TECHNICALITIES

BY DON SHERMAN

Divine Guidance

“Three years ago, when I joined the Corvette group, I saw a major opportunity,” recalls Jim Mero, the vehicle dynamics engineer responsible for ride and handling development. “The sixth-generation was born with good but not great steering. Working on other programs at GM, I had witnessed what could be accomplished when a capable supplier teams with development engineers to create a world-class steering system.”

The engine may be the heart of a sports car, but the driver senses a car’s soul through its steering. Porsches, BMWs, Ferraris and other bluebloods earned their renown by providing impeccable steering sensitivity. Now that Corvette plays in the world-class league, every aspect of its performance — including steering feel — is measured by a platinum yardstick.

Mero used a “build it and they will come” tactic to get the steering upgrades he sought approved. “Talk gets you nowhere. Until you install improvements in a car that anyone can drive and experience for themselves, there are no believers,” he explains.
Corvettes are equipped with Magnasteer variable-effort rack-and-pinion steering manufactured by Delphi Steering Systems. The ‘magna’ part of the name refers to one of the two channels by which power assistance is tuned to achieve the desired dynamic characteristics. A magnetic field surrounding the pinion shaft is adjusted by an electronic controller to alter effort as desired. The second channel is hydraulic. As effort rises, the control valve attached to the pinion shaft closes to deliver hydraulic pressure supplied by an engine-driven pump to an assist cylinder integral with the steering rack.

Magnasteer’s electronic controller monitors car speed and lateral acceleration via sensors. Corvette engineers program the controller to reduce the amount of steering assistance as speed and cornering g’s rise. This allows the car to feel responsive to the driver’s inputs without sacrificing stability and predictability. Mero adds, “Generally speaking, lower steering friction yields a clearer communication between the tires and the driver. Steering effort must rise in a linear progression to send a clear message to the driver that the car is working harder in a corner.”

Mero sought improvements in linearity, sensitivity and precision beyond what could be achieved by routine external calibrations. To achieve these gains, he challenged Joel Birsching, a product engineer at Delphi, to dig deep inside the Corvette’s steering gear.

To hit the ambitious performance metrics GM engineers established, Birsching’s team changed every major internal component. Friction throughout the system was analyzed and materials and processes were altered as necessary to meet GM’s requirements. The gear set was redesigned with new geometry to improve how precisely and smoothly the teeth mesh together. Operating clearances were tightened. A new algorithm was created for the Magnasteer controller to provide higher on-center stiffness. (Engineers define stiffness as the amount of motion at the steering wheel rim before the car responds with a change in direction. Less motion equals higher stiffness.)

A concerted effort paid handsome dividends. Steering feel is notably improved in the 2008 Corvette. Chalk up one more category where America’s favorite sports car meets or exceeds the blueblood standard.

Winter-Technicalities-Art

photo by Blair Bunting

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