Gears multiply torque by moving the receiver gear-the one connected to the wheels or output-slower than the pinion gear, the one connected to the driveshaft or input. You cannot increase engine torque with gearing; you can only increase the torque experienced by the output shaft or wheels. To calculate increases in effective torque, you need only know the original gear ratio, the new gear ratio and the engine's output torque.
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Things you need
- A calculator
Multiply the engine torque by the original gear ratio to get effective torque. Example: if your engine has 300 foot-pounds of torque and your final drive ratio is 2.5:1, the effective torque to the axle is 750ft-lbs.
Multiply engine torque by intended gear ratio. Example: you apply 3.5:1 gearing to that same 300ft-lb. engine, your effective torque comes out to 1,050ft-lbs.
Subtract new effective gear torque from old effective torque. In our example, an increase in gearing from 2.5:1 to 3.5:1 on an engine with 300ft-lbs of torque yields an increase of 300ft-lbs. to the wheels.
To Calculate Torque
Determine the desired effective (wheel) torque. In this example, we'll say that you have the same 300ft-lb engine and want 1,200ft-lbs. of torque to the wheels.
Divide the desired effective torque (1,200) by the engine's output torque (300). In this example, that comes out to a gear ratio of 4:1.
Order a set of gears as close to that number as possible. For many domestic vehicles in an example like this, the closest thing to 4:1 comes out to 4.12:1.
To Calculate Ideal Gearing
Tips and warnings
- One major reason for changing gear ratio is to compensate for larger tires. Large tires make the axle turn slower per revolution, reducing effective torque to the ground. To compensate for larger wheels and tires, divide the new tyre's circumference (2 x 3.14 x the tyre's radius) by the old tyre's circumference. Multiply your old gear ratio by that number to arrive at your new ratio.
- Example: To calculate the percentage difference in size between 24-inch tall tires and 36-inch tall tires, you must first get the new tyre's circumference. You can calculate it by multiplying the 36 inch tyre's radius (18) by two and again by pi (3.14); the end result is a circumference of 113 inches. Apply the same calculation to the 24 inch tire to arrive at a circumference of 73.36 inches. Divide 113 by 73.36 to arrive at 1.49. If your old gear ratio was 2.5:1, multiply that by 1.49 to arrive at 3.74. Thus, with 36 inch tall tires you'd need 3.74:1 gears to maintain the same effective torque that you had with 24-inch tall tires and 2.5:1 gears.
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