(R) Straight-Line Braking Test for Truck and Bus Tires
|Publication Date:||1 March 2021|
This SAE Recommended Practice (RP) describes a test method for determination of heavy truck (Class VI, VII, and VIII) tire force and moment properties under straight-line braking conditions. The properties are acquired as functions of normal force and slip ratio using a sequence specified in this practice. At each normal force increment, the slip ratio is continually changed by application of a braking torque ramp. The data are suitable for use in vehicle dynamics modeling, comparative evaluations for research and development purposes, and manufacturing quality control.
This document is intended to be a general guideline for testing on an ideal machine. Users of this RP may modify the recommended protocols to satify the needs of specific use-cases; e.g., reducing the recommended number of test loads and/or pressures for benchmarking purposes. However, due care is necessary when modifying the protocols to maintain data integrity.
For this RP, truck tires are defined as tires mounted on heavy commercial over-the-road trucks (Class VI, VII, VIII) and buses. Vehicles which use heavy truck tires include: tractor and semi-trailer combinations, rigid trucks (e.g., flat beds, dump trucks, refuse, and refrigerated vans), buses (e.g., school, city and coaches), heavy trailers (including semi-trailers, full trailers, platform trailers, and dollies), and special purpose vehicles (e.g., truck mounted concrete pumps, military trucks, and fire fighting appliances). Tires mounted on other types of vehicles with a GVWR of 8845 kg (19500 pounds) or less (Class I through V) are explicitly excluded from consideration in this document. However, when testing tires used on larger off-highway vehicles the protocols outlined in this RP may provide a useful reference provided the tire sizes and road surfaces are similar to on-highway conditions.
Effects Not Considered
The effects of non-zero inclination angle and non-zero slip angle or any combination of non-zero inclination angle, non-zero slip angle, and spindle torque with normal force are not considered in this document, but the accuracies of machines including inclination angle and slip angle are referenced.
This RP is test machine neutral. It may be applied to any type of test machine capable of fulfilling the specified requirements. For example, specific data used to develop this RP came from both an indoor flat-belt type machine, the CALSPAN Tire Research Facility (TIRF) (as it stood in 2004), and an outdoor over-the-road dynamometer, the University of Michigan Transportation Research Institute (UMTRI) Mobile Tire Dynamometer. The ideal machine defined in 1.3.1 is not required, but a test machine's performance must be fully defined over its operating range.
NOTE: The UMTRI Mobile Tire Dynamometer is decommissioned and no longer available to test tires. It is mentioned because much of the supporting data for SAE J2673, SAE J2429, and SAE J2675 was obtained from tires tested using the UMTRI machine and experience with it shows the quality of information that can be obtained with a machine of this type. Similar over-the-road test machines have since been commissioned like the TÜV measuring trailer II (Europe) and ARRB tyre-pavement test-trailer (Australia). These machines have increased vertical loading capacity (70 to 90 kN) compared to the UMTRI machine (45 kN) as well as hub braking (30 to 40 kNm) and slip angles up to ±25 degrees and ±10 degrees of camber in the case of the ARRB machine.
This RP references an ideal machine capable of fully matching every requirement of SAE J2673, SAE J2429, and SAE J2675. Such a machine neither exists currently, nor is it certain that the technology to build a machine to the desired accuracy exists. However, this RP does not require an ideal machine. Useful data can be, and has been, gathered on existing machines. However, for repeatability and for situations when data from different machines might be compared, it is important to document the capability of each machine and to provide a method for assessing performance; for example, through benchmarking machine performance using a Standard Reference Test Tyre (SRTT) such as the ASTM F2870, 315/70R22.5 154/150L Radial Truck SRTT or other specified procedure.
NOTE: The ideal machine, ideal test operating range, and ideal accuracies are goals, NOT requirements.