Corrugated roads

I spent the weekend (plus a couple of other days) at Lake Waikaremoana, enjoying the bush and the scorching sunshine (yes, evern in Te Urewera). There’s not a lot of physics that goes on there; maybe that’s why it is so relaxing, with the science focussed towards earth-science (I hope that rock doesn’t fall on us) and biological-science (is that a weka? – No, don’t be silly – yes, I’m sure it is). But I will pull out some physics, namely the state of the road.

State Highway 38 is hardly what you’d call a motorway, though if your definition of a road is something that you can drive a car along it fits the bill, just. One very frustrating thing on metal roads such as this one is the way it becomes corrugated – lots of equally spaced ridges – which makes it in places like driving on corrugated iron. It can’t be good for the suspension. The cause of these, as far as I’m aware, is over-enthusiastic acceleration.

But why does this cause lumps? Surely, on average, car wheels going over the road will result in the level of the stones being smoothed out, not forming into regular corrugations. This is an example of a broad class of physics phenomena that can be collectively titled as ‘symmetry breaking’. We might expect the natural state, to be nice and symmetrical (in the case of the road, nice and smooth) but in actual fact something happens that breaks the symmetry, and we get patterns forming (like the corrugations).

Once the corrugations start to form, the action of an accelerating car is to grow the ridges that exist, leaving less touched the grooves between them. So a weak pattern of corrugation becomes progressively stronger and stronger, and, over the course of time, the pattern will inevitably occur.

A simple model of this is trying to balance a pencil on its point. If you get it spot on, it will work (in theory), but the slightest deviation from vertical will be emphasized and it will become a large deviation, and ultimately the pencil will fall. It is utterly inevitable. Which way it will fall you can’t tell until it starts moving, but fall it will. Same with corrugated roads. Once those ridges start appearing,  it’s hard to stop them.

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