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Automotive Industry

CAE Drives Ford Chassis Dynamics

Automotive Industries, June, 1999 by John McCormick

Extensive use of customized software helps engineers create vehicles that `drive like a Ford' -- and even better.

Jump into a new car or truck and you should know within 150 feet whether it has the right stuff. That's the view of Richard Parry-Jones, Ford's group vice president of vehicle development. "A vehicle must communicate to the driver in the first 50 meters a feeling of being fun to drive," asserts Parry-Jones. "We want to create an instantaneous recognition that `this car drives like a Ford.'"

Wanting your products to be recognized for top-notch driving dynamics is one thing; making it happen is quite another. An experienced rally driver himself, Parry-Jones aims to turn around Ford's hit-or-miss reputation for vehicle handling, ride and roadholding, with a new focus on the science of vehicle dynamics.

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To that end Ford's development engineers are leaning more heavily on the established tools of their trade -- objective and subjective testing, plus utilizing the latest software to carry out objective tests of virtual vehicles. In the field of computer aided engineering (CAE), as it applies to multi-body dynamics (the dynamics of any multi-part system, such as a suspension comer), Ford contends it has a distinct edge on the competition.

Their primary computer weapon is ADAMS, a CAE dynamics software developed by Ann Arbor, Mich.-based Mechanical Dynamics Inc. (MDI). "Everybody in the industry uses ADAMS (which stands for Automatic Dynamic Analysis of Multibody Systems) to do this kind of work," says Greg Stevens, Ford's North American vehicle dynamics manager. "But we've been the biggest users of that software for over a decade. Vehicle dynamics CAE is not like FEA (finite element analysis) or some of the other CAE technologies that are more mature. This multi-body dynamics is a much smaller field."

Ford has 50 dedicated ADAMS users globally -- reckoned to be the software's most extensive automotive OEM usage in the world. Every one of the automaker's new vehicle programs (including those of Jaguar and Mazda) now uses CAE dynamic testing. Ford currently builds and tests roughly 100,000 models a year on the computer -- 100 times more simulations than six years ago.

75,000 Lines of Custom Code

Two key evaluation processes -- determining the effects of variations in component tolerances and checking large numbers of buildable design combinations -- are simplified by Ford's use of vehicle dynamics CAE.

"It is very easy for us to build and simulate large numbers of models," explains Stevens. "If you want to see how tolerances were effecting vehicle performance with actual hardware, you would have to build up a lot of vehicles and see how (suspension) toe curves, for example, varied. That would be tremendously time consuming and costly."

Instead, the engineers run ADAMS simulations by formulating a description of the vehicle based on all the design information. "The software takes that information and creates a set of non-linear differential equations. Those equations are very difficult to solve and that's really what MDI is selling us, a package that will solve those equations. Those can take from 10 minutes to half an hour to solve."

Though such a process is far superior to the old method of educated guessing, the time element is still too long when it's necessary for engineers to give feedback on possible changes at a design review meeting.

For that reason, the virtual dynamics team has mapped out a large number of the possible design changes in advance and created a database of information on how a vehicle's responses can change, depending on different combinations of spring rates, sway bar rates, shock rates, and other design permutations.

"You can draw relationships from that database with simple curve-fitting techniques and they give you algebraic equations," explains Stevens. Armed with DOE (Design of Experiments) spreadsheets on laptop computers, the engineers can attend review meetings and produce answers within seconds.

ADAMS Rib

Among the dynamic metrics for which virtual CAE testing can give particularly accurate data are pitch in braking and accelerating maneuvers, as well as understeer and oversteer characteristics. The veracity of the ADAMS simulations can be checked through correlation exercises with kinematics and compliance test rigs, plus instrumented prototype vehicles tested on skid pads.

The depth of Ford's involvement with MDI (see sidebar) can be gauged by the fact that the car manufacturer has actually modified ADAMS to suit its own purposes.

"We always worked closely with MDI, but we often found that we needed some special capability in the tool that wasn't in their generally supported package," says Stevens. "I believe at last count we had written something like 75,000 lines of custom code that works with ADAMS to tailor it to fit our needs." He says it's relatively expensive to develop code for your own company and Ford wanted some of its Tier 1 suppliers to use the same software. So the automaker entered into an agreement with MDI to re-license a lot of the custom software that Ford had written.

Automotive Industry

CAE Drives Ford Chassis Dynamics

Automotive Industries, June, 1999 by John McCormick

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