How to Calculate Torque to the Shaft With Multiple Gears
- 1). Gather the data regarding your car's transmission, transfer case and rear differential gearing as well as the tire diameter. All of these have an effect on output torque as measured through the gearing.
- 2). Multiply your engine's torque by the transmission's gear ratios. For example: a given engine produces 400 foot-pounds of torque, and its four-speed transmission has ratios of 3.5-to-1 for first gear, 2.0-to-1 for second, 1-to-1 in third and an overdrive ratio of 0.50-to-1. In this case, output shaft torque would be 1,400 foot-pounds in first, 800 in second, 400 in third and 200 in overdrive.
- 3). Multiply the transmission's output torque by the gear ratio in your transfer case or bolt-on overdrive unit. For example: The transmission above uses a transfer case with a 1-to-1 direct-drive "high" range and a 3.5-to-1 low range. High range output would be the same as transmission output, and torque output in low range would be 4,900 foot-pounds in first, 2,800 in second, 1,400 in third and 700 in overdrive.
- 4). Multiply your transfer case output by the final drive gear ratio in your rear end. On this vehicle, we'll assume a 2.90-to-1 final drive ratio. That comes out to 4,060 foot-pounds in first gear with the TC in high range, 2,320 foot-pounds in second gear, 1,160 in third and 580 in overdrive. With the TC in low range, that would be 8,120 foot-pounds in first, 4,060 in second, 1,400 in third and 700 in overdrive.
- 5). Calculate wheel torque when changing tire sizes by dividing your old tire diameter by the new to get a percentage of loss or gain in wheel torque. For instance, switching from 30-inch-tall tires to 36-inch tires give you 0.83, which equals 83 percent torque or a total loss of 17 percent torque to the wheels. Switching from 28-inch-tall tires to shorter 24-inchers gives you 1.16, or a total gain of 16 percent torque to the wheels.
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