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116 Build Your Own Electric Vehicle• Transmission—Another handy item in today’s EV conversions, thetransmission’s gears not only match the vehicle you are converting to a varietyof off-the-shelf electrical motors, but also give you a mechanical reversingcontrol that eliminates the need for a two-direction motor and controller—againsimplifying your work. In the future, when widespread adoption of AC motorsand controllers provides directional control and eliminates the need for a largenumber of mechanical gears to get the torques and speeds you need, today’stransmission will be able to be replaced by a greatly simplified (and even morereliable) mechanical device.• Driveshaft, Differential, Drive Axles—These components are all used intact intoday’s EV conversions. Because contemporary, built-from-the-ground-upelectric vehicles like General Motors’ Impact use two AC motors and placethem next to the drive wheels, it’s not too difficult to envision even simplersolutions for future EVs, because electric motors (with only one moving part)are so easily designed to accommodate different roles.Going through the GearsThe transmission gear ratios, combined with the ratio available from the differential (orrear end, as it’s sometimes called in automotive jargon), adapt the internal combustionengine’s power and torque characteristics to maximum torque needs for hill-climbingor maximum economy needs for cruising. Figure 5-8 shows these at a glance for atypical internal combustion engine with four manual forward gears—horsepower/torque characteristics versus vehicle speed appear above the line and RPM versusvehicle speed appear below. The constant engine power line is simply equation 5, hp 5FV/375 (V in mph), less any drivetrain losses. The tractive force line for each gear issimply the characteristic internal combustion engine torque curve (similar to the oneshown in Figure 5-7) multiplied by the ratios for that gear. The superimposed inclineforce lanes are the typical propulsion or road-load force components added byacceleration or hill-climbing forces (recall the shape of this curve in Figure 5-5). Theintersection of the incline or road-load curves and the tractive-force curves are themaximum speed that can be sustained in that gear. The upper half of Figure 5-8illustrates how low first gearing for startup and high fourth gearing for high-speeddriving apply to engine torque capabilities.The lower part of Figure 5-8 shows road speed versus engine speed—for each gearappears. The point of this drawing is to illustrate how gear selection applies to enginespeed capabilities. Normally, the overall gear ratios are selected to fall in a geometricprogression: 1 st / 2 nd 5 2 nd / 3 rd , etc. Then individual gears are optimized for starting(1 st ), passing (2 nd or 3 rd ), and fuel economy (4 th or 5 th ).Table 5-9 shows how these ratios turn out in an actual production car—in this casea Ford 1989 Taurus SHO. Notice the first two gear pairs are in a 1.5 ratio, whereas thenext two move to 1.35. Table 5-9 also calculates the actual transmission, differential, andoverall gear ratios (overall equals transmission times differential) for the 1987 FordRanger pickup truck that will be later used in the design section. Notice that the Rangeris optimized at both ends of the range but lower in the middle versus the Taurus,reflecting the difference in car versus truck design.