Magic Numbers in Design of Suspensions for Passenger Cars

Design specifications for suspensions of passenger cars for ride and handling in the past half century were amazingly associated with some “Magic Numbers.” These parameter values are needed for mathematical models which describe the dynamic motions of a vehicle on the road, help researchers to validate the overall performance of their simulations. Moreover, these numbers should be viewed as a guide for evaluation of vehicle design practice for moderate payload and driving conditions. Number 1, for instance, is the magic number associated with bounce resonant frequency of the sprung mass, but also related to the desired value for the dynamic index for ride quality and directional control. Number 10, on the other hand, is the magic number associated with wheel hop resonant frequency and the desired total mass/-unsprung masses ratio. This paper demonstrates through some illustrative, examples, computer simulation, laboratory and road test results, that since the introduction of the Independent Front Suspension (IFS) in the automobile industry more than 50 years ago, a series of specific numbers govern vehicle design for ride and handling. Moreover, these numbers will continue to control our design philosophy in the next decade regardless of high technology concepts such as active suspension and four-wheel steer. These magic numbers are timeless. They are the product of the so-called “Past Experience” and/or “The Slide-Rule Generation” in the automobile industry. In this respect, despite innovations in suspension design the reality is that there is no chanse in these design specifications.