If Corvette chief engineer Tadge Juechter said “Corvette is the poster child for continuous improvement. Making our car look, feel and perform better is what we do every day.”

“Last year (2008), we added a premium steering gear in response to owners who felt that Corvettes could benefit from improved on-center feel and precision,” adds Juechter.

For 2009, we found another means of making Corvette steering even better.
Quicker steering ratios make a car feel nimble and responsive. But if the ratio is too quick, the car is nervous at highway speeds. Juechter’s ‘even better’ development is a variable-ratio rack-and-pinion steering gear that provides the best of both worlds.

Terrel Johnson, a Corvette engineering team member for five years, is the lead steering design engineer.

Johnson explains:

“The new variable-ratio steering provides the same 17.1 to 1 on-center ratio that we’ve used successfully for years. But when the steering wheel angle exceeds 15 degrees of left or right turning, the ratio begins speeding up.
The change is so gradual that the driver notices nothing unusual, but after a little more than half a turn of the steering wheel, the ratio has changed to 14.6 to 1. This trims the number of turns lock-to-lock from 2.78 to only 2.54, a 9-percent improvement.

“With multiple ratios in one steering gear, we’re able to tune the on-center zone for excellent stability and reduced sensitivity at highway speeds without making Corvettes feel sluggish around town. The quicker off-center ratio helps the car feel more nimble and highly maneuverable while parking or negotiating a U-turn.”

Asked exactly how the variable-ratio gearing is achieved, Johnson answered with two words: helix angles. Some digging revealed what he was talking about.

Inside the rack-and-pinion steering system, the pinion gear teeth encircle their shaft in a spiral called a helix. This configuration increases the number of pinion teeth in contact with rack teeth.
The greater the gear-to-gear contact area (more teeth engaged), the less the likelihood of lash when the rack-and-pinion assembly is under load. Lash is steering precision’s bitter enemy.

Pinion gear teeth must be evenly spaced because each tooth contacts the rack at least twice during the two-plus turns lock-to-lock.
But, since each rack tooth is engaged only once throughout the rack’s full travel, how the rack teeth are configured can vary.

At the center of the steering rack, the teeth are cut at a 14-degree helix angle. This causes the rack teeth to engage the pinion teeth at their roots, yielding minimal rack motion for each increment of pinion rotation.

At the end of the rack, the teeth are cut at an 18-degree helix angle. Now the rack teeth engage the pinion gear teeth near their outmost tips. That larger-radius point of contact causes the rack to move substantially more for each increment of pinion rotation. Presto: a faster steering ratio.

Variable-ratio steering is another example of a seemingly miniscule change that yields a noticeable improvement in the Corvette’s daily-driving behavior.

“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.