How do rail car wheels manage to go round the curve?
Recently I remembered a puzzle I thought about a few years ago: „How do rail car wheels manage to go round the curve?“. My friends and I, we were in a train returning from a trip to the mountain. The train had a small incident, something related to a short-circuit and electric failure: there was a bolt of electricity which could be seen from under the train reaching a considerable distance and touching a nearby house. The train came to a stop and there was some smoke, but we were fine and we got back home safely. Oh yes, going back to wheels, well, this event made us more conscious that we are actually traveling in a train, on a set of wheels on a track, and someone posed the question.
Take a normal car, the one that can be driven on the road. Have you observed that, while you're inside it, when the car steers, a part of the vehicle moves more than the other? Consequently, when in a curve, some of the wheels have to roll more than the others. To make this possible, cars are equipped with differentials placed between them on the axis, allowing wheels to turn independently one from another.
Rail vehicles, however, use no differentials: the axis and two wheels are all connected in one piece. This shows that the problem of trains negotiating curves is not as easy as one can initially think. My first guess was that tracks are tilted in curve, allowing trains to bank when needed. It turns out that this is true, this type of curves are called cants1. They allow trains to change their direction while maintaining a greater speed. However, we're not done yet, there's something more at work…
Another instance of curved tracks I observed is that which is used by tram-ways. There is at least one place in Bucharest where the tracks lead trams in a tight curve (it's actually a U-turn at the end of the line). They slowly pass through it, making a lot of noise resulting from the friction of metal. In this case, the wheels are spinning at the same speed, except superelevation (or banking) and the centrifugal force cause more weight to be placed on the inside wheels. Since there is less force placed on the outside wheels, they skid over the rail (producing the friction noise).
But leaving the trams aside and going back to trains, the most interesting fact, seems to be this one: the wheels and track have such a geometry that, as the car enters a curve, the lateral displacement causes the rolling radius of the left and right wheel to change. This means that the wheels start to behave as a cone rather than a cylinder, allowing the car to steer freely through the curve.
So yes, have you ever wondered why are railroad wheels shaped like that? it seems that indeed there is a purpose to the geometry of the wheels and the rail profiles as well.
1: „http://en.wikipedia.org/wiki/Cant_(road/rail)“
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