Tech Briefs
Ford driving dynamics

Ford vehicle dynamics definitions Vehicle Characteristics Slip angle—the angle between the direction a tire is pointed and the direction it is moving. Some slip angle is required to generate cornering force. Understeer—a cornering condition in which the front slip angle is greater than the rear slip angle. A vehicle with extreme understeer will tend to keep going straight when the steering wheel is turned. Understeer is sometimes referred to as "push" or "plow." A slight amount of understeer is desirable for stability. Oversteer—a cornering condition in which the rear slip angle is greater than the front slip angle. A vehicle with extreme oversteer will tend to spin out in a hard corner. An oversteer vehicle is sometimes referred to as "loose." Ford production vehicles are designed with slight understeer. Neutral steer—a cornering condition in which the front and rear slip angles are equal. A neutral steer configuration provides maximum cornering performance but makes the vehicle less stable and more difficult to drive than a vehicle with slight understeer. Pitch—a vehicle is said to pitch when the front end rises while the rear end falls, or vice versa, such as in a fore-and-aft rocking motion. Nose-dive is an example of pitch. Roll—rotation of a vehicle about its longitudinal axis. An example of roll is body lean to the left or right during a cornering maneuver. Yaw—rotation about the vertical axis. A vehicle spinning out in a turn is an example of extreme yaw motion. Boom—an undesirable noise caused by high-frequency vibrations resonating in the body of a vehicle. Ride—low-frequency body motion and vibration (up to 5 hz) when a vehicle drives over imperfect roads. Ride most often refers to vertical motions, but pitch and roll motions also contribute to ride. Shake—intermediate-frequency vibrations (5-25 hz). Harshness—high-frequency vibrations (25-100 hz). Chassis Attributes Jounce—wheel motion that compresses the suspension (wheel moves up relative to body). Rebound—wheel motion that extends the suspension (wheel moves down relative to body). Ride height—distance between vehicle wheel lip (fender) and the road surface. Roll stiffness—rigidity in a vehicle suspension which resists roll motion. High roll stiffness results in less body lean during cornering. Ride steer—the tendency of a wheel to steer slightly to one side or the other due to jounce and rebound motions of the suspension. Kinematics—a description of the path of the wheel center and the changes in toe, caster, and camber as the suspension is moved through jounce and rebound motions. Compliance—wheel orientation will change with cornering and vertical loads due to suspension deflection. Compliance refers to the movement of the wheel center and the changes in toe, caster, and camber as forces are applied to the wheel. Wheel Alignment Steering axis—the line through the center of the ball joints in a front suspension, sometimes called the kingpin axis. Camber angle—in a front view of a vehicle, the angle a tire leans from the vertical. Positive camber means the top of the tire is leaning away from the vehicle. Negative camber means the top of the tire is tilted in toward the center of the vehicle. Caster angle—in a side view of a vehicle, the angle of the steering axis relative to vertical. Positive caster means the top of the steering axis is leaned toward the rear of the vehicle. Toe angle—in a top view of a vehicle, the angle of the tire relative to a line running along the length of the body. Toe-in means the forward portion of the wheels are pointed slightly in toward the vehicle center, while toe-out means the wheels are pointed slightly away from the vehicle center.

Acar or truck should reassure the driver with a sense of stability, precision and comfort, while also delivering spirited agility, according to Richard Parry-Jones, Ford Group Vice President - Product Development.

Ford vehicle dynamics engineers make use of three disciplines in their development process: objective testing, subjective testing, and computer-aided engineering (CAE). Objective testing includes data gathering at Ford's Kinematics and Compliance labs as well as on-road and on-track testing of instrumented prototypes. Subjective testing includes extensive driving of prototypes and benchmark vehicles on a variety of road surfaces.

CAE is objective testing of virtual vehicles. CAE models give engineers almost unlimited flexibility in analyzing and optimizing component and subsystem options on the computer to achieve breakthrough dynamic performance. These virtual prototypes are critical to Ford's goal of further reducing product development times and costs.

Dynamic attributes that Ford engineers focus on include: agility, comfort, precision, and stability. Early on, the vehicle team sets target values for each of these attributes that will define the vehicle's "dynamic identity." Customer expectations are an important consideration. For example, comfort is especially important on a Lincoln Town Car; agility is essential to the Puma's dynamic identity.

The four dynamic attributes are closely linked. Agility is the attribute that enables a car to be fun to drive. A responsive and energetic vehicle invites the driver to find a winding road. To optimize a new vehicle's agility, development engineers must be especially mindful of stability and precision—or else the vehicle may become "nervous" or "darty." Stability gives the driver confidence because it makes the vehicle's dynamic performance predictable.

Precision as a dynamic attribute applies to steering, handling, and braking. A vehicle's steering response from on-center and off-center should be immediate, yet it should have a strong center-feel. Braking should be fade-free. Brake pedal travel, effort, and response should be well connected.

Comfort to a Ford dynamics engineer means removing float and shake, minimizing harshness, and controlling roll and pitch to acceptable limits.

Virtual vehicle output by Ford vehicle dynamics engineers has grown a hundred-fold in the past five years. Virtual vehicle prototypes—computer simulations—are used to design and test numerous combinations of components that define a vehicle's unique ride, handling, steering, and braking characteristics.

"We build and test roughly 100,000 models a year on the computer, compared to maybe 1000 in 1993," explains Greg Stevens, the CAE (computer aided engineering) supervisor who leads the development of the software tool set now used worldwide by some 50 Ford vehicle dynamics engineers in North America, Europe, Australia, and Japan. "This common tool set is used on every new vehicle program at Ford, as well as at Jaguar and Mazda."

"CAE has many strengths, but no computer can ever totally replace the feel, talent, and intuition of an experienced development engineer," says Stevens. "It is important to use each of the three tools in the area where it is most beneficial."

CAE modeling is especially helpful to:

• tune designs early in the development cycle, before physical prototypes are available
• test designs in an environment without test-to-test variability or "noise"
• determine ways to make designs less sensitive to variations in component tolerances
• evaluate the large number of buildable design combinations found on most vehicle programs.

After initial data gathering, many design possibilities can be pre-tuned and evaluated quickly and inexpensively in CAE before the first fully developed physical prototypes are built. "It's just a matter of minutes to build and test a CAE model," says Stevens. "And if I want to change the suspension geometry, like where a control arm attaches to the frame rail, I just update the data-set, re-run the test, and get the analysis back in 10 minutes.

"We can make changes to the design much more efficiently compared to working with a real prototype, where we'd have to take it into the shop, cut off the old brackets, weld on new ones, and fabricate a new control arm."

Another way in which CAE testing helps with problem-solving is by enabling individual components to be changed and tested with no extraneous "noise" to skew the results. "If we change the stabilizer bar, we know that any changes in vehicle response happen just because of actual physics," says Stevens, "not because a real prototype out on the track caught a sudden wind gust."

CAE's capacity to build and test thousands of virtual prototypes helps Ford engineers evaluate the many possible buildable combinations of a particular vehicle, as well as the ways driving dynamics can be affected by minor variations in the manufacture of various components.

"Each part can be slightly different from another one and still be within tolerance," says Stevens. "But when you stack up all the differences in all the different parts, does that affect the vehicle performance? With CAE we can determine that very easily. If we had to rely only on physical prototypes, we'd have to build and test thousands of vehicles.

"In the same way, we use CAE to evaluate a design with all the different build combinations, such as long wheelbase versus short wheelbase, sedan versus station wagon, 4x2 versus 4x4, and all the different springs, dampers, bushings, stabilizer bars, and tires.