Getting the Drift

The physics of losing traction

Recently I was watching the animated movie Cars with my automobile-obsessed four-year-old son, when an interesting and unexpected physics item made an appearance in one of the scenes.

Lightning McQueen, the arrogant young protagonist race car, is astonished when he can't make a left turn on a dirt track. When "Doc" explains that McQueen must turn right to go left, Lightning is annoyed and dumbfounded by the seemingly ridiculous logic of Doc's proposition. But Doc is right (no pun intended). What he is describing is the phenomenon known as "drifting."

The video above shows some good examples of drifting -- a popular technique among automotive thrill seekers (and recently featured on the big screen in movies like "Tokyo Drift"). Notice how the drivers turn the front wheels opposite to the direction of the turn. Why do they do this and how does it work?

Basically, in order to maneuver a turn, there must be friction between the tires and the road. Try driving around a turn on ice and you'll see what I mean. The friction force gripping the tires acts centripetally, pulling the car into its circular path around the curve in the road. The car's natural tendency is to go straight (see Newton's First Law on interia) but when you turn the front tires static friction grips them, preventing the car from sliding out along the straight path, and pulling it into a curved one. So normally when you want to turn left, you turn the front tires to the left and the friction force pulls you leftward.

However, in certain situations -- for example when traveling on loose gravel, encountering a patch of ice on the road, or hitting a turn too fast -- the rear tires lose their grip on the road. If that happens, then as you steer into the turn, the rear of the car will over-rotate in the direction of the turn, causing it to go into a spin. To compensate for this, you have to turn the front tires away from the spin, or opposite the direction of your turn. In the video, the drivers purposely use a variety of techniques to lose traction on the rear tires and start to drift.

It's a lot more disconcerting when the drifting is accidental.

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.