Science And Cycling: The Pothole

Written by Boston Biker on Feb 10

As you probably have noticed there seem to be a lot of new potholes around. Potholes might be the wrong term, is there a word that means “giant wheel swallowing death traps filled with ice cold salt water?” Pothells?

I nearly ended up like the guy above yesterday as a brand new pit had sprung forth on a street that had been bereft of them the day before. I was able to swerve and avoid the front wheel and bunny hop the back wheel over it, in the most ungraceful way possible.

So where do all these monsters come from? And why do they seem to appear so suddenly? That my friend is a job for science!

To understand potholes you have to understand water. Most things shrink when they get cold, and expand when they get warm. Water is one of the few things that actually gets bigger as it gets colder. The reason for this is the way in which water molecules link up as they become ice.

The next thing you have to understand is that the road surface is not a single sold object. It feels pretty solid when you fall off your bike onto it, but its actually very porous. Black top is made of gravel and tar, concrete is comprised of gravel and cement. The road has lots of little spaces for water to soak into.

The road and everything else that is embedded into it, (utility covers, grates, train tracks, etc), shrink and expand as they heat and cool. Just to make it complicated they all expand and shrink at different rates, opening even more holes for water to soak into. Even things like white painted lines on the road can affect the rate at which things heat up and cool down.

The water soaks into tiny cracks in the road during the day when it is above freezing, at night when it cools down the water expands and pops open that hole a little more. The next day MORE water gets in there, when it freezes that hole gets bigger, repeat till your front wheel will fit into the hole. Metal objects only make this process faster, which is why you often see the worst potholes around utility hole covers and grates.

Just for good measure add a bunch of heavy vehicles driving over the surface and the smooth city streets are soon infested with a legion of evil potholes.

The recent spat of warm weather we had combined with the recent spat of extremely cold weather has caused a bumper crop of potholes. So many in fact that it seems every street now has one. Be vigilant riders, for a dark evil lurks below your wheels. Be sure to glance down once in a while to make sure a road once loyal is trustworthy still!

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Posted in education, fun | 3 Comments »

3 Responses to “Science And Cycling: The Pothole”

  1. By cycler on Feb 10, 2011 | Reply

    You’ve been having lots of adventures in physics recently between the exploding wheel and the creation of potholes.

  2. By Jonathan on Feb 10, 2011 | Reply

    “Mayor Thomas M. Menino’s Office of New Urban Mechanics, the team that launched the Citizens Connect iPhone app in 2009, is working on an application for smartphones that would automatically report potholes without the need for a driver to dial a number or send a text message.”

  3. By 2whls3spds on Feb 12, 2011 | Reply

    So from what I gather…we need to limit heavy vehicle use on streets until all the pavement has undergone a thorough inspection for weak spots? (tongue firmly planted in cheek)


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