INSIGHT: IMSA DPi vs IndyCar engineering with CGR’s Shepherd

Danielle Shepherd, lead engineer for the No. 02 Chip Ganassi Racing Cadillac in the IMSA WeatherTech SportsCar Championship, is enjoying a stellar year in her first season engineering in sports cars. Until recently ensconced in CGR’s IndyCar team — and still lending a hand there, as she did at the Indianapolis 500 with Tony Kanaan — she assisted in Scott Dixon’s 2018 championship and was the simulation engineer for Alex Palou’s title run last season. On the sports car side, she led the No. 02 with Earl Bamber, Alex Lynn and Neel Jani to the 2022 Mobil 1 Twelve Hours of Sebring victory, and Bamber and Lynn are in the thick of the DPi title fight.

As the WeatherTech Championship heads to the streets of Belle Isle for the Chevrolet Sports Car Classic — its third track that it shares in common with the NTT IndyCar Series after Long Beach and Mid-Ohio and the second doubleheader with the IndyCar — RACER talked with Shepherd about some of the similarities and differences, both between her current and former duties and the challenges of engineering the two different platforms.

“I was doing simulation on the 10 car with Alex Palou last year. That was more of the computer simulation program, assisting the race engineer and trying to help them develop the tools on the simulation side that the race engineer can actually use during the event,” she explains. “It was more of the the preparation and helping the race engineer have what they need rather than doing the race engineering role.

“We’re still working to get the simulation program up on [the IMSA] side, getting ready for LMDh next year. We’re really putting some effort into that, so there is quite a bit on the simulation side that we’re working on here. So it definitely does translate, and having done it for a while, I feel like I have a good grasp on it and know what questions I can ask them and type of answers I can get out of a good sim program.”

In IndyCar, barring the different Chevrolet and Honda engines, everyone is using the same equipment. With DPi being a homologated class, the situation isn’t entirely different; the majority of the competitors are using the same Cadillac DPi-V.R, with a couple of Acuras in the mix as well. Both have limits to what the engineer can do. The strategies, even when race distances are similar, are rather different, however.

“It is kind of a different game. The rules between the different series make different strategies more valid,” she says. “In a race situation, given the two cars, you are obviously playing by different rules here. You have a different strategy in DPi than you do in IndyCar racing. That influences the way you approach the race and how you think about when you’re going to stop. You’ve got two drivers in one, you’ve only got to worry about one driver in the other. It’s still engineering a race car, but there are different tools that work better for the DPi, and there are different tools you tend to use on the IndyCar.”

The most obvious of those different tools is the adjustable front wing on the IndyCar. It’s an easy way to adjust balance during a practice session, or even mid-race — give the wing a couple of turns and you’re off. But on the DPi, Shepherd notes, it requires adjusting the whole car to move the center of pressure — and that front wing is the one tool an IndyCar has that she would love to have on a DPi. Mechanically, though, the two platforms are aren’t that different.

“The overarching things you can change are still the same. It’s still front and rear roll center, you’ve still got dampers you can adjust, you’ve got third springs you can play with, the Xtrac gearbox … it’s still the same overall mechanical pieces in both car,” Shepherd explains. “The IndyCar has finer gear ratios, so it’s easier to tune, but also harder because you have more gear ratios to choose from. On the DPi side, we have fewer options, but then I feel like we’re compromising more based on whatever stack we pick.”

At Detroit — where both series are racing for the final time on the Belle Isle circuit before the race moves downtown in 2023 — it’s all about getting the car to work over the bumps, something the Cadillac generally excels at compared to the Acura. Cadillac has won three of four races at Detroit in the DPi era, including a Ganassi win in 2021. One of the key components to managing the bumps is the skid wear, the plywood piece under the car that helps protect the bottom, but is also a component of the rules — that piece can’t be worn more than 5mm in any spot after the race.

“You want to get the car as low as you possibly can, but you have to deal with the bumps. You’re trying to optimize the aero on the car and you’re trying to optimize the ride, but you can’t run it anywhere you want because you have to abide by the skid wear rules,” she says.

Ask Shepherd which is easier, engineering IndyCar vs. DPi, and you’ll get an engineer’s answer: “It’s just different.” But would having more tools to play with off more ways to screw something up?

“More toys to play with, you’re going to have more toys on the floor,” she agrees with a laugh. “But you can also screw it up just as much with just a few. You can still take all your toys out of the toy box.”