Warmth of Friction Sets Wheels Spinning

Alaskans often feel as if they're getting nowhere when they step on the gas at an intersection. The speedometer needle jumps, but it's the only thing moving forward. The car's tires spin in place. We all know we're slipping on ice, but what really happens down there where tire meets road?

For the answer I dialed Samuel Colbeck, a man who studies the slipperiness of ice and snow. Colbeck is a geophysicist who works at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire.

One of Colbeck's main interests is snow friction on ski bottoms, but he also commutes to work in a car and knows the physics behind what's happening when his tires spin on a snowy road.

To illustrate one of the main forces making roads slipped, Colbeck told me to rub my hands together. I did. My hands heated up.

The warmth of friction generated by a tire spinning on snow creates a microscopic layer of water that lubricates the space between tires and road surface.

Part of the problem is that a car's immense weight is supported by just a few square inches where each tire meets the ground.

"Putting all of that energy in a small area, you generate an awful lot of melt-water, Colbeck said.

Skis, snowboards and ice skates are fun because they glide on this microscopic film of water. When Tommy Moe is downhill racing, each of his ski bottoms are putting out as much heat as three 100-watt light bulbs, Colbeck said.

Roads are less slippery when it's cold. At 40 below, for example, our main concern is making it to an intersection rather than sliding through it. When it's that cold, we don't slide as much because the heat generated by tires is quickly absorbed the by the frigid air.

Cold snow is also more "aggressive," which means it's harder and has a more angular structure that tends to provide spikes of traction. Cold snow becomes slicker, though, when it's packed down.

Packed snow has less friction than unpacked snow. After a few cars drive over deep snow, the tires polish the surface, changing the snow's texture from spiky to glassy. This causes tires to spin easier. And once they begin spinning, it's time to search for the tow rope.

In Alaska, gravel is often used to combat slippery snow. Gravel acts as a solid physical link between tires and the road surface, with each chunk acting as a little extension of the road surface for tires to grip.

Although it's outlawed for use in the city of Fairbanks, salt is often dropped on roads in other areas of Alaska and the Lower 48. Salt reacts with ice and snow to lower its melting temperature, but it's not much good in cold climates.

The eutectic temperature of salt---the point at which it stops reacting with ice and snow---is rather high, about minus 5 degrees Fahrenheit. Salt isn't much good for cleaning up roads in the Interior, because it's rarely above that temperature on midwinter days.

Colbeck said we're better off without salt, because it's so corrosive that many cars in the Lower 48 have fenders that look like they've been used as shotgun targets. Given a choice between the damage caused by gravel or salt, I think most Alaskans would pick a cracked windshield over a corroded chassis.