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Between which points is the forward distance of a semi-trailer measured?
半挂车的前向距离是在哪两个基准点之间测量的？
A semi-trailer's forward distance is measured from its rear axis to the centre of the kingpin.
半挂车前向距离从后轴基准线量到牵引销中心。
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It is not measured from the front or back of the body to an axle. Full trailers and rigid vehicles use different forward-distance definitions.
它不是从车体最前端或最末端量到车轴；全挂车与刚性车辆的前向距离各有不同量法。