TECHNICALITIES

BY DON SHERMAN

Teaching ZR1 not to “dance” during takeoff
Hip Hop. Not.

When your mission is accelerating from zero to 60 mph in practically nothing flat, every 10th of a second counts. During ZR1 development, Corvette engineers were dismayed to find that zero-to-60 performance was less than anticipated. Major increases in power, torque and traction over the Z06 were not delivering the expected reduction in acceleration times.

“The problem,” reveals Corvette engineering development manager Dave Wickman, “was power hop.” During launch, the ZR1’s rear tires were sticking and releasing in an unfavorable manner, which excited the entire drive-line into a torsional resonance.

All mechanical systems resonate at some natural frequency. Think of a plucked harp string, a ringing church bell or the rim of a martini glass stroked by a damp finger. From the crankshaft to the rear tire patches, the ZR1’s driveline constitutes a mechanical system with a particular natural frequency. When excited by the staccato torque/no-torque conditions that follow an abrupt first-gear clutch engagement, developmental ZR1 mules were resonating — hopping — at the driveline’s natural frequency.

The traction characteristics of the new Michelin Pilot Sport 2 radials, combined with the torque characteristics of the new LS9 supercharged V8, the suspension dynamics provided by ZR1’s softer spring rates and the damping properties of the electronically controlled magneto-rheological (MR) shock absorbers, had driven the new Corvette prodigy into an uncharted corner of the performance map. Something had to be done.

Wickman’s development team went to work. “Substantially increasing the driveline’s stiffness would shift its natural frequency away from the resonance point,” notes Wickman. “Unfortunately, greater stiffness usually means added mass, something we strive to avoid.

“The smarter solution was decoupling the left and right sides of the driveline by using half-shafts with significantly different natural frequencies. In production, the ZR1’s left half-shaft is a 40mm (1.57-inch) solid steel rod while the right one is 33mm (1.30-inch) in diameter, yielding a 1.5 times difference in stiffness and, therefore, natural frequency.”

Programming the MR dampers was the second half of the solution. Corvette ride and handling engineer Jim Mero explains: “With the car at rest, the dampers provide essentially no damping. So we created a lift-dive algorithm (software) that would automatically configure the suspension for optimum launch traction.”

Following experimentation, the optimum arrangement turned out to be the full 100 percent of available rebound damping in front and 30 percent of maximum jounce damping at the rear. So, when the ZR1 settles back on its haunches following an abrupt clutch engagement, the rearward pitch motion and resulting damping forces supplement the rear tires’ static load. That maximizes tire-to-road adhesion.

The icing on the cake is ZR1’s traction control system. Mike Petrucci, responsible for chassis controls development, explains: “The section of the traction control logic applicable to launching the ZR1 aggressively from rest is programmed to allow a fruitful amount of rear-wheel slip as long as the steering is pointed dead-straight ahead. This allows owners to experience entertaining street performance. By keeping the traction control actively engaged, they should be able to come very close to the 3.4-second zero-to-60 acceleration figures we’ve measured in ZR1s under ideal circumstances.”

Problem solved.

ZR1

photo by Evan Klein

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