Aerodynamic and Dynamic Study of a Hillclimb Car

Topics: Resonance, Damping, Harmonic oscillator Pages: 4 (1148 words) Published: August 15, 2011
Choosing baseline damper curves for ride. Last time, we discussed the process of choosing proper baseline spring rates. In this paper, we will continue the discussion on suspension components by simplifying the process of choosing appropriate baseline damper settings for ride. What is damping? In a spring-mass system, any displacement and release of the mass from its equilibrium position will cause the mass to oscillate. If the system were ideal, the mass would continue vibrating at a given frequency (its natural frequency) indefinitely with unchanged amplitude. Introducing damping into the system causes the oscillation to trail off and forces the system to reach a steady state value. Going back to physics, the simplest equation of a damper is F = CV, where F is the force exerted by the damper, C is the damping coefficient, and V is the velocity of the damper. So why did we go through all this? In order to introduce the damping ratio – this is where you want to start in determining what kind of damper to use. Damping ratio The damping ratio, usually designated as ζ, is defined as the ratio of actual damping coefficient to the critical damping coefficient. The reason why we work with damping ratios instead of actual damping coefficients is so that we can normalize the discussion for all dampers. Choosing a damping ratio is a generally a tradeoff between response time and overshoot (you want to minimize both). Typically, passenger cars will use a damping ratio of around 0.25 to maximize ride comfort. For a racecar, the damping must be considerably higher for road holding and control of the unsprung mass motion. Data has shown that for racecars, a good baseline for damping ratio is between 0.65 and 0.70. Transmissibility There is one more concept that should be understood before introducing the damping curve – the concept of transmissibility. Transmissibility is defined as the ratio between output and input amplitude. In our application, the input amplitude will be...

References: Brisson, Samuel & Giaraffa, Matt. “Tech Tip: Spring & Dampers, Episode Four”. OptimumG, Denver, CO. Milliken, Douglas L. & Milliken, William F. Race Car Vehicle Dynamics. Society of Automotive Engineers, Warrendale, PA, 1995.
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