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Comment by cstross | Hacker News Reader

Actually, the TGV record was set by a specially-rebuilt train set that was heavily customized -- and the test crew reportedly couldn't stand/walk around the instrument car at speeds over 500km/h due to vibration. (The track they ran the test on was newly-laid and not yet open to regular traffic: they had to replace most of the overhead cables from the stretch where the train hit maximum speed due to arcing and shock waves.)

The big difference is that the TGV record set pretty much an upper limit on steel-wheel speed -- horrible vibration, and the wheel rims were close to supersonic, which would have led to more shock waves if they'd pushed it faster. Similarly, Formula One cars go a lot faster than regular road vehicles, but you wouldn't want to commute to the office in one! Whereas this maglev test demonstrated a technology that would be comfortably usable at this speed.

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Interestingly, the TGV record was broken by an unmodified Velaro-D train recently. Standard rolling stock, used the week before for normal transport, used a week after for normal transport.

Are you sure? I'm not finding anything about this.

The TGV record is about half the speed of sound, which means the wheel rims are bordering on supersonic, so it's not a simple thing to talk about exceeding it.

Why would the wheel rims rotate at twice the speed of the train? If the train advanced along the track 1m, surely the part of the wheel in contact with the track must also have moved 1m?

The bottom of a wheel is stationary relative to the ground. (X - X)

The middle a the wheel moves at the same speed as the vehicle relative to the ground. (X + 0)

The top of the wheel moves at twice the speed of the vechile relative to the ground. (X + X)

Put another way, the top of the wheel must go faster than the vehicle or it does not rotate.

I think the idea is that the top of the wheel is basically super sonic compared to the ground. This has to be the case: the train is connected to the middle of the wheel at speed, the bottom of the wheel is going 0 so long as it's not slipping, so the top of the wheel is screaming along at near-sonic speeds.

It's an odd diagram to draw out, as that result is pretty counter-intuitive.

Pretty sure you're mistaken. The Velaro-D reached 403km/h without modifications[0], the Velaro-E did the same back in 2006. That's not the untuned record either, these belong to China's CRH380BL.

[0] http://www.mobility.siemens.com/mobility/global/en/interurba...

"the wheel rims were close to supersonic"

This doesn't make any sense. The record is around 575 km/h. The speed of sound is 1225km/h. How would the wheel rim be travelling faster than the train? The while rim rotates 1m for every 1m the train travels, surely?

Can you explain what you mean?

The top of the wheel is moving faster than the train. If the contact point between the wheel and rail is not slipping, the relative velocity between the wheel at that point and the rail is 0.

To maintain that condition, the top of the wheel has to travel at twice the speed of the train. Imagine a point on the wheel. Each time the train moves ones circumference forward, that point also makes a circular trip of 1 circumference.

So at the top, when the motion of the point relative to the train is full forward, the net velocity is twice the train. At the bottom, when the motion of the point relative to the train is full backward, the net velocity is 0.

There are diagrams and further explanation here: http://physics.stackexchange.com/questions/48234/why-is-the-...

Your link follows to a pretty good set of animations on the topic (linked by the OP of that thread):

http://www.animations.physics.unsw.edu.au//jw/rolling.htm

Good stuff, and even has an interesting note about how train wheel rims acually go backwards as the wheel turns!

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