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In the Garage: Why Is Everyone Talking About “Balance” At INDYCAR?

Will Power's mechanic works on a spare rear wing for the Team Penske driver. [Eddie LePine Photo]

Will Power’s mechanic works on a spare rear wing for the Team Penske driver. [Eddie LePine Photo]

What’s up with all this talk about “balance” in INDYCAR?

Setting aside ideas of life/work harmony and competitive parity, INDYCAR drivers are mainly referring to how their cars are working with regard to both mechanical (center of gravity) and aerodynamic (center of pressure) aspects of performance.

“When you have a car that’s balanced—I’m not sure if you watched Helio Castroneves’s car in Long Beach during qualifying,” said reigning Indianapolis 500 champion Ryan Hunter-Reay on Wednesday, “—but you can see the car (a Chevrolet) is very well balanced: the front is working, it’s cornering well and it’s putting the power down well.”

Race car balance is affected by multiple inputs: engine, tires, suspension, road, aerodynamics, and of course the driver.

All other things being equal, the input that has been modified most obviously in the 2015 INDYCAR season is the aerodynamic interaction between the various body components.

Chevrolet and their aerodynamics partner Pratt and Whitney have taken a straightforward wings and tunnels approach; while Honda has tuned its body kit to some exotic formations that borrow from the past and suggest the future.

Regardless of philosophy, the result is a significant, distinct competitive edge for Chevy on the track and at the podium (so far) by various methods to create and shape downforce such as inverted wings, diffusers, and vortex generators.

Hunter-Reay drives a Honda, of course.

So what has Chevy done that makes a championship-level driver like Hunter-Reay envious?

To put it simply Chevrolet and its teams have learned where to apply the external downforce generated by a number of aero parts across their cars onto the best places, where they produce the greatest gain in performance for the way their drivers drive.

In case you are wondering, the best places to apply downforce are the four tires, those decidedly last-century pieces of technology at the corners of the race car. And by so doing they have pulled more performance from their tires than the Honda crew have done (so far).

By controlling the downforce distribution between the front and rear wheels, the vehicle stability is altered favorably, and by relying on the tires’ increased performance rather than on engine, suspension, road or driver, Chevy is bringing home trophies.

So how does Honda catch up?

Typical total downforce and percent of front downforce (%F) requirements for various race track conditions are fairly consistent across open-wheel formulae and widely available.

At the beginning of the season Honda was earnest in insisting its aero changes to the Dallara DW12 add approximately 1000 lbs of additional downforce, bringing the maximum downforce on the chassis to roughly 4500-5000 lbs.

These numbers are not significantly different from Chevrolet’s.

Published literature, which is rare in open-wheel racing as teams tend to be secretive in their data-collection, suggests the following front wing downforce parameters for varying track lengths and geometries:

• Road course 45%
• Short oval 35%
• Long oval 35%
• Super speedway 33%

This means that the aerodynamic sweet spot that engineers refer to as the “center of pressure” must be behind the vehicle center of gravity. The resultant “seat of the pants” feel to the driver is referred to as balance, or more appropriately the proper ratio of downforce between the front and rear tires.

One might ask why Honda just doesn’t set the front/rear wing downforce ratios as suggested above and be done with it.

The answer is that aerodynamic downforce can be generated both by adding wings or by using the vehicle’s body.

If you open an aerodynamics book the text will explain it thus: coupled configuration downforce is much larger than the combined (but far apart) contribution of the body and the wing alone.

Again, all things being equal (e.g., ground clearance and wing angle) the interaction of the various components on the external surface of the race car create the strongest interactions to influence performance.

The author is making such a big deal of this because if you notice, on the Honda, there is a dorsal fin running along the top and rear of the engine cowling.

That fin, which was first added to race cars nearly 80 years ago, is intended to influence rotational stability at high speed.

The earliest racers were limited to the extreme by their tires. As a means of controlling the sideways slip of early tires at the limits of traction manufacturers experimented with these longitudinal wings to act as windbreaks and help keep their cars on line in an arc around a high-speed corner.

In situations of large side-slip, such as cornering at Indianapolis Motor Speedway at speeds well over 200 mph, the smaller variation in yaw (roll) derived from the longitudinal fin should result in lessened rotational moment and afford a slightly greater tire patch in contact with the diamond-ground surface.

In lay terms, a more stable car through the corners than the Chevrolet, and more rubber meeting the road when the driver unwinds the wheel and the throttle is pounded home out of the turn.

Computational fluid dynamics and shaker-rigs and conjecture can’t make up for time on the track; and it won’t be until the INDYCAR series tests at IMS on May 3rd that we will have any evidence that the kind of balance known as “parity” is restored when super-speedway racing commences.

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