How to Make the C8 Corvette Handle Even Better
In technical director Kim Reynolds’ figure-eight testing report on the 2020 Chevrolet C8 Corvette Z51, he spoke of strong understeer, combined surprisingly with a fair amount of snap oversteer in this unique handling-limits test, one that MotorTrend has used for years as a standard of comparison.
I’d like to point out that a good chassis tester like Kim focuses on the negatives, elucidating where it can be better. It’s something I’ve always done as a racer, too, and is a key to my successes. So I’m guessing that the Corvette engineers also read this report, and as a result, we were quite honored that they contacted us and offered to show us how their new supercar works when tuned more for handling at its limits.
Enter a team of MT testers and Chevy engineers at Virginia International Raceway.
What they did, as reported here, was to alter the wheel alignment with a small tweak to the front caster, and large increases in negative camber, front and rear. This is where the tops of the tires lean in, to compensate for the cornering loads and body roll of high g-force cornering. The only other modification was to decrease cold tire pressures by 6 pounds in front and 5 pounds at the rear, significant amounts, because racetracks heat tires much more that street driving does. Heating tires raises their pressures, and for optimum grip, it’s important not to let those pressures get too high.
So … how would I choose to tune the C8 for the track?
Well, the added camber is a real aid to tire grip, spreading the corner load more evenly across the tires, which will last much longer, too, as a result. But there were a couple dynamic quirks that beg for more attention.
I found that the esteemed Mr. Reynolds’ recognition of significant midcorner understeer to be still present at VIR, but it is less intrusive with the track’s increased camber and because the corners at VIR are mostly not the long, steady-state turns consistent with our figure eight.
VIR has a great majority of fast esses and 90-degree turns, with a lot less direction change. The quick initial steering response and improved shock control was very effective in these types of maneuvers.
However, the handling issue that surfaced on the racetrack was that the C8’s rear could come unglued when off power—something we call trailing throttle oversteer. But applying moderate power immediately stabilizes the chassis, a very beneficial characteristic. This is fascinating because it is the exact opposite behavior of the front-engine C7, with its terrific corner entry but snap power oversteer.
More weight to the rear helps traction under acceleration in the C8. It puts down the LS power very well from low speeds, but here’s another chance to be even better: There’s still too much midcorner understeer .
My hypothesis is that Team Corvette is working to compensate for the off-power oversteer tendency. My humble armchair advice to Mike Petrucci and his engineering team is to use the terrific limited-slip e-diff, a trick learned from years of racing the tail-heavy Porsche 911.
A limited-slip is a very strong tuning device when on or off throttle, as opposed to steady-state cornering. Tighten it up, and the car will want to go straighter and will be more predictable. I rather brashly suggested that they increase the lockup off throttle on that wonderfully tunable limited-slip e-diff. He countered that they wanted the car to point to the apex midcorner. “Oh, it does, sir, too much,” said I. “It’s nervous. Your customers will spin it. Make it more stable for when they do a sudden lift off the gas, which they will.”
Petrucci, who is a still-waters-run-deep type, calmly and quizzically replied, “I have never heard our drivers mention off-throttle oversteer.” Well, it’s there, big as life. (I’m not at all sure I said that out loud, but I sure thought it.)
Keep in mind these Chevy testers have been driving the twitchy C7 for years, and that colors their impressions. Also, power immediately stabilizes this chassis, to its credit. Finish braking and go right to power, and the issue mostly disappears (opposite of the C7, which makes it worse). And stability control covers it up. Also, there may well be a forest-for-the-trees effect. When the same testers drive the same cars for hours every day, they learn very well what it wants. But in a perfect car, it’s a natural thing, no adaptation needed. And isn’t perfect the goal?
With more stability from the action of the diff, Team Corvette can reduce the steady-state understeer simply by the book, with softer front or stiffer rear springs and bars. And as for the 80-plus-mph oversteer, hook the e-diff up longer on power, too, to reduce that tendency. This car is not powerful enough to be wheelspinning at that speed.
The capabilities of an electronically controlled limited-slip differential may be the magic bullet for squeezing the very best from the Corvette C8 at the track. It’s a fine machine, and these suggestions could put an even finer point on it, leaving no negatives at all on which to focus.
Editor’s Note: Randy Pobst is a two-time class winner of the 24 Hours of Daytona, and with more than 90 professional race wins, he’s a multiple-time class champion in myriad racing series. He has been a factory-supported driver by Porsche, Volvo, Mazda, and Audi. Numerous automakers have hired his diagnostic and testing services as a dynamics consultant during vehicle development.
Source : Erika Pizano Link