Situation: A “rough idle” problem was harming the sales of, and creating high warranty costs, for Ford’s Escort model in the early 1990’s. At idle speed, the car’s engine generated dynamic forces at a frequency close to the natural frequency of the steering column, resulting in a violent shaking of the steering wheel. These “rough idle” vibrations became especially objectionable after adding a driver air bag. A rather costly reinforcement of the steering column alleviated the shaking, but its intensity was still unacceptable. A damper – a one-pound lead inertia mass – was attached to the steering column, but the shaking was still much more violent than in other compact cars in the market. No space was available for a larger inertia mass. Increasing the idle rotational speed was also ruled out, since that would impair fuel efficiency. A task force comprised of senior engineers from the engine, transmission and electrical divisions, worked for several months and mapped out long-range projects (maxi-problems) to solve the problem. Millions of dollars in warranty costs, however, were mounting monthly. The TRIZ Group was hired to resolve the problem.
Actions: Since an immediate solution was required, the only quick, practical approach was to suppress the shaking with an effective damper. The conflict: A damper with a large inertia mass could adequately abate the shaking of the steering column, but it was unacceptable. According to TRIZ, the ideal inertia mass is absent, but its function is fully performed. This called for removing the existing lead block and assigning its function to a resource component elsewhere in the car, preferably to the steering column itself. The vehicle’s structure was evaluated, and massive structural components that could be used as inertia mass were identified (air bag, spare tire, battery, etc.). Tests showed that the air bag was the most effective component. To use the air bag as a damping inertia mass, it was separated from the steering column and reattached with flexible tunable connectors.
Results: The shake amplitudes of the steering wheel were reduced 6-7 times below those of the (then) best-in-class Toyota Corolla. A U.S. patent was granted for this solution.