《SAE-TPS-731999-01-3781.pdf》由会员分享,可在线阅读,更多相关《SAE-TPS-731999-01-3781.pdf(12页珍藏版)》请在三一文库上搜索。
1、400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A.Tel: (724) 776-4841 Fax: (724) 776-5760 SAE TECHNICAL PAPER SERIES 1999-01-3781 Heavy and Medium Duty Vehicle Suspension- Related Performance Issues and Effective Analytical Models for System Design Guide Ragnar Ledesma and Shan Shih Meritor Au
2、tomotive International Truck 2) ride comfort study of vehicles with solid axle suspensions and inde- pendent suspensions; 3) trailer axle roll-steer perfor- mance model; 4) steering system vibration model; 5) suspension articulation and deflection vs. U-joint angle model; and 6) a suspension compone
3、nt durability study. Discussions on each subject include the engineering background, the modeling requirements, and the problem solving methodology. INTRODUCTION The main purpose of a vehicle suspension system is two- fold: 1) to provide a vertically compliant element between the sprung mass and uns
4、prung mass that will reduce sprung mass motions and allow the tires to maintain con- tact with uneven ground; and 2) to provide a mechanism that will allow the proper attitude of the wheels with respect to the vehicle and with respect to the road sur- face, and transmit longitudinal and lateral forc
5、es and torques from the tires to the chassis in a controlled man- ner. General desirable design features of suspension systems are limits on maximum deflections (rattlespace), and the compatibility with other components in terms of overall ride quality, relative constant natural frequency between la
6、den and unladen conditions, minimum weight, and cost 1. Medium-duty and heavy-duty suspension systems can be categorized into leaf spring suspensions and air sus- pensions. These suspension systems differ from passen- ger car suspensions in the sense that the ratio of laden spring loads to unladen s
7、pring loads is much higher in medium-duty and heavy-duty vehicles than in passenger cars. Leaf spring suspensions can be classified into sin- gle-stage springs wherein the leaf spring has a linear force-displacement relation, or multi-stage springs which exhibits an increasing spring rate with incre
8、asing load. Leaf springs exhibit hysteretic damping due to interleaf friction. Air suspensions are inherently nonlinear springs whose stiffness increases with load such that the wheel hop frequency remains relatively constant as the load changes. Unlike leaf springs, air suspensions exhibit very lit
9、tle damping. Suspension system assemblies typically include components such as control arms, torque rods, brackets, bushings, shock absorbers, bump stops, and rebound stops. Over the years, hundreds of design variations of suspen- sions for single-axles and tandem-axles have been intro- duced in the
10、 medium-duty and heavy-duty vehicle industry. Examples include variations of the following types of suspensions: walking beam suspension, inverted spring suspension, two-spring or four-spring suspensions, four-bar, six-bar, or eight-bar suspensions, trailing-arm air suspensions, combined leaf spring
11、 and air suspensions, and parallelogram air suspensions. Each variant has been designed to fulfill requirements that are specific to a particular vocation or application. However, there are some general considerations that are common in evaluating the performance of a suspension system and its relat
12、ionship to the total vehicle system. Some of these include sprung mass and unsprung mass natural frequencies and damping ratios; rattlespace or con- straints on static and dynamic deflection, ride comfort or isolation of the sprung mass from road roughness as well as minimization of sprung mass moti
13、on; road holding or minimization of dynamic tire load variation; handling or enhancement of cornering and braking ability; load equalization for tandem axles; and weight minimization of the unsprung mass. 2 VEHICLE SYSTEM DYNAMICS IN SUSPENSION SYSTEMS DESIGN The challenge for the suspension system
14、design engi- neer is to make sure that all general considerations men- tioned above are taken into account when choosing a particular design of a suspension system, in addition to fulfilling requirements that are specific to a particular vocation. In the search for an optimum design, compro- mises h
15、ave to be made among conflicting design consid- erations. For example, it is well known that an improvement in ride comfort can be attained at the expense of reduction in handling capabilities (this conflict exists for both passive and active suspensions). Further- more, the design of a suspension s
16、ystem may affect the performance of related vehicle subsystems such as the steering system, brake system, or powertrain (axle and driveline) system. It is now recognized within the engi- neering community that the most effective approach in evaluating the performance of suspension systems and relate
17、d subsystems is through vehicle system dynamics. Such an approach allows the engineer to efficiently assess the impact of the suspension design on the per- formance of the total vehicle in terms of ride comfort, vehicle handling, noise and vibration, and durability loads on every component. Another
18、benefit is that vehicle sys- tem dynamics can be performed throughout the product development process, starting from the conceptual design stage where specific design decisions still have to be made, to the final design stage where component dimensions are optimized. Several factors have contributed
19、 to a greater need for vehicle system dynamic analysis today than in the past. From the engineering design perspective, we now have more design alternatives from various types of suspen- sion systems. From the customers perspective, we have seen an increase in demand for reducing noise and vibration
20、 in the medium-duty and heavy-duty vehicles. From the business perspective, we have heard the man- date for lower product development costs and faster time to market, as well as the push for lower warranty costs. These demands point to the need for more vehicle sys- tem dynamic analysis up-front for
21、 improved durability and reliability prediction capabilities, reduction in NVH prob- lems, and consideration of the best design among all possible design alternatives. In this paper, the aspects of vehicle suspension system design where vehicle system dynamic analysis are shown to make a huge contri
22、bution are categorized into the following three areas: function, NVH, and durability. Examples of suspension design con- siderations that fall in each of these categories are listed in Table 1. Function, as used in this paper, is a generic term that refers to making sure that the proposed design wil
23、l per- form its intended function. Included in this area are con- siderations for component sizing and packaging, contact and interference, mechanical lock-up, profile synthesis for cams and slots, and other kinematic synthesis applica- tions. A good example of kinematic synthesis is the pre- dictio
24、n of Ackermann error as a function of turn angle for a particular design of a front steer axle. Also included in the function category are suspension-related vehicle per- formance issues such as rattlespace constraints, vehicle handling and vehicle stability. Automotive NVH (noise, vibration, and ha
25、rshness) refers to the judgment of the driver and passenger with regards to the degree of comfort while riding on the vehicle. Ride quality is a term often employed in referring to tactile and visual perception of vibrations, while noise is the term used when referring to the aural perception of veh
26、icle vibration. A more quantifiable dichotomy of vehicle vibra- tions is to classify vibrations under 25 Hz as ride, and vibrations in the 25-200 Hz range as noise 2. Automo- tive NVH is a vehicle system performance issue, and vehicle system dynamic analysis is an indispensable tool in solving probl
27、ems in this area. As far as suspension systems and related subsystems are concerned, NVH issues include (but are not limited to): 1) sprung mass vibration isolation; 2) unsprung mass vibration; 3) effect of suspension articulation and deflection on powertrain torsional vibration; 4) steering system
28、vibration such as Table 1.Some considerations in the design of suspensions Design Area Design Consideration Function vehicle stability cornering and braking response shimmy stability tire wear packaging and mechanical lock-up sprung-to-unsprung mass ratio load transfer dive, lift, squat NVH ride com
29、fort sprung mass isolation engine isolation frame compliance powertrain/suspension interaction steering system vibration brake noise natural frequencies and damping ratios Durability component and joint forces tire forces and moments component flexibility material selection: toughness and ductility
30、bump stop/rebound stop selection bushing and shock damper selection 3 tramp, wobble, and shimmy; and 5) effect of suspension system on brake noise. Durability loads prediction is another area where vehicle system dynamic analysis can make a significant impact to the product development cycle. Vehicl
31、e system dynamics can be employed to simulate either staged events or random events, and obtain estimates of the dynamic loads that act on any component. Needless to say, the analytical models have to be validated and corre- lated with vehicle tests. The big payoff is that once the simulation models
32、 have been updated to correlate with field tests, the model can be used to predict dynamic loads for a wide array of operating conditions without having to conduct field testing. These predicted loads can be subsequently used in the stress analysis (FEA) of components and even predict the fatigue li
33、fe of the com- ponent if an estimate of the duty cycle is available. INDUSTRY PRACTICES IN VEHICLE SYSTEM DYNAMICS So far, we have discussed the need for a systems approach in several suspension-related system perfor- mance issues and the benefits of using vehicle system dynamics in addressing these
34、 issues effectively. In this section, we will attempt to outline the processes involved in this approach, as well as the prerequisites of being able to apply this approach. Every application may have its own set of procedures, but some general procedures are common to all system dynamic analyses. So
35、me of the basic procedures involved in tackling suspension- related, system performance issues with the system dynamics approach include the following: define the engineering problem and the objective of the analysis define the physical system define the idealized system build the mathematical model
36、 perform the appropriate analysis validate the model and verify the results update the model if necessary perform design sensitivity studies and design of experiments optimize the design The first three items require the suspension design engi- neer to have a good understanding (through experience)
37、of the products function in order to define the objective of the inquiry and to define the boundaries of the vehicle subsystems that are important to the phenomenon under investigation. Building the mathematical model and per- forming the appropriate analysis, either through commer- cial codes or th
38、rough in-house programs, require that the engineering analyst should have solid analytical skills and an expertise in using the computer program. Validat- ing the model and verifying the results, usually through field tests, requires some expertise in experimental meth- ods and test data analysis. T
39、he last two items on the list which pertain to design optimization requires that the designer should have some insight as to what design parameters can have a significant impact on the perfor- mance of the system. The varied skills required to suc- cessfully complete a vehicle system analysis and de
40、sign can be fulfilled through a team approach where each team member, each being highly skilled in a specific area, can contribute to the process. Besides personnel skills, other resources are necessary to be successful in applying the system dynamics approach in a medium-duty and heavy-duty vehicle
41、 pro- duction environment. First, computer hardware with ade- quate capabilities must be available. Second, software such as commercial codes or programming software, appropriate for solving the problem at hand, should be available. Third, in a production environment, core mod- els for each applicat
42、ion should have been developed. These models should be easy to maintain and modify in order to accommodate changes in design configurations. Fourth, a database that can support the core models is crucial if the model is to be successfully used in tight product development schedules. The database sho
43、uld be continuously maintained and updated because products and practices evolve over time. Finally, there should be adequate resources for field testing, model correlation, and model updating to ensure that predictions obtained from the simulation models are valid, especially for new products or ne
44、w designs. EXAMPLES OF DYNAMIC SYSTEM MODELS USED IN SUSPENSION-RELATED SUBSYSTEM DESIGN At Meritor Automotive, vehicle system dynamic simula- tion is conducted through the use of the commercial multibody dynamics code ADAMS along with the com- mercial finite element code ANSYS, while test data anal
45、- ysis and model updating are performed through in-house codes developed in the Matlab environment. The finite element code is used in modeling component flexibility and in estimating the modal properties of the vehicle components. In the following paragraphs, we describe six applications that demon
46、strate the use of vehicle system dynamic analysis in evaluating the performance of sus- pension-related vehicle subsystems. The components included in the system, model outputs, and other model features are summarized in Table 2. BRAKE CHATTER Brake chatter is an example of the dynamic interaction b
47、etween the suspension system and the brake system. It is characterized by self-sustained oscillations induced by friction between the brake shoes and the brake drum. Brake chatter occurs when the brake shoe radial motion mode coalesces with the tangential mode (or rocking mode) of suspension/brake a
48、ssembly due to leaf spring wrap-up. Mode coupling occurs when the difference between the generalized stiffness of radial and tangential modes get below a threshold, the value of 4 which depends on the coefficient of friction and contact properties between the brake shoe and the brake drum. The gener
49、alized stiffness of the radial mode depends on the brake pressure which is a time-varying state variable, while the generalized stiffness of the tangential mode depends on the wrap-up stiffness of the leaf springs and on the torsional stiffness of the axle. When mode cou- pling occurs, a point on the brake shoe lining follows an elliptical path such that the amount of frictional energy entering into the suspension, axle, and brake assembly is positive, thus giving rise to the limit cycle phenomenon 3. Figure 1 shows the finit
链接地址:https://www.31doc.com/p-3793961.html