The 12 Hours of Sebring: The Night and Day Physics Test

Audi R15 TDI
Audi has dominated the 24 Hours of Le Mans during this decade, winning the French classic every year since 2000 except 2003. The past three years, the diesel-powered Audi R8 TDI has triumphed, a showcase for Audi's diesel technology with its turbocharged, 600-horsepower, 5.5-liter V12 diesel. The Le Mans regulations for 2009 have changed, restricting the air inlets for diesel engines by ten percent and boost pressure by seven percent. Audi has changed with it, creating the R15 TDI.

The centerpiece of the car is its turbocharged 5.5-liter V10 diesel, a lighter, more compact version of the 90-degree V12. As before, it uses a five-speed gearbox. It still produces more than 600 hp and it develops 774 pound-feet of torque. And as before, it uses filters to reduce particulate emissions, a measure to demonstrate Audi's corporate endorsement of clean diesel performance. The R15 chassis features a F1-style high-nose aerodynamic package, and the components have been packaged close to the center line in F1 fashion as well. In addition, the 1,984-pound car has a longer wheelbase than before, a measure to move more weight toward the front of the car, as the tail-heavy R10 diesel (essentially a converted R8) understeered in slow- and medium-speed corners.

The most interesting choice is the roadster configuration of the Audi R15 TDI. Sleek aerodynamic coupes are now permitted by the rules and should theoretically produce higher top speeds, but Audi believes a coupe complicates of driver exchanges, limits visibility, and adds weight.

Peugeot 908 HDi FAP          
First unveiled in 2006, the Peugeot 908 HDi FAP didn't appear at the 24 Hours of Le Mans until 2008, as its team sought to carefully develop its speed and reliability. Last year, its turbocharged 5.5-liter V12 diesel developed more than 700 hp and was rated at 890 lb-ft of torque, about ten percent more than the Audi V12. It carries a six-speed gearbox.

Peugeot chose a coupe configuration to maximize the car's top speed at Le Mans, and further benefited from structural rigidity from its chassis. A coupe is also required by the rules to have an air-conditioning system to create a healthy environment for its drivers, and a larger air inlet is allowed to compensate for the engine power required to run the system.

Just as at the 2008 24 hours of Le Mans, the Peugeot 908 showed a useful margin of top speed over its completion from Audi. As a result, it raced its German rival head-to-head for virtually the whole 12 hours. But just as at Le Mans, its aerodynamics appear to be very critical, and sudden reductions in downforce can make the car difficult to drive and also reduce tire temperatures sufficiently to compromise lap times. Though a complement of F1-trained drivers has been hired to wheel the car, even they couldn't keep it pointed forwards all the time, while cooler nighttime temperatures seemed to affect tire temperatures.

Acura ARX 02a
The Acura 0ARX 02a unexpected earned the pole position at the Sebring 12 Hours in the hands of former Indy 500 champion Gil De Ferran, driving his first endurance race. Designed by Nick Wirth for competition in the U.S.-based ALMS series, the ARX 02a is meant to do its best at the mix of medium-speed road courses and street-based circuits in America.

Its key feature is the use of front tires that are the same dimensions as the rears, dramatically improving cornering speed. Special measures had to be taken to improve the durability of the 02a's power steering and front suspension to withstand the load, and indeed there has been concern about the impact on driver fatigue that might be produced by consistent cornering loads that approach 4.0g.

By winning the pole at Sebring, the Acura ARX 02a proved that it can uses its radical chassis and normally aspirated 4.0-liter V8 to turn a quick lap time. In the race, however, its didn't have enough top speed to be able to hold back the Audi R15 and Peugeot 908 long enough to get an advantage with its cornering speed.

Lola B09/86-Mazda MZR-R
The LMP2 class is typically composed of proprietary chassis powered by pure racing engines, and the Acura ARX 01b and Porsche RS Spyder that dominated last year's ALMS events. Mazda's entry the last two years has been a notable exception, as its turbocharged MZR-R engine uses the same inline-4 cylinder block seen in its production cars and adapted to everything from single-seat Formula Atlantic cars to 1,500-hp drag racing cars.

It will be interesting to see if this Cosworth-prepared turbocharged version of the MZR will be able to combine with the aerodynamic advantages of a coupe chassis and first-class preparation from Dyson Racing to get on terms with the highly developed Acura ARX 01b. Not at Sebring, however, as both Mazda entries failed to finish.
  


BMW M3 GT2
The basis of sports car racing lies with the kind of cars that you see on the street, so it has been refreshing to see the growth of the ALMS GT2 class, which features truly production-based cars, unlike the expensive, high-tech GT1 cars. Indeed, Chevrolet is developing a Corvette C6 GT2 car to replace its GT1 Corvette C6.R, which has dominated its class at both the 24 Hours of Le Mans and in the ALMS.

A BMW M3 GT2 developed by BMW Motorsport and fielded by Rahal Letterman Racing joined the GT2 field at Sebring, which features the Aston Martin DB9 GT2, Doran Ford GT40, Ferrari 430 Berlinetta, and Porsche 911 RSR. The 2,745-pound (dry) is recognizable as a street car beneath the decals, but the dry-sump 4.0-liter V8 is rated at more than 450 hp and 380-lb of torque. -Michael Jordan