SAE J695-2024
Turning Ability and Off Tracking - Motor Vehicles

Standard No.

SAE J695-2024

Release Date

2024

Published By

Society of Automotive Engineers (SAE)  US  /  SAE

Status

Be replaced

Replace By

SAE J695_202401

Latest

SAE J695_202401

Introduction

Standard Overview and Update Background

The latest revision of the SAE J695 standard (January 2024) primarily corrects the inconsistency between Equation 8 and Figure 4, and also updates the format of the title of Figure 1, the title of Section 9, and the reference format of Section 2. This standard provides a unified method for determining the steering capability and deviation tracking of motor vehicles.

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Definition of Core Terms

Term	Definition	Graphical Reference
Turn Center	The theoretical center point around which all vehicle components rotate at a given radius. Ideally, the extension lines of each wheel axis should pass through this point.	Figure 1
Turning Diameter	Twice the turning radius, including the curb-to-curb diameter (150mm high curb passability) and the wall-to-wall diameter (maximum vehicle projected envelope)	Figure 1
Offset Tracking	The difference in radius from the centerline of the vehicle's fore-aft axle to the steering center reflects the trajectory offset during turning.	Figure 6

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Steering geometry calculation principle

Based on Ackermann steering geometry (Figure 2-3), the standard provides three types of calculations:

1. Turning diameter for a given wheelbase and steering angle (Formulas 3-5)
2. Front axle configuration required to achieve the target turning diameter (Formulas 6-7)
3. Curb clearance increment calculation (Formula 8)

Key parameters include: wheelbase T, kingpin inclination angle KI, steering lever length LL, etc. The calculation formula for kingpin center distance PC is:
PC = KS + 2(OS·cosCa + TOS·sinCa)·tanKI

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Measurement Process Specifications

Steps	Operation Points	Precision Control
1. Steering System Calibration	Check steering geometry alignment and ensure minimum 20mm tire interference clearance	±2mm
2. Load Conditions	Load to Maximum Design Gross Weight	±1% GVW
3. Track Marking	Complete 2 full turns at low speed, passing the tire drip mark path	Ground marking error ≤50mm
4. Measurement Method	Use a plumb line to measure the curb clearance radius at a height of 150mm.	Angular error ≤1°

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New Offset Tracking Calculation Method

The standard adopts the "Sum of Squares Method" proposed by WHI RCR.3 to simplify the calculation:

Maximum offset = (∑L²)/2R, where:

• L: Distance between each axle group (including hinge point distance)
• R: Turning radius

Typical three-axle vehicle handling principle: The rear two axles are regarded as the midpoint equivalent to a single axle. Actual tests show that the steering center will be slightly offset to the rear.

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Implementation Recommendations

1. For multi-axle vehicles, it is recommended to use the quick reference curve in Figure 7 to determine the offset.
2. The steering system design should balance the relationship between Ackermann geometry error and tire wear.
3. Negative offset (such as tractors with long rear overhangs) requires separate evaluation.
4. During computer-aided design, it is recommended to retain a 10% steering margin.