Speed Not The Problem For IRL

Speed management is a high-profile issue in NASCAR when the series travels to its fastest venues, like Daytona International Speedway and the track where they are racing this weekend – Talladega Superspeedway. But while NASCAR tends to restrict engine horsepower to control velocity, the Indy Racing League tweaks its aero package to keep the cars under control.

The importance of establishing appropriate speeds for the bigger tracks is imperative, and one only has to look at last year’s Firestone Firehawk 600 at Texas Motor Speedway to see why. The CART FedEx Championship series race was cancelled after many drivers experienced dizziness and light-headedness while traveling around the high-banked, 1.5-mile oval at speeds nearing 236 mph.

While Nazareth Speedway’s low-banked, 1-mile oval is configured differently than Texas Motor Speedway, aerodynamic considerations will still be very important for the safety and success of the IRL drivers racing in the Firestone Indy 225 this weekend.

"An IRL race car is shaped and tuned to make maximum use of ground effect," said Kevin Bayless, a chassis and aerodynamics consultant for GM Racing. "Ground effect is the interaction between the moving vehicle and the stationary track surface.

“The underside of the sidepods forms a venturi between the car and the track. The air beneath the sidepod accelerates as it moves through the venturi, creating a low-pressure area. The difference in pressure between the upper and lower surfaces of the sidepod produces downforce, or negative lift.”

Downforce is a very desirable characteristic to have when racing because it compresses the tires against the track, allowing cars to corner quicker, resulting in lower lap times. The amount of downforce an IRL car produces is heavily regulated by the series, with mandatory minimum rear-wing angles and wicker bills (right-angle bends at the rear of the wings) for most events.

But with the benefits of downforce also come the disadvantages.

"The downside of downforce is that it almost always comes with the penalty of drag," Bayless said. "Drag is the fore-and-aft force that pushes back against the car and slows it down."

At a technical track like Nazareth, with an elevation change of 34 feet, teams will set up the cars to provide a high level of downforce.

"The amount of downforce an IRL race car produces can be adjusted to suit various tracks," Bayless said. "At a high-speed track like the Indianapolis Motor Speedway, the cars are trimmed out to produce 1,700 to 2,000 pounds of downforce with minimum drag.

“On the other hand, at a one-mile track like Nazareth, the cars are set up to produce up to 4,000 pounds of downforce. On a short oval, the increase in corner speeds produced by the extra downforce more than compensates for the speed penalty produced by additional drag on the straights."

Besides adjusting wings to obtain optimum downforce at any given track, teams also have to factor in the exposed wheels of IRL cars into their setup equation.

"A rotating cylinder like a tire and wheel combination creates both drag and lift in a moving air stream," Bayless said. "The spinning tire has very high velocity at the top and piles up air at the bottom, two conditions which produce lift. There also is tremendous turbulence behind the tire that creates drag.

"The turbulence produced by the open wheels disrupts the airflow around the rest of the car, affecting the aerodynamic performance of the sidepods and wings. The ramps in front of the rear wheels are designed to deflect the air to minimize these lift and drag effects. Of course, the ramps are also regulated by the IRL rules."

It’s a complicated process that changes from track to track. But come Sunday, IRL drivers will be turning quick laps around Nazareth Speedway, their downforce dialed in.

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