From the November 2015 issue
There’s no control in a car more central to driver involvement than steering. And there’s no control that juggles more variables to get satisfying feel—that just-right balance of how much force the driver has to apply to the steering wheel to change direction; how immediately and linearly the car responds to that input; and how much that effort and response change according to road conditions and grip levels.
We call this trifecta “effort, response, and feedback.” They’re particularly tricky to master now that electric power steering (EPS) is consigning hydraulic assist to history. EPS improves fuel economy, but the addition of an electric motor to the steering system has not benefited drivers. The drag and inertia of the motor and its connecting mechanisms create new challenges for engineers hoping to coax engaging, informative sensations through a car’s steering wheel. We spoke with engineers from three manufacturers—Cadillac, Mazda, and Porsche—currently turning out the best steering on the market. All three emphasize that all-around great steering starts with the mechanical layout: stiff structure, favorable suspension geometry, and minimal friction. To wit:
A Solid Foundation
Good steering starts with a rigid structure. Deflections anywhere in the suspension connections to the chassis cause delays, reduce steering response and precision, and complicate the job of providing feedback to the driver. Tony Roma, chief engineer on the Cadillac CTS and V cars, tells us: “On the V models, we added underbody shear panels and tower-to-tower braces. We swapped bushings for rod ends to sharpen steering response.” Porsche goes even further on cars with its optional Porsche Dynamic Chassis Control (PDCC). Manfred Harrer, Porsche’s director of vehicle dynamics and performance for production cars, says that PDCC quickens response without changing the steering calibration by tightening the dampers and anti-roll bars.
Compromise
Dave Coleman, vehicle development engineer at Mazda, explains the biggest challenge: “The problem is that what you and I call feedback, most people call disturbance.” That makes cultivating feedback a balancing act between the stiff components that sharpen response and the more compliant ones needed for a quiet and comfortable ride. Reducing friction throughout the steering system is critical; it allows engineers to fine-tune the damping in the system through software calibration. Good EPS systems even incorporate programming to compensate for the electric motor’s inertia. That’s just one of as many as 150 variables in EPS control software for engineers to adjust. Cadillac’s Roma says: “In tour [mode], the car should operate
with comfortable effort and minimal feedback. But in track mode, we turn off the algorithms for artificial lane centering, crown compensation, etc. That enables the EPS to focus on feedback to the driver.”
Geometry
The next step is optimizing wheel alignment. Caster is the angle between vertical and the axis about which the front wheels steer, and it’s critical for good steering. With greater caster, the tires’ contact patches move farther laterally as you steer the front wheels. That extra displacement produces larger self-aligning torque, which means greater self-aligning in the steering and a more natural effort. Mazda’s cars run seven degrees of caster. Porsche 911s run eight degrees. Most cars run half that much.
Rack Be Nimble
Porsche 911s with rear-wheel steering—the Turbo and GT3—use a quicker rack up front to improve steering response, relying on rear steering to maintain high-speed stability.
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