评论
打印
Bicycles are one of the most efficient and versatile human-powered means of transportation we have yet devised.
自行车是我们所设计的最有效、最通用的人力交通工具之一。
But perhaps even more incredible than humans riding bicycles is the fact that bicycles can ride themselves.
但也许比人类骑自行车更令人难以置信的是，自行车可以自己骑。
Yes, once they’re set in motion at a sufficient speed, bicycles can stay upright without any human intervention.
是的，一旦它们以足够的速度启动，自行车就可以在没有任何人为干预的情况下保持直立。
A common misconception is that bikes stay up because of conservation of angular momentum – that is,
一个常见的误解是，自行车之所以能立起来是因为角动量守恒——也就是说，
since the wheels are spinning, if the bike tips to one side
因为轮子在旋转，如果自行车倾斜到一边
there’ll be some sort of countering force from the wheels that keeps the bike upright.
车轮会产生某种反作用力，使自行车保持直立。
But there’s an easy way to see this explanation is wrong:
但有一种简单的方法可以看出这种解释是错误的：
simply lock the handlebars in place and a moving bike will fall over just as easily as a stationary one.
只要把车把锁在适当的位置，移动的自行车就会像静止的一样容易摔倒。
Another common misconception is that bikes stay upright because of their forward momentum.
另一个常见的误解是，自行车保持直立是因为它们向前的动力。
However, if you knock a ghost-riding bicycle sideways,
然而，如果你碰一下自己跑的车子，
it’ll change directions and then continue merrily on its way – plainly changing its momentum, but nevertheless staying upright.
它会改变方向，然后愉快地继续它的旅程——简单地改变它的势头，但仍然保持直立。
What we do know about how conventional bikes stay upright on their own is this:
关于传统自行车如何保持直立，我们知道的是：
when a moving bike starts leaning to one side, it also automatically steers towards that side a little bit.
当一辆移动的自行车开始向一边倾斜时，它也会自动向那一边倾斜一点点。
The result is that the wheels come back underneath the center of mass, keeping the bike balanced.
其结果是，车轮回到重心下方，保持自行车平衡。
And there are three main mechanisms responsible: First, because of the backwards tilt of a bike’s steering axis,
有三种主要机制负责：首先，由于自行车的转向轴向后倾斜，
its front wheel actually touches the ground slightly behind that axis.
它的前轮实际上在轴的后面接触地面。
This means that when the bike leans to the left, the upward force from the ground acts to turn the wheel and handlebars to the left,
这意味着当自行车向左倾斜时，来自地面的向上的力作用使车轮和车把向左转动，
helping the bike steer its wheels back underneath its center of mass.
帮助自行车将车轮转回重心下方。
Second, the weight of a bike’s front wheel and handlebars is generally distributed slightly in front of the steering axis,
其次，自行车前轮和车把的重量通常轻微地分布在转向轴的前面，
so when the bike leans to the left, the downward pull of this mass also helps turn the front wheel to the left,
所以当自行车向左倾斜时，这个物体向下的拉力也会帮助前轮向左转动，
the same way divining rods will turn towards whatever direction you tilt your hands.
同样的方法，探测棒将转向任何你的手倾斜的方向。
Third, there is indeed a gyroscopic effect from the wheels, but it doesn’t keep the bike upright on its own.
第三，车轮确实有一个陀螺效应，但它不能保持自行车自己直立。
Instead, it helps steer: as Destin and Carl demonstrate excellently in this video about how helicopters work,
相反，它有助于引导：德斯坦和卡尔在这个视频中出色地演示了直升机是如何工作的，
trying to tilt a spinning object makes the object tilt as if you pushed it at a point 90° away from where you did – it seems spooky,
试图倾斜一个旋转的物体会使物体倾斜，就好像你把它推到一个90度远的地方——这看起来有点吓人，
but basically the effect of your torque lags behind where you push.
但基本上扭矩的作用滞后于你推的地方。
Now imagine this happening vertically on a bike,
现在想象一下在自行车上垂直地发生这种情况，
and you can see that the gyroscopic precession from the bike’s leftward lean makes the front wheel turn to the left,
你可以看到，自行车向左倾斜产生的陀螺仪进动使前轮转向左边，
again helping steer its wheels back underneath its center of mass.
再次帮助控制车轮回到它的重心下面。
In short, a normal bicycle is stable thanks to a combination of the front wheel touching the ground behind a backwards-tilted steering axis,
简而言之，一辆普通的自行车之所以稳定是因为前轮与地面的接触与转向轴向后倾斜，
the center of mass of the front wheel and handlebars being located in front of the steering axis,
前轮和车把的重心位于转向轴的前方，
and the gyroscopic precession of the front wheel, all of which help the bike automatically steer its wheels back underneath it when it leans.
还有前轮的陀螺仪进动，所有这些都能帮助自行车在倾斜时自动将车轮转回下方。
At least, when it’s moving forwards at the correct speed.
至少，当它以正确的速度前进时。
If the bike’s going too slow, it won’t turn quickly enough to keep from crashing into the ground.
如果自行车走得太慢，它转弯的速度就不够快，无法避免撞向地面。
And if you push the same bike backwards, the gyro effect will reverse but the other two effects won’t,
如果你向后推同一辆自行车，陀螺仪效应会逆转，但其他两种效应不会，
with the result that the wheels are steered out from under the bike when it leans.
其结果是，当自行车倾斜时，轮子从车底转向外面。
What’s more, none of these three mechanisms is, on its own, the secret to bike stability:
更重要的是，这三种机制本身都不是自行车稳定性的秘密：
here’s a bicycle that has no gyroscopic effect and whose front wheel touches the ground in FRONT of the steering axis yet which is stable without a rider.
这是一辆没有陀螺仪效应的自行车，它的前轮在转向轴的前面触地，但它在没有骑手的情况下是稳定的。
Here’s a stable rear-steering bike, and here’s a design for a stable bike where the steering axis tilts forward instead of back.
这是一个稳定的后转向自行车，这是一个设计的稳定自行车，其中转向轴向前倾斜，而不是向后。
On the other hand, I made my own bike totally unstable just by adding some extra weight behind the front fork.
另一方面，在前叉后面增加一些额外的重量，我让自己的自行车完全不稳定。
There are clearly a lot of different variables that can be combined in various and surprising ways to make stable and unstable bicycles.
显然，有很多不同的变量，可以以不同的和令人惊讶的方式结合，来制造稳定和不稳定的自行车。
Adding a human to help with steering and balance can sometimes make unstable bikes stable,
增加一个人来帮助转向和平衡有时可以使不稳定的自行车稳定下来，
and I imagine a rider would also make some stable bikes unstable.
我想骑手也会让一些稳定的自行车变得不稳定。
But amazingly, even for a riderless bike, science currently doesn’t know what it IS about the special combinations of variables that enables a bike to stay up on its own.
但令人惊讶的是，即使是一辆无人骑的自行车，科学目前也不知道是什么特殊的变量组合使一辆自行车能够保持直立。
We just know that some combinations work, and others don’t.
我们只知道有些组合可行，有些则不行。
