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The Right Car Handled the Right Way Can Drive You up the Wall

Young physicists find the specs to keep a race car off the beaten track



Matt Collins

Any kid who ever clutched the wheel of a parked car and vocalized engine noises such as “vroom, vroom, bbbbbbb, nehnehnehnehneh, vroooom” has thought about this basic physics question: If I went fast enough, could I drive along the racetrack's wall without falling?

In the spring of 1978 I actually went to the Indianapolis 500. And my biggest question was whether I could negotiate the incredibly slippery bathroom floors without falling. When you have hundreds of thousands of inebriates assembled before 10 in the morning, the bathroom floors will be slippery, trust me. As the great sportswriter Dan Jenkins described the Indy atmosphere in his 1991 semiautobiographical novel You Gotta Play Hurt, “There were cars and people as far as we could see, and the infield was already running with rivers of vomit, beer, grease, and smoke.” To be fair, the bathrooms were not all that greasy.

My second biggest question was whether some part of one of the vehicles was about to fly off into the stands and turn my obituary into a sidebar of the race coverage. But I digress.

The short answer to the driving-up-the-wall problem is, of course, that a car of the right mass moving at sufficient speed on a curved vertical surface could stay up there. Then again, Jenkins noted that “the wall had won more Indy 500s than A. J. Foyt, Wilbur Shaw, and all of the Unsers combined.” Therefore, professional drivers usually try very hard to avoid contact with the wall.

Nevertheless, four intrepid physics students at the University of Leicester in England crunched the numbers in the university's 2013 Journal of Physics Special Topics. (The publication gives Leicester's future physicists a place to ponder issues such as the “implications of our moon being made of cheese,” specifically Wensleydale. The upshot of that dire dairy debate: a cheese moon of the same volume would be less dense, thus imparting smaller gravitational forces and weakening the earth's tides. Hey, they said these topics were special.)

For the vertical-driving analysis, the students model their track on the Indy speedway. And as do all great physicists, they include some simplifying assumptions: “The track is circular rather than oval, [and] the vehicle is already traveling at a given speed on the vertical banking.” (Vertical banking also describes how your financial institution's service fees send you up a wall.)

At this point, the sideways driver is at the mercy of four forces: static friction between the tires and the surface; the normal force (basically the force with which the surface pushes back at an object, insisting on annoying the surface with its presence); gravity; and the “downward force” (also sideways in this case) of the car, which should have aerodynamic qualities that make it stick more to the wall the faster it goes.

And we're talking really, really fast. “The cars were whining by so fast,” Jenkins wrote, “we couldn't make out the decals.” At my Indy 500, I sat between the third and fourth turns, a short and therefore slow section of the track, and the cars were indeed still going so fast that even Rick Mears's were smears. As were the decals of Al Unser, Sr., the eventual winner.

The Leicester kids consider two vehicles, an Audi TT road car with a mass of 1,390 kilograms and an open-wheeled Penske-Reynard-Honda racing car, at 700 kg. (“It's impossible to escape the Penske logo at Indy,” Jenkins wrote. “You see it in your sleep.”) And at a dawdling 150 miles per hour, the Penskemobile will stick to that wall like an ExxonMobil ad on a driver's fire-resistant uniform. Although the heavier Audi will drop and roll.

The young physicists conclude, “Given the right vehicle, the vertically banked race track would be feasible. However, it is unlikely to ever become a reality as such a track would likely be both hugely expensive and very dangerous in the event of a crash.” Jenkins also considered a hypothetical: “I [offered] a suggestion on how to make automobile racing more interesting and prove who the best driver was, really. Two-way traffic.”

This article was originally published with the title "Being Driven Up the Wall."

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