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122 Build Your Own Electric VehicleTorque Required WorksheetTable 5-10 computes the torque required data for a 1987 Ranger pickup, the vehicle to beconverted in Chapter 10. You’ve met all the values going into the level drag force before,but not in one worksheet. Now they are converted to torque values using equation 11,and new values of force and torque are calculated for incline values from 5 percent to25 percent. Conveniently, these correspond rather closely to the acceleration values for1 mph/sec to 5 mph/sec, respectively, and the two can be used interchangeably. Thevehicle assumptions all appear in Table 5-l0. If you were preparing a computer spreadsheet,all of this type information would be grouped in one section so that you could see theeffects of changing chassis weight, C dA, C r, and other parameters. You might also want tograph speed values at 5 mph intervals to present a more accurate picture.Torque Available WorksheetThere are a few preliminaries to go through before you can prepare the torque availableworksheet. First, you have to determine the horsepower of an electric motor using thefollowing equation:hp motor5 (Torque wheel3 mph 3 Revolutions/mile)/(315120 3 n o)Plugging in real values for the Ranger pickup with P185/75R14 tires and using thetorque value from the Torque Required Worksheet at the 50-mph design center speed(recall that overall efficiency for rear-wheel-drive manual transmission was 0.90 fromTable 5-8) gives you:hp motor5 (152.04 3 50 3 808)/(315120 3 0.9) 5 21.66This corresponds quite nicely to the capabilities of the Advanced DC Motors modelFB1-4001 rated at 22 hp. From the manufacturer’s torque versus speed curves for thismotor driven at a constant 120 volts and using this equation:hp 5 (Torque 3 RPM)/5252 5 (25 3 4600)/5252 5 21.89This motor produces approximately 22 hp at 4600 RPM at 25 ft-lbs of torque and 170amps. Next, calculate Wheel RPM using the following equation:Wheel RPM 5 (mph 3 Revolutions/mile)/60 5 (50 3 808)/60 5 673.33You can then calculate the best gear using the next equation:Overall gear ratio 5 RPM motor/RPM wheel5 4600/673.33 5 6.83From Table 5-9, the 2nd gear overall ratio of 7.18 for the Ford Ranger pickup comesquite close to this figure, which means it should deliver the best overall performance.Now you are ready to prepare the torque available worksheet shown in Table 5-11.This worksheet sets up motor values on the far left. Wheel torque and vehicle speedvalues for the 1st through 5th gears go from left to right across the worksheet, and athigher values of torque and current from top to bottom of the worksheet within eachgear. The net result is you now have the wheel torque available at a given vehicle speedfor each one of the vehicle’s gear ratios. A spreadsheet would make quick work of this;you could see at a glance the effects of varying motor voltage, tires sizes, etc.