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车轮克服阻力而向前滚动,或是因为轮轴上有推力(或拉力)在作用(图1a),或是因为轮子上有力矩(转矩)在作用(图1b)。马车车轮属于前一类;汽车的驱动车轮属于后一类。普通汽车只有后轮是驱动的,就是说,只有后轮承受发动机经过传动系统传来的转矩;前轮是从动的,跟马车车轮一样。轮子上的转矩如何成为推动汽车前进的牵引力呢?
The wheel rolls forward against its resistance, either because of thrust (or pull) on the axle (Figure 1a) or because of the torque (torque) on the wheel (Figure 1b). Wagon wheels belong to the former category; the car’s driving wheels belong to the latter category. Ordinary cars are driven only by the rear wheels, that is, only the rear wheels withstand the torque transmitted by the engine through the transmission system; the front wheels are driven, just like the carriage wheels. How does the torque on wheels become the traction that drives the car forward?