Rolling Friction and Hydroplaning

posted: 04/11/12
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Aside from helping to smooth out the bumps in the road so you don't bounce around while driving, your tires' basic job is to keep your car moving. They can roll -- and you can steer -- because of friction.

Friction is a complicated phenomenon, but in everyday situations it typically boils down to a simple rule of thumb. Surfaces have imperfections that cause them to resist sliding against each other. The rougher the surface is, the greater the friction and the harder it is to slide. For example, wool socks are nubby, and a carpet is made of loops of coarse, fibrous material, so you can't really slide across carpet in your stocking feet. A freshly waxed floor, on the other hand, is smooth, and sliding across it in your socks is a breeze.

Where Rubber Meets Road

When you're driving, the friction between your tires and the road comes from invisible imperfections in the tires' rubber and tiny variations in the road's surface. The biggest imperfections you can see in tires -- the treads -- don't add to the friction on a dry road. They do the opposite, reducing friction by cutting down on how much rubber touches the road. But that reduction in friction pays off as soon as it starts to rain. Then, the treads give the water somewhere to go, shuttling it away so the rubber stays in contact with the pavement.

In addition to its imperfections, the rubber contributes to the tires' rolling friction by changing shape. The weight of the car presses down on the tires, flattening them on the bottom. As they flatten, the rubber presses into the road's indentations. The bottoms of the tires become constantly changing patches of friction that the car uses to push itself forward.

When Rubber Meets Rain

But the very traits that keep tires rolling on a dry road can also lead to hydroplaning in the rain. If the tires roll too fast, the treads can't move water out of the way fast enough. A wall of water builds up in front of the tires, and at the right speed they can roll on top of it instead of through it. Water is much smoother than pavement, and the tires can't get a grip. Instead, they skate across the surface of the water in an uncontrolled skid. According to a 2004 article in Science News, rubber's pliability is one of the culprits when this happens. As the tires roll, the rubber seals off water-filled pits in the road. These topped-off pits combine to form a dangerously slippery surface.

Recovering from Hydroplaning

There are lots of factors at play in whether a car hydroplanes or doesn't. NASA has developed an equation that predicts when a hydroplane will happen -- after all, aircraft have tires just like cars do. NASA's equation relates to tire pressure and speed, but other factors play a part as well. The size and shape of the tread and the water depth are involved, as are the smoothness of the road surface and whether it's grooved to help divert water.

The biggest factor is speed. The faster the tires roll, the less time water has to get out of the way. This is why the most common piece of advice for recovering from a hydroplane is to take your foot off the gas. But don't try to slow down by braking -- you could lock your tires and make the skid worse.

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