Figure 72
10.4 SERVICEABILITY — After the vehicles are assembled, the system is evaluated for ease of service-ability (Figure 73). A proven technique involves the use of ranking each maintenance function versus com-petition with any rank less than any competitor sending the designer back to the drawing board to provide a competitive edge. The serviceability review should stress ease of periodic maintenance, quick removal and replacement of parts during unscheduled maintenance, minimum use of special tools, and the elimination of possible misassembly after servicing.
10.5 SYSTEM FUNCTIONAL CHECKS — Before the vehicles are moved out of the assembly area in the garage, system functional checks should be made to insure that the following conditions are met: the link-age provides specified wheel-cut, no toggle conditions exist, nominal clearances are satisfactory, linkage stops are properly set, front suspension is correctly aligned, power steering pressures and relief valve set-tings are correct, and preloads are set at the proper
conditions.
10.6 STATIC EVALUATION — The next phase of evaluation covers those development checks which
are performed in a static condition. It is important that these events occur before dynamic evaluation to insure that the system is performing to design level during the driving tests.
10.6.1 CAB INTERIOR — A manikin drop is per-formed to assure that the seat is located to vehicle package specifications and the following evaluations are then performed: pedal to steering column clear-ance; steering wheel to seat and knee clearance for ingress, egress and pedal operation; visibility of instru-ments (Figure 74); location of trailer brake handle on tractor models; adjustment of moveable columns;
clearance of steering wheel to dash panel and other close objects; and steering wheel “feel” including rim size and section, spacing and configuration of finger grooves on the underside, and surface finish.
10.6.2 FRONT END ALIGNMENT — Caster, cam-ber, toe-in, king pin inclination, and front ride height are checked in both the empty and loaded vehicle
10.6.3 SUSPENSION AND LINKAGE ARTI-CULATION — The vehicle is placed on a hoist and
Figure 73 67 levels.
Figure 74
depending upon anticipated usage, the front suspension indicated, the system can be disconnected progressive-is placed in the appropriate articulation modes shown ly from the right front wheel up to the steering wheel below (Figure 75): to identify the problem.
Full Slam Both sides of the front axle at metal to metal contact
Full Rebound Both sides of the front axle at full rebound
Modified Jounce One side of the front axle at normal load position and the other at metal to metal contact
In addition, information regarding static steering efforts versus wheel cut in normal load configuration is obtained on an exterior concrete surface for subse-quent plotting of a steering effort curve (Figure 76).
At the same time, linkage deflection under load is also examined.
Full Jounce One side of the front axle at full re-bound and the other at metal to metal contact.
10.6.5 SYSTEM RATIO — The overall system ratio is established by the formula shown below and com-pared to the ratio calculated during system design:
OSR SWT Eq. (22)
The front bumper should be maintained in a horizontal WC position to insure that the frame will not twist, so that the distances measured on the vehicle during the sus-pension articulation evaluations can be accurately re-lated to the dimensions on the layouts which are drawn with a nondeflected frame.
Where:
OSR = Overall system ratio
SWT = Degrees of steering wheel turn from full left to full right turn
While in the articulated conditions, the steering system is cycled from full letf turn to full right turn and back again and the following items are evaluated:
tire clearance to sheet metal, chassis parts, and steering linkage; ball stud angles for cam out; linkage clear-ances; toggle conditions; steering gear travel; and length and routing of power steering hoses.
WC = Degrees of road wheel-cut from full left to full right turn
10.6.4 STATIC EFFORTS — The system is cycled with the wheels on rotating plates to insure that all joints are functioning properly. If a malfunction is
10.7 DRIVING EVALUATION — The vehicle is now ready for driving evaluation to prove whether or not the driver must concentrate on the act of steering the truck while performing normal maneuvers and to confirm the presence or absence of any undesirable dynamic characteristics (Figure 77). Because of the subjective nature of a driving evaluation, it is advisable to have several knowledgeable drivers rank each of the
=
Figure 75
Figure 76 69
following characteristics by using the jury rating scale given in Figure 71:
•
•
•
When driving straight ahead, does the vehicle track without undue steering corrections?
When driving over rough roads, is the steering wheel movement objectionable?
When cornering, does compliance steer cause oversteer or understeer? The same for axle roll steer and changes in lateral acceleration.
Does the vehicle return from a curve without un-due driver input or excessive steering wheel spin?
Are the steering efforts acceptable?
•
• •
Is the steering wheel correctly placed with re-spect to the seating package and cab interior?Does the vehicle exhibit wheel shimmy at any particular speed or when driven over a bump?
•
Does the truck swerve with severe brake applica-tions?•
Does the power steering system exhibit dynamic instability or bias?•
Is the power steering road feel acceptable?•
Does the power steering pump provide adequate flow during the following maneuvers: static stop to stop turn, 90o turn from a standing stop, lane change, and evasive maneuvers.10.8 DYNAMIC MEASUREMENTS — In addition to the driver evaluation, the following dynamic
char-acteristics will be measured in quantified terms: turn-ing radius, steerturn-ing effort, tie rod clearance in windup and jounce, and any close clearances noted in the static evaluation.
10.9 POWER STEERING TEMPERATURES — The best method for checking power steering system temperatures is in a wind tunnel where the conditions can be accurately controlled. If this facility is not avail-able, actual road tests in typical ambient temperatures can be utilized. If a wind tunnel is used to check stabi-lized maximum operating temperature, the test condi-tions would be: engine at governed speed and full load, wheels straight ahead, and 5 MPH to 15 MPH wind velocity. This simulates a long pull up a steep grade and will result in the most severe sustained tempera-tures anticipated in normal operation.
Because the vehicle configuration has a great effect on stabilized maximum temperature, this test must be run on a vehicle in the wind tunnel or on the road.
However, this is not the case for the full speed, max-imum pressure, zero output pump burn out test. There-fore, this test is usually run on the bench in the labora-tory.
11. SUMMARY
This paper has been written as the first step in the journey from beginner to expert in the science and art of commercial vehicle steering design and develop-ment. It has been a general discussion in order to cap-ture an overview of the total system. However, it is the fine points of design which achieve product
improve-Figure 77
•
ment. Therefore, it is hoped that the information pre-sented here will serve as a foundation of knowledge upon which the young engineer will build his own accomplishments of technical excellence as he designs steering systems from the hand wheel to the road wheel.
12. ACKNOWLEDGEMENTS
As noted in the introduction, a commercial vehicle steering system combines a great diversity of “industry standard components” into a specific vehicle system.
Therefore, the knowledge of the total system lies not with one man or group of men from one company. In-stead, it exists in the minds and experience of many men throughout the industry. Just as a steering system is a compilation of specialized parts, so is this paper a synthesis of the knowledge of the many technical ex-perts in the steering field. The author wishes to express his sincere gratitude for the generous expenditure of time, effort, and talent which the following men have contributed in a consulting capacity in this undertaking:
Automotive Control Systems Group, Bendix Corp-oration, Mr. D. E. Runkle; Fluid Power Division Mar-shall Plant of Eaton Corporation, Mr. C. A. Serle;
Ford Motor Company, Mr. W. Bergman, Mr. J. J.
Duffy, Mr. W. J. Graczyk, Mr. G. E. Johnson, Mr.
K. G. Moss, Mr. R. J. Parker, Mr. R. L. Raymond, Mr. K. W. Schipper; Michigan Division of TRW Inc., Mr. R. W. Dymond, Mr. E. J. Herbenar; Ross Gear Division of TRW Inc., Mr. W. L. Adkins, Mr. V. J.
Bhatia, Mr. C. V. Gagen, Mr. B. C. Hudgens, Mr. D.
C. Shropshire, Mr. R. K. Thelen; Saginaw Steering Gear Division of General Motors Corporation, Mr.
G. L. Enszer, Mr. M. M. Frank; Sheller-Globe Corp-oration, Mr. F. E. Young; R. H. Sheppard Co., Inc., Mr. W. Cuny; Vickers Mobile Division of Sperry Rand Corporation, Mr. J. C. Jones, Mr. G. E. Koch, Mr.
N. Lichtenberg, Mr. D. West. In addition, the follow-ing men must be recognized for their efforts, expertise and exceptional enthusiasm in the preparation of the technical analysis: Dr. H. J. Bajaria, Dr. M. E. Chang, Mr. G. D. Channel, Mr. V. R. Denham, Jr., Mr. J. F.
Schauer.
13. REFERENCES
1. K. W. Schipper and K. G. Moss, “The Systems Ap-proach to Heavy Truck Steering.” Paper 700880 presented to the National Combined Fuels and Lubricants and Transportation Meetings, Novem-ber 1970.
2. T. J. Budzynski and R. J. Parker, “Heavy Truck Steering System Analysis.” Paper 660431 presented to the SAE Mid-year Meeting, June 1966.
71
K. M. Koch, “Center Point Steering Axles.” Paper 368B presented to the SAE Summer Meeting, June 1961.
J. A. Davisson, “Design and Application of Com-mercial Type Tires.” Paper SP-344 presented at the Fifteenth L. Ray Buckendale Lecture, January 1969.
J. J. Taborek, “Mechanics of Vehicles.” Machine Design, Volume 29, Number 11, May 30, 1957, pp. 60-65; Volume 29, Number 12, June 13, 1957, pp. 130-135; Volume 29, Number 13, June 27, 1957, pp. 92-100.
C. A. Searle, “A New Power Steering Pump for Heavy Duty Trucks.” Paper 700881 presented to the National Combined Fuels and Lubricants and Transportation Meeting, November 1970.
W. L. Adkins, “Truck Integral Power Steering.”
Paper 700882 presented at the SAE National Com-bined Fuels and Lubricants and Transportation Meetings, November 1970.
“Automotive Steering Linkages.” TRW Michigan Division, Troy, Michigan.
W. H. Baier, “Vehicle Steering Fundamentals.”
Paper 218A presented at the SAE National Farm, Construction and Industrial Machinery Meeting, September 1960.
P. R. Basal, Jr., Edited by, “Mobile Hydraulics Manual.” Sperry Rand Corporation, September 1967.
D. Bastow, “Steering Problems and Layout.”
Proc. Institute of Automotive Engineers, Volume 32, p. 124-175.
W. Bergman, “The Basic Nature of Vehicle Under-steer-Oversteer.” Paper 957B presented at the SAE International Automotive Engineering Con-gress, January 1965.
A. J. Coker, “Automotive Engineers Reference Book.” London: George Newnes Ltd., 1959.
W. H. Crouse, “Automotive Chassis and Body.”
New York: McGraw-Hill, 1959.
D. A. Eaton and H. J. Dittmeier, II, “Braking and Steering Effort Capabilities of Drivers.” Paper 700363 presented at SAE International Auto-mobile Conference Compendium, P-30, June 1970.
E. Favary,“Motor Vehicle Engineering: The Chas-sis.” New York: McGraw-Hill Book Co., 1922.
10.
J. W. Fitch, “Motor Truck Engineering Hand-book.” San Francisco: J. W. Fitch, Publisher, 1969.
J. G. Giles, “Steering, Suspension and Tyres.”
London: Iliffe Books Ltd., 1968.
P. M. Heldt, “The Automotive Chassis (Without Power Plant).” Nyack, N. Y., P. M. Heldt, 1952.
G. T. Kless, “Attenuating Device.” U.S. Patent No. 3,323,305, June 6, 1967.
P. Kyropoulos, “Human Factors Methodology in the Design of the Driver’s Workspace in Trucks.”
Paper SP-367 presented at the 18th L. Ray Buck-endale Lecture, January 1972.
T. I. Monroe, “Discussion of Steering Problems on Modern Heavy TruckS.” Paper 368A presented at SAE West Coast Meeting, June 1961.
K. Newton and W. Steeds, “The Motor Vehicle.”
London: Iliffe & Sons Ltd., 1950.
D. L. Nordeen, “Vehicle Handling: Its Depen-dence Upon Vehicle Parameters.” Paper S405 presented at the SAE Junior Activity Meeting, February 1964.
M. Platt, “Front-Steered Cars.” Automotive Engi-neer, Volume 30, No. 394, February 1940, pp.
35-37.
G. E. L. Walker, “Directional Stability.” Auto-motive Engineer, Volume 40, No. 530, August 1950, pp. 28l-285; Volume 40, No. 533, Novem-ber 1950, pp. 370-376.
15. APPENDIX I — A C K E R M A N N G E O M E T R Y
15.1 GEOMETRIC PROOF — Ideal steering condi-tions for a forward steering, four wheeled vehicle exist when the axes of rotation of the forward wheels meet on the axis of rotation of the rear wheels at a common point, TC, as shown in Figure 78. When this occurs, all tires experience pure rolling action about TC. It is apparent from the geometry in this figure that the fol-lowing conditions exist:
Where the terms for the above are defined as: 2 CLG
OTA = Outside wheel turn angle
ITA = WB = X = CC =
Inside, wheel turn angle Wheelbase
Distance from turning center to centerline of vehicle
Distance between rotation points of the front wheels
Subtraction of equation 24 from equation 23 above de-rives the basic Ackermann geometric relationship stated below:
cot (OTA) - cot (ITA) = CC WB
This analysis has been based on the industry accepted practice of using zero caster in the layout of the Acker-mann linkage. Therefore, CLYL is equal to the wheel-base.
It may be further shown that the relationship be-tween angles ITA and OTA must result in the inter-section of lines CLP and CRP at the common point P on the locus of points formed by connecting points G and YL if the conditions of Ackermann Geometry are to be satisfied by angles ITA and OTA.
The proof follows:
Eq. (25) CLN = CLG – NG Eq. (26) CLG = GCR
Substitution of GCR for CLG in Equation 26 results in the following:
CLN = GCR – NG Eq. (28) Substitution of CLN as defined in Equation 28 into Equation 25 results in the following:
cot (ITA) = GCR – NG
NP Eq. (29)
The definition of cotangent (OTA) in similar terms is readily apparent from examination of Figure 78 as:
GCR + N cot (OTA) NP
Subtraction of Equation 29 from Equation 30 derives the following:
cot (OTA) – cot (ITA) = 2 NG
Because triangle GNP is similar to triangle GCLYL, we can write Equation 31 which is expressed in terms of triangle GNP as the following equation in terms of triangle GCLYL:
BASIC GEOMETRIC
Equation 32 is recognized as the basic Ackermannrelationship as expressed in Equation 13. Therefore, it has been proven that line GYL represents the locus of points for the intersection of lines CLP and CR P when angles ITA and OTA fulfill the requirements for Achermann Geometry.
15.2 LAYOUT PROCEDURE — The tie rod link-age system will provide ideal steering geometry in straight ahead driving and at one specific angle of wheel-cut. This portion of the appendix proves this statement and presents a graphical method for deter-mining how any proposed tie rod layout compares to the ideal Ackermann condition established in Section 15.1 of this appendix.
The graphical technique begins by drawing the side-view, rear side-view, and plan view of the front axle as shown in Figure 79, and establishing the following points in all views:
C – Intersection of king pin axis with the ground
E – Ball joint on the tie rod arm
V – Point of intersection of a line drawn from E perpendicular to the king pin axis The technique then proceeds by the following steps.
S tep 1 — Swing an arc around VL as the center with radius VL EL from the straight ahead position of EL to the maximum left turn wheel-cut position eL.
73
Step 2 — Divide the arc established in Step 1 into four equal segments and label them 1,2, 3, eL.
Step 3 — Swing an arc around VR as the center with radius VR ER .
Step 4 — Using the tie rod length EL ER and points 1, 2, 3, eL on the arc of locus of points for the left tie rod arm ball joint, establish the corresponding points 1,2,3, eR on the locus of points on the arc for the right tie rod arm ball joint established in Step 3.
Step 5 — Using any convenient radius, draw a con-struction arc about points CL and CR.
Step 6 — Establish Point G midway between points CL and CR in the plan view.
7 — Establish points 1', 2', 3', eL' on the left con-struction arc drawn in Step 5 by the following angular equalities:
E LV Ll = GCLl' E V 2 = GC 2' E V 3 = GC 3' E V e = GC e '
Step 8 — Repeat Step 7 for the right side to establish 1', 2', 3', eR' on the right construction arc with center at C R drawn in Step 5.
Step 9 — Connect Point CL with straight lines to 1', 2', 3', eL' on the left construction arc about CL.
Step