Tire slip-angle force measurements on winter surfaces
Introduction
Tire lateral force measurements are of interest to vehicle designers for stability control and ABS braking applications. They are also needed in vehicle computer models and vehicle simulators to generate realistic tire forces as a vehicle maneuvers over different ground surfaces. Especially of interest to the military is vehicle response in off-road and all-season conditions. One of our main goals in this study was to develop an accurate Vehicle Terrain Interface (VTI) that extends the existing two-dimensional ground contact models used by the US Army Tank Automotive Research, Development and Engineering Center’s (TARDEC’s) Ride Motion Simulator (RMS) to include lateral forces under winter, low friction conditions. The VTI can be used in vehicle modeling applications to include robotics.
The vehicle tire is a complicated component and classic tire modeling is a very complex endeavor, part art and part science. These authors are unaware of any one model that can fully explain tire forces for all conditions. The more complex tire/terrain models are usually computationally intensive and do not lend themselves well to real-time virtual simulations that need to be computationally efficient and quick to perform. Experimentally determined tire lateral force data can be useful through regressed equations in virtual simulations as well as for providing the more complex physical models with necessary verification data.
It is difficult to obtain tire lateral force measurements on winter surfaces as the traditional laboratory technique of lowering an instrumented tire onto a moving belt surface makes it impossible to use naturally occurring snow or ice as the surface material. Thus there are few published measurements of tire lateral forces on snow and ice [1], [2], [4] and the techniques employed vary with the resourcefulness of the testers. Instrumented vehicles [3] are frequently used in these measurements.
This paper will describe a winter measurement technique for lateral tire force measurements on a free rolling tire that uses the CRREL Instrumented Vehicle (CIV) as a towed sensor. The technique is best limited to slow speed measurements, less than 4.5 m/s, but uses both front tires in the measurement procedure. We have used the technique for two years (2005 and 2006) of measurements at the Keewenaw Research Center (KRC) in Houghton, Michigan. The first year’s work was presented at the Society of Automotive Engineers (SAE) 2006 World Congress [4]. This paper will present data from new snow conditions and compare and discuss trends between the two data sets. We will look at how different snow conditions affect the lateral force versus slip-angle data.
Section snippets
Test terminology
The tire slip angle is defined in this study as the angle between the course of the vehicle and the heading of the tire itself. The course of the vehicle was constrained to be the same as the vehicle longitudinal axis by the way we conducted the tests and therefore the tire slip angle is also the tire steer angle, the angle between the vehicle longitudinal axis and the tire heading. Fig. 1 shows the tire slip angle conventions and Fig. 2 describes the tire force conventions.
The lateral force is
Data analysis
Fig. 6 shows a time series plot from a typical towed slip angle test. The longitudinal, lateral and vertical forces from each front tire along with slip angle are recorded and through Eq. (1.1) are converted into lateral coefficient of friction versus slip-angle curves. Shoop and Coutermarsh [4] shows typical raw data plots of lateral coefficient of friction versus slip angle which can be noisy due to the tire either dragging on the packed surfaces or forcing through the built up snow under and
Conclusions
The data presented here and from our previous work, Shoop and Coutermarsh [4] show that the type of snow or ice condition will affect not only the magnitude of the maximum lateral coefficient of friction but also the shape of the coefficient versus slip-angle curve. This would be important when programming tire response in dynamic vehicle simulations.
The packed snow surfaces gave the highest lateral coefficient of friction and the values compared very well between the 2005 and 2006 test years.
Acknowledgements
Much of the work presented here was accomplished under the U.S. Army Technology Objective (ATO) “High Fidelity Ground Platform & Terrain Modeling” (HGTM). This ATO was jointly performed by the Engineer Research and Development Center’s (ERDC) Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH, and the Geotechnical and Structures Laboratory (GSL) in Vicksburg MS along with the US Army Tank Automotive Research, Development and Engineering Center (TARDEC) and the Army Research
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