Tire slip-angle force measurements on winter surfaces

Abstract

Tire lateral force data on winter surfaces cannot be obtained with the traditional laboratory test technique of an instrumented tire on a moving belt surface. Furthermore, changing snow and ice conditions can drastically change the tire/surface interaction. In this study the Cold Regions Research and Engineering Laboratory’s (CRREL’s) Instrumented Vehicle (CIV) was used in a unique configuration to measure tire lateral force versus slip-angle data on ice and snow at various temperatures, moisture contents, depths, and densities. The vehicle is instrumented to record longitudinal, lateral, and vertical force at the tire contact patch of each wheel as well as vehicle speed, tire speed, and front tire slip angle. The tests were conducted at the Keweenaw Research Center (KRC) in northern Michigan in February 2005 and March 2006. Tests were conducted on ice, packed snow from 0.50 to 0.58 g/cc, remixed snow depths of 2.5–20.3 cm at 0.43 to 0.48 g/cc and freshly fallen snow with depths of 0.5–17 cm at 0.07 to 0.23 g/cc. Surface air temperatures during testing ranged from −14 to 1.6 °C. The data collected show that peak lateral force and the shape of the lateral force versus slip-angle curve are related to snow properties and depths.

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.