the same way as in this figure. You'll find that, because its diameter is half of the circle's in the figure, it has to complete two full rotations to cover the same 4-inch line. This explains why two gears, one half as big as the other, have a gear ratio of 2:1. The smaller gear has to spin twice to cover the same distance covered when the larger gear spins once.
Most gears that you see in real life have teeth. The teeth have three advantages:
They prevent slippage between the gears. Therefore, axles connected by gears are always synchronized exactly with one another.
They make it possible to determine exact gear ratios. You just count the number of teeth in the two gears and divide. So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.
They make it so that slight imperfections in the actual diameter and circumference of two gears don't matter. The gear ratio is controlled by the number of teeth even if the diameters are a bit off.
So, to get the actual gear ratio of a specific gear box, from 1-2 you count the number of teeth on the 1st gear and divide it by the number of teeth on the 2nd gear....and so on (2-3, 3-4, 4-5...).
The higher the initial gear ratio, the more pull off the line the car can have.
- The final drive gear ratio is the number of times the motor needs to turn to power the wheels for one full rotation.
Here's our gear ratio setup:
Final-drive ratio: 4.24:1
Gear, Ratio, Mph/1000 rpm, Max test speed
I, 3.54, 4.7, 29 mph (6200 rpm)
II, 2.05, 8.2, 51 mph (6200 rpm)
III, 1.33, 12.6, 78 mph (6200 rpm)
IV, 0.97, 17.2, 107 mph (6200 rpm)
V, 0.78, 21.6, 127 mph (5900 rpm)
For our final drive to be 4.24:1, that means our motor has 4.24 full revelutions to make just to get the tire to make 1 full revelution.
Compared to a Porsche 911 Turbo:
Final Drive 3.44:1
1st Gear Ratio 3.82:1
2nd Gear Ratio 2.05:1
3rd Gear Ratio 1.41:1
4th Gear Ratio 1.12:1
5th Gear Ratio 0.92:1
6th Gear Ratio 0.75:1
Amazingly, our gear ratio isn't far off from the 911 setup. It has a higher 1st gear ratio, so it will explode more from the start, has more in the middle gears as well and a similar final gear setup. Only thing is, it's got a final drive of 3.44 so it takes about 1 less engine rotation to get the tire spinning than it does ours.
As you can see, it's all like a bike gear setup. You start off in the biggest gear b/c that's where you get more of a 1:1 (not exact but closer) engine to power delivery conversion. If you started in the highest gear of a bike, you'll notice that it takes less rotations to spin the rear tire but a LOT more force to get it spinning. Same concept in cars. Start on the big gear b/c from the start, it takes less power to get spinning and can get you moving faster with less power. As you move up in the gears, the ratio actually drops b/c the gear size from the previous gear to the next is getting smaller. With the engine (or the person on the bike) in full rotation now, the smaller the gears means that the engine can utilize the power it's creating by allowing the smaller gears to spin the tires at a higher rate with less actual rotation.
As you get closer to the end, the gears are much smaller than when you started, which allows the engine to spin less but to have more power delivered to the ground with less effort. In the FINAL drive ratio, you are in the smallest gear and that's where (as stated above) you are spinning the engine as little as possible to get the most forward motion out of the vehicle.
Please ask questions if necessary.