# Can Wind Really Turn a Stock Car?

Equilibrium. It’s more than just a neat word. It’s the holy grail for a racecar driver.

Brian Vickers lost his car on the first lap of yesterday’s Michigan Sprint Cup Race. Vickers said:

“I was going into Turn 3 and expecting to follow the 48 in there and the 22 jumped inside of us and it just came around,” Vickers said. “I mean I just lost it. I have no idea what happened or why. The car just got really loose into three and I chased it all the way up to the wall. I thought I had it saved and it just came all the way around.”

A racecar driver’s goal is to keep the car exactly at equilibrium. Equilibrium means that all the forces acting on a object equal out.  For example, I’m sitting in a chair at my desk. Gravity pulls me down with a force equal to my weight. The chair pushes up with a force equal to my weight. If you add them up, they equal out. If the chair were to break, it would exert less force on me than gravity and I would accelerate downward.

The chair is actually capable of exerting a much larger force than my weight (which I know because people heavier than me have sat in this chair and it didn’t break.) Most things we use have a safety factor – they’re much stronger, or capable of exerting a greater force, than we will ever need.  We’re not even close to having to worry about equilibrium.

Racing is the act of keeping the car exactly at equilibrium. I like to think of equilibrium as applied to racecars like this:

When the forces are exactly balanced, you’re living up to car’s potential and getting it to go as fast as it’s capable of going.  If you’re not pushing the car to the limits of the tire’s traction, you’re giving up speed.  If you push the car beyond the tire’s limits, you crash. Look at the in-car cameras during a race and see how tenuous the connection is between the car and the track.

With the car perched on the top of an unstable equilibrium like the one diagrammed above, all it takes is a little perturbation and the car moves off the peak position. If the perturbation is small, the driver may be able to recover. But it doesn’t have to be very large – a good wind is more than enough – and the driver is caught in a spin. The side of a racecar presents a huge area for the wind to push on. It’s not surprising that a good wind, hitting at exactly the wrong angle, could spin out even a 3,480 lb racecar – because the racecar is already on the edge of crashing.

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