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[h=2]IMSA: A closer look at Cadillac, Mazda DPis[/h] Marshall Pruett (words and images)

One fine gift from IMSA over the holidays has been parsing through the visual differences between the three new Daytona Prototype international models that will go head to head in its WeatherTech SportsCar Championship Prototype class.


Cadillac’s angular DPi-V.R and Mazda’s flowing RT24-P were seen in action during the Dec. 13-14 IMSA test at Daytona, and with the shots revealed of the Nissan Onroak DPi during its shakedown run at Sebring on Dec. 21, images of all three are now available. We’ll hold off on exploring the Nissan, however, until it’s available for technical photos. And with the Nissan exposed, an aerodynamic review of the three cars will also be forthcoming.
That means more images and comparisons will follow at the upcoming Roar Before the 24 on Jan. 6-8, so for now, here are some additional views and annotations of the Cadillac and Mazda to enjoy until the full DPi field returns to Daytona next week.

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1. With the shift to an all-P2-based formula, the DPis use a diffuser (below, top) that is far more streamlined and simple than the giant diffuser used to add downforce to DPs starting in 2014 (below, bottom).

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2. Although we haven’t seen the Nissan without its nose to look at its suspension arrangement, we do know the Dallara-built Cadillacs and Riley/Multimatic-built Mazdas use completely different suspension philosophies.
The DPi-V.Rs come with small, lightweight torsion bars (red) inside their housings at the front instead of bigger coilover springs. The front suspension pushrod connects to a rocker arm (purple) that twists the torsion bar, which replicates the act of compressing a spring. By using the torsion bar, less size and weight is carried at the front of the car, and on the other side of that rocker arm, a traditional damper (green) is used to control oscillations and manage ride characteristics. The anti-roll bar (yellow) is also seen up front spanning the dampers. It connects to both sides of the suspension through drop links attached to the rocker arms.

Take a look at the back of the Cadillac and a completely conventional suspension system is found – a pushrod (red) that turns a rocker arm (blue) that compresses a coilover spring and damper (green). With plenty of space in the engine back and no concerns for packaging, it works just fine. Up front, with a desire to reduce the DPi-V.R’s size and weight, the torsion bar gives Cadillac everything it was seeking.

For those interested in seeing different torsion bars, here’s a shot from the Ford GT pits showing pairs of bars with different spring rates to test. The thinner the torsion bar shaft is between the mounting splines (bottom), the lower the spring rate; the thicker, the higher (top), as shown below.

3. The Mazda, which had its suspension covered for the mid-December test (or at least every time our camera was nearby), makes use of an old but proven Riley suspension system. The packaging at the front of the car is also one of the great accomplishments on the car.
The stock Riley Mk 30 WEC P2 chassis comes with the same suspension, so we’ve used it to show what the Mazda has front and rear. The system, which Riley unveiled on its championship-winning IMSA WSCs in the mid-1990s, uses a large pushrod-activated (green) rocker arm (red) that compresses the spring (blue) and damper (yellow) separately. A tidy anti-roll bar (purple) is mounted low in the chassis and runs through the bellhousing.

Like the Cadillac, the wide open space at the back of the car makes it easy to implement its preferred suspension system. At the front of the car, Riley has built a recess into the top of the tub – above the driver’s feet – to fit the springs and dampers in place. The anti-roll bar (and more) is also included in the packaging. A carbon-fiber hatch (below) is used to cover the suspension and is easily removed to allow changes.

The split spring and damper system allows fast setup changes and also makes it easier for the team to try different motion ratios.
4. Yet another philosophical split comes in how the brakes are mounted on the Cadillacs and Mazdas. Having weight as low as possible in the car helps improve its center of gravity, and to aid this piece of physics, the RT24-P has its brake caliper and pads located almost at the bottom of the front suspension upright (below, top), and just a bit higher at the rear. The Cadillac, conforming to the polar moment of inertia approach, went for a more familiar rear mounting to centralize the DPi-V.R’s weight (below, bottom).

5. Returning to the Cadillac, and this feature is seen on all three DPis, we have a raised keel. To help move air below the cars and out and around the sidepods, Dallara, Onroak (Ligier) and Riley raise the front portion of their tubs to create a void below to manipulate the oncoming air.
It’s another old and proven design characteristic in the LMP1 and LMP2 cars, but for drivers accustomed to DPs that were not allowed to use raised floors to create keels (blue) and had their legs and feet angled down to touch the floor, nearly all have had to become comfortable with the new seating position where their legs and feet are now elevated to chest height.

The distance from the bottom of the Dallara tub (red) to the keel (green) illustrates how DPi drivers now sit in a position like they are performing an abdominal crunch.

6. The Dallara has an interesting hinged cockpit center piece – replacing the DP’s removable helmet net – that can be flipped up out of the way (green) during driver changes and snapped back into place with ease.

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7. Big cooling is required for the DPi brakes, and in addition to seeing a version of its splitter and underwing, the DPi-V.R’s brake duct hole and matching duct affixed to the chassis can be seen to feed the carbon discs and pads.

8. The one question we received more than any other after the mid-December test involved Wayne Taylor Racing’s unique chassis setup tools. The use of gym room weights on the front and back piqued the interest of many intrepid fans and most figured out their purpose – the plates are used as ballast to make up for the weight lost by removing the bodywork and wheels/tires to perform chassis setup.
Most teams perform their setups with only the driver’s weight accounted for (usually by placing sand bags or lead weights in the seat), but WTR prefers to account for the exact weight as-raced when making its tweaks and changes.

There’s a lot more to uncover and explain once the Roar arrives, so we’ll save our next installment for an in-depth look at all three cars. We’ve seen a lot of the Cadillacs, and there’s still more to show, but next time, there will be a heavy emphasis placed on the Mazda and Nissan.