Technical Analysis of a Proposed Shock Absorber Design Standard 2016-01-1543
One important part of the vehicle design process is suspension design and tuning. This is typically performed by design engineers, experienced expert evaluators, and assistance from vehicle dynamics engineers and their computer simulation tools. Automotive suspensions have two primary functions: passenger and cargo isolation and vehicle control. Suspension design, kinematics, compliance, and damping, play a key role in those primary functions and impact a vehicles ride, handling, steering, and braking dynamics. The development and tuning of a vehicle kinematics, compliance, and damping characteristic is done by expert evaluators who perform a variety of on road evaluations under different loading configurations and on a variety of road surfaces. This “tuning” is done with a focus on meeting certain target characteristics for ride, handling, and steering One part of this process is the development and tuning of the damping characteristics of the shock absorbers. This process if quite involved as there are many variables to adjust and while some characteristic may be good for one type of road or circumstance they be less desirable on others. This leads to a process of evaluating and tuning over a number of surfaces and conditions to develop the proper package.
Recently, in a series of three American Society of Mechanical Engineers (ASME) papers published by employees of an Arkansas firm (Engineering Institute or EI), a new and supposedly “novel” approach to shock absorber damping tuning was presented [1, 2, 3]. The papers propose a simple procedure which supposedly provides an automotive engineer with a method by which rear suspension shock absorber damping could be easily selected to provide appropriate damping to the vehicle. The work is based on experiments where three large rubber blocks are glued to single tire so that when the vehicle is driven it forces the rear suspension to hop and tramp. Several maneuvers modeled after the test standard SAE J266, Steady-State Directional Control Characteristics for Passenger Cars and Light Trucks , are then run. An objective metric based the maximum difference in steering from the vehicles calculated Ackerman angle is proposed to gauge vehicle performance. The authors of those papers even went as far as to declare vehicles with a “maximum steering delta” in their proposed test that exceeds four degrees as unsafe and defective. Additionally a theoretical model that assigns a percent critical damping value to a given set of shock absorber damping values, estimated suspension inertia, stiffness, and geometry was used. The authors of that paper have stated that vehicles with less than twenty percent critical damping, using that model, are unsafe. The authors attempt to support this opinion by plotting the “maximum steering delta” versus the calculated percent of critical damping and claiming a linear relationship.
This paper performed an in depth study into this proposed shock absorber damping design standard and vehicle performance metric as well as an in depth study on the supporting tests and studies that were used to support the theory. Additional testing was conducted on several production vehicles to evaluate the proposed methodology and metric. The mainstream technical community has endorsed the proposition that any test adopted as a standard for vehicle performance must be: reliable, repeatable, objectively quantifiable, and possess criteria that reasonably relates to vehicle usage in the real world. This paper employs that approach in a technical analysis of the proposed damping metric as well as the protocols for the tests themselves.