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Tandy D.F.,Tandy Engineering and Associates Inc. | Beane S.,Ford Motor Company | Pascarella R.,Ford Motor Company
SAE Technical Papers | Year: 2014

There have been many articles published in the last decade or so concerning the components of an electronic stability control (ESC) system, as well as numerous statistical studies that attempt to predict the effectiveness of such systems relative to crash involvement. The literature however is free from papers that discuss how engineers might develop such systems in order to achieve desired steering, handling, and stability performance. This task is complicated by the fact that stability control systems are very complex and their designs and what they can do have changed considerably over the years. These systems also differ from manufacturer to manufacturer and from vehicle to vehicle in a given maker of automobiles. In terms of ESC hardware, differences can include all the components as well as the addition or absence of roll rate sensors or active steering gears to name a few. Like in the development of passive suspensions and steering systems, a development engineer must take into account the mission of a vehicle. There is no need to tune an ESC system on a two door sports car for off road driving or trailer towing but work may be focused on maximum cornering characteristics whereas a commercial four wheel drive pickup truck will require tuning that accounts for its anticipated load-carrying duty cycle. This paper puts forth a methodology that a vehicle dynamics development engineer might consider when tasked with developing and/or evaluating the stability-control-related steering, handling, and stability characteristics of a given vehicle. Copyright © 2014 SAE International.

Tandy D.F.,Tandy Engineering and Associates Inc. | Hanba S.,Tandy Engineering and Associates Inc. | Pascarella R.,Ford Motor Company
SAE International Journal of Passenger Cars - Mechanical Systems | Year: 2016

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 [4], 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. Copyright © 2016 SAE International.

Tandy D.F.,Tandy Engineering and Associates Inc. | Neal J.,Tandy Engineering and Associates Inc. | Pascarella R.,Tandy Engineering and Associates Inc. | Kalis E.,Ford Motor Company
SAE Technical Papers | Year: 2011

Recently, papers have been published purporting to study the effect of rear axle tramp during tread separation events, and its effect on vehicle handling [1, 2]. Based on analysis and physical testing, one paper [1] has put forth a mathematical model which the authors claim allows vehicle designers to select shock damping values during the development process of a vehicle in order to assure that a vehicle will not experience axle tramp during tread separations. In the course of their work, "lumpy" tires (tires with rubber blocks adhered to the tire's tread) were employed to excite the axle tramp resonance, even though this method has been shown not to duplicate the physical mechanisms behind an actual tread belt separation. This paper evaluates the theories postulated in [1] by first analyzing the equations behind the mathematical model presented. The model is then tested to see if it agrees with observed physical testing. Finally, the validity of the physical test methods are evaluated. Among the issues of concern were the discovery of significant algebraic and mathematical errors, questionable assumptions in the modeling of the suspension system, and procedurally flawed physical testing. This study found that the results from the referenced analysis and testing are invalid. Review of U.S. government analysis and conclusions further prove that no relationship exists between the purported suspension system damping levels and crash statistic where a tread separation is involved. Copyright © 2011 SAE International.

Tandy D.F.,Tandy Engineering and Associates Inc. | Coleman C.,Tandy Engineering and Associates Inc. | Ray R.,Exponent, Inc.
SAE International Journal of Passenger Cars - Mechanical Systems | Year: 2013

This paper explores tire placement with given tread depths on vehicles from two distinct perspectives. The first area explored is an analysis of crash data recently reported by the National Highway Traffic Safety Administration (NHTSA). In this report, thousands of tire-related crashes were investigated where the tread depth and inflation pressure were logged for each tire and assessments were made as to whether tire condition was a factor in the crash. The analysis of the data shows that in regards to accident causation, it is not statistically significant which axle has the deepest tread. What is significant is that a tread depth at or below 4/32″ anywhere on the vehicle leads to an increased rate of crashes. To understand the physics implied by the NHTSA data, a study was performed on how the placement of tires of various tread depths affects the steering, handling, and braking performance of a modern sport utility vehicle. The test vehicle was instrumented with on board video equipment and a computer with transducers to measure driver inputs as well as vehicle responses during the testing. The vehicle was tested on a uniform wet test surface at a test track specifically designed for this purpose. Specific repeatable tests were performed to study the wet surface steady-state and transient handling performance as well as the straight line stopping distance during limit braking. These tests included an SAE J266 one hundred foot circle test, a closed loop single lane change, a slowly increasing steer test, and a limit ABS brake stop. These tests were each performed three times on the vehicle with tires of various makes and manufacturers and different levels of real world customer wear. A pair of "shaved" tires was also evaluated. The results show that placement of tires has a definite effect on the vehicle dynamic performance of the utility vehicle tested and that deeper tread depth on the rear suspension is not always the best configuration for overall vehicle performance. © 2013 SAE International.

Tandy D.F.,Tandy Engineering and Associates Inc. | Ault B.N.,Tandy Engineering and Associates Inc. | Pascarella R.,Tandy Engineering and Associates Inc.
SAE Technical Papers | Year: 2012

In this study, tests were performed with modified tires on twenty-two different vehicles to measure their steering and handling capacities with a fully separated tire. Vehicles were tested with delaminated tires (i.e. tires where the tread and upper steel belt were removed) placed on the front suspension as well as the rear. These tests were performed using open loop steering evaluations at highway speeds and according to the Society of Automotive Engineers procedure J266, which includes testing to measure the steering required to follow a circular path at ever increasing speeds until the limits of tire traction are exceeded. The SAE J266 one hundred foot circle test was performed with both good tires and with a front or rear tire that had a totally separated tread and upper belt. In the tests with good tires, the vehicles could achieve a maximum lateral acceleration in the range of 0.65 g to in excess of 0.80 g in both clockwise and counterclockwise directions. In constant radius testing with the missing tread tire on the rear suspension, the vehicles could achieve in the range of 0.50 to 0.75 g lateral acceleration in a turn with the separated tire on the inside or unloaded side of the vehicle. However, when the vehicle was driven in a turn with the separated tire on the rear and outside or loaded side of the vehicle, the lateral acceleration capacity of the vehicle was reduced and resulted in an oversteer related condition on all vehicles tested. When the delaminated tire was place on the front suspension, all vehicles experienced a significant increase in understeer and decreased cornering capacity with the separated tire on the outside of the turn. This study found that separated tires reduced the handling capacities and fundamentally changed each test vehicle's handling behavior. These results are vehicle independent and confirm all vehicles will have diminished capacity when such tire problems occur. However, all vehicle tested could be steered or braked to a stop with a delaminated tire at any position. Vehicle architecture changes such as vehicle type, drive option, wheelbase, and shock absorber location have no real effect on how a vehicle's handling characteristics change with separated tires. Copyright © 2012 SAE International.

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