Monday, November 8, 2010

What Do Streetcars Have In Common With Beaches?

Posted by Alexandru Nicolae


I've been a train enthusiast for quite some time, so I thought I would write an article to inform everyone about a very obscure piece of science that you probably encounter everyday if you live in a big city and use streetcars frequently, yet you've probably never even noticed it. Streetcars are amazing pieces of technology that we take for granted everyday, but one of their oddest features has to do with the physics behind their movement. So to skip ahead and answer the question, what do streetcars and beaches have in common? The answer is sand.

Now, you're probably thinking what on earth does a 35,000 pound vehicle have anything to do with common beach sand? Streetcars have a variety of designs but they all function similarly. They run on metallic wheels that ride on metallic rails, both are finely constructed so as to minimize any bumps or grooves on their surface. This is all done to reduce the friction between the wheels and the tracks. If you'll recall some basic physics, a vehicle that large would take a lot of energy to move on a road, but if you make it glide on a surface by reducing contact friction you can power even the largest of vehicles with relatively little energy input; think of a those 50-car freight trains and it starts to make a little more sense.

This lack of friction allows for relatively easy acceleration and high top speeds for any rail-bound vehicles like streetcars. But for every up-side there is a down-side. Reducing friction to the extent required for trains and streetcars causes, well, loss of friction! Friction is important in tires and wheels for gripping a surface, if the wheels don't grip anything the train ends up behaving like a clumsy figure skater: uselessly spinning its wheels and not moving anywhere. The problem becomes even worse when any uphill climbs need to be made. Streetcars are notoriously bad for not being able to climb very steep grades (angled hills really). If they can't get a good grip climbing up hill they literally end up rolling back down the hill like a useless shopping cart. So how do streetcars tackle this problem? This is where the sand comes in...

Remember how we said that the wheels, and the streetcar tracks are all polished nicely and are for all intents and purposes perfectly smooth? Well sand is the opposite of that, it's rough and harsh and increases that lovely friction stuff we were talking about. With increased friction there is less chance of the wheels slipping and malfunctioning. There is also the possibility of climbing up slight angles now without having to worry about falling backwards and having a class-action lawsuit filed against your transit company. Now that we've determined how sand could be useful, how do streetcars actually utilize it?

The answer is astoundingly simple. Streetcars have a box full of sand, which the operator can control through a button in their cabin. Pushing the button literally floods the front of the streetcar wheels with sand to increase the friction of the upcoming section of track. It's that simple, yet elegant in a way. They use this when the wheels aren't gripping the rails properly (like in winter), or when climbing up steep gradients.

I have a task for you, the reader, now that I've informed you of this wonderful little piece of science and engineering. The next time you're waiting for the streetcar I want you to look down at the tracks, especially the grooves next to the tracks, and tell me what you see. I guarantee you'll find the exact same kind of sand you see at the beach. Look hard, you'll find it.

References:

Bow J. (2010). The TTC's Surface Rail Work Car Fleet. Transit Toronto.on.ca. Obtained November 8, 2010 from

Transportation Research Board (2005). Track-related research: flange climb derailment criteria and wheel. Lubrication and Wheel Slide. Transit Cooperation Research Program; 5(71): A38, A1.6.

The use of sand for improved track adhesion goes back a long way:
See The Electrical Engineer; volume 7. available on Google books. pages 26, 315, 347, 393, 394.

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