Driving demands and chassis set-up vary greatly between the up-and-down Barber Motorsports Park and the exceptionally flat Indianapolis Motor Speedway road course and, as Chip Ganassi Racing Teams engineer Brad Goldberg says, the pair of long straights at IMS drive how the 25 cars are tuned for the inaugural Grand Prix of Indianapolis.
"IMS has such long straights it's really a matter of how much aerodynamic drag you can stand to take off the cars without losing overall grip," said Goldberg, the race engineer of the No. 83 Novo Nordisk car driven by Charlie Kimball. "Therefore, unlike Barber, with the rolling hills and the need to support the car on all the compressions and long corners that generate massive loads in the car, everything at IMS is generally a slower corner where the support generally isn't needed. The balance act is biasing your set-up for slow-speed mechanical grip, with low drag for all-out speed."
The Chevrolet- and Honda-powered Verizon IndyCar Series cars approach 200 mph entering Turn 1 of the 2.439-mile, 14-turn circuit and close in on 190 mph on heading to Turn 7, making straightline speed a priority. The IMS road course also features a number of high-speed corners, but with low-downforce settings on the cars, drivers are definitely holding on during fast laps and toward the end of long stints.
"So far, we've been trimming out and trimming out and going faster and going faster," said Justin Wilson, driver of the No. 19 Dale Coyne Racing car. "So it seems like the less downforce the better, but it's a quite high-grip surface so it is still high commitment for the corners. The car isn't squirmy or moving around. It's an abrasive surface so it wears out the tires quickly, but we did a lot of laps on our set (May 8 in practice)."
Altering the chassis set-up to go quick in the corners while surrendering some top speed, as Goldberg explains, wouldn't be the best solution to try this weekend.
"It would be very hard to have a high downforce set-up and not get passed on the straights in the race, especially since the corners leading onto the straights are slower speed," he said. "Downforce is generated by air speed relative to the car, and if your airspeed is low, downforce is low, and low downforce equals low aero grip. In qualifying, it might be a bit more advantageous to have a higher downforce as you are looking for the best outright lap time. For instance, at IMS you are full throttle for roughly 62 percent of the lap, while at Barber it's only 56 percent. Six percent is actually quite a big difference."
Most natural-terrain road courses are known for having a significant number of bumps that disrupt a car's balance, but with the newly-paved IMS road course engineers have adjusted their set-ups to account for the smooth surface.
"We make these tuning changes through the dampers," Goldberg explained. "At bumpy tracks, the focus shifts a bit to support the car over and across the bumps, which may not be the best for grip. With IMS being so smooth, the focus shifts solely on to grip. IMS also allows you to lower the ride heights as you are not worried about bouncing over bumps. Lower ride heights generate more downforce, and between the high-speed straights and the lack of bumps, IMS is a completely different animal that we have to tackle. To me, it is pretty interesting since the Indy oval is a completely different oval animal, which adds to the uniqueness of this month."