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    2015-4-23 11:33










    When you turn a car's steering wheel, whether you go forwards or backwards. So it seems like it should be just as easy to steer going backwards as going forwards.But clearly,it's not.Suprisingly,the difficulty of driving backwards has almost nothing to do with the direction you're facing and everything to do with the fact that you steer using the front wheels.

    When driving forwards,a car goes in the direction the front wheels are pointed.And when driving backwards,the car goes where the back wheels are pointed.Simple!But the difference is that going forwards,the direction of travel is determined by the same wheels you steer with,so you just point them where you want to go,and that's where you go,the back wheels follow automatically.

    In physics,we call this a "stable"system.like dangling a pencil from your fingers-move your hand around and the pencil follows,no thought required.However,when driving backwards,the direction of travel is determined by the back wheels but you steer with the front wheels.

    So rather than just pointing the front wheels where you want to go,you have to point them in the direction that will get the back wheelsto point in the direction you want to go-it's an additional level of separation before the car does what you want.

    And in fact,if you don't make any corrections,then over time the back wheels point farther and farther away from the direction you want to go:instead of following the front wheels,they want to run away!This is an unstable system,like trying to balance a pencil on your fingertip.It's incredibly finicky and requires a ton of coordinated hand-eye feedback to maintain.
    事實上,如果你不采取修正措施,隨著時間流失,后輪就會越來越偏離你預期的方向:不但不隨前輪運動 而且還會失控!這是一個不穩定系統,就像嘗試平衡立在指尖上的鉛筆一樣。這相當講究,需要不停的進行手眼反饋協調來完成。

    That's why you constantly have to readjust your steering when driving backwards-mathematically,your car is like an upside-down pencil! Of course,there are some successful rear-steering wheeled vehicles-forklifts,for example,only drive slowly and choose rear-steering for greater control of their forks.
    這就是倒車時你需要不斷調整輪胎方向的原因-數學說法就是 你的車就像東歪西倒的鉛筆!當然 也有一些很棒的后輪控制轉向的車子-例如,叉車,行駛緩慢 并且選擇后輪主控叉子。

    But let's go in the other direction and add a TRAILER to your car steering backwards has gained ANOTHER level of difficulty:you use your front wheels to get the back wheels to point in the direction needed to get the TRAILER to point in the direction you want to go.Not complicated enough?It becomes worse the more trailers you add each one adds another level of separation between the direction of your steering and the actual direction of travel.

    Which is why it's nearly impossible to push on one end of a chain or a rope and have the other end go where you want it to,and why you will never,ever blance a flexible rope or chain upright on your finger.which makes backwards driving seem downright easy!