Keywords

1 Introduction

With the aim of designing and developing safe and efficient vehicles, tire characterization covers a crucial role. Multi-body models employed for vehicle handling simulations require detailed descriptions of forces and moments exchanged at the tire-terrain contact area [10]. Pacejka’s Magic Formula (MF) is the most employed tire model for simulating vehicle handling in different conditions, offering a good compromise among accuracy, reliability and computational effort. However, MF model requires the identification of a set of coefficients, which has to be done by means of a curve fitting procedure over a large number of physical tests. For this reason, proper testing facilities are mandatory to accurately measure tire forces and tire slip in well defined loading conditions [12]. Concerning with tire testing facilities, both indoor and outdoor test rigs can be found [10, 12]. Indoor-type testing facilities allow to replicate tire working conditions within a controlled laboratory environment. On the other hand, outdoor test rigs allow for tire testing under more realistic operative conditions, i.e. on real road surfaces like asphalt or tarmac. Such kind of machines are generally mounted on moving trailers that can be driven on public roads like the Simcenter Tire (formerly TNO/TASS Delft Tyre) semi-trailer [11] and the skid trailers by Dufournier Industries [4]. The Simcenter Tire semi-trailer, developed by the TU-Delft university, features a structure able to accommodate up to two wheels (one per side), one side has car tire measurement setup, while the other allows to apply camber angles up to 70\(^\circ \) for motorcycle tire measurement. The Dufournier skid-trailer test rig is composed by a dual-axle lightweight trailer and it is able to characterize C1 and C2 car tires. The wheel is mounted on an actuated tower, while 6 axes hub sensor is located on the spindle and allows to measure tire contact forces and moments. In the mentioned outdoor test rigs, dedicated actuators are able to control vertical load, camber angle, steering angle and braking force. However, the main limitation of these laboratories lies in the fact that longitudinal forces can be applied only by braking, meaning that only longitudinal positive slip conditions can be actually investigated. In this paper, an innovative Moving Laboratory for Automotive components Safety (MoLAS) assessment is presented. MoLAS is a moving laboratory for tire testing and its aim is that of providing a comprehensive suite of testing conditions within a single unit. Its main innovation with respect to current state of the art test rigs relies in the possibility of testing tires both in traction and braking slip condition, allowing for a complete characterization of the tire. This is obtained thanks to the introduction of an internal combustion engine (ICE) in the drive line, enabling testing the tire also in traction slip conditions. The MoLAS is able to characterize tire sizes ranging from 16 to 24 in., with a width up to 345 mm. Tire forces are measured by means of a 6-axis measuring wheel mounted on the spindle.

2 MoLAS Equipment

The basic structure of the moving laboratory is represented by a semi-trailer frame, characterized by three different containers: the electric generator and warehouse container, the measuring unit and the control room. The total length of the semi-trailer is approximately 11 m, with a total mass of 17000 kg (Fig. 1, left).

Fig. 1.
figure 1

MoLAS test system (left), MoLAS measuring unit structure (right).

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The core structure of the measuring unit (Fig. 1, right), is represented by a double-framed solution

  • a fixed frame (orange in Fig. 1, right) linked to the semi-trailer chassis, providing stiffness and supporting all the suspended parts;

  • a rotating frame (gray in Fig. 1, right), pinned to the fixed frame and able to rotate to apply the desired camber angle.

A double stage suspension system, designed to filter out vertical disturbances coming from road irregularity, holds the wheel. The vertical load (up to 20kN) is imposed by means of a pneumatic spring placed on top of the wheel supporting structure (see the red arrow in Fig. 1, right).

The camber angle is provided by an electro-mechanical linear actuator mounted on top of the structure (see the green arrow in Fig. 1, right). The linear actuator is moved by a three-phase 230 V electric motor coupled with a linear ball screw drive, reaching a maximum force of 13 kN and a camber angle in the range \(\pm 5^\circ \). The steering angle, varying between \(\pm 20^\circ \), is controlled by means of a second electric motor mounted on the steering axis. The electric motor features an output power of 3 kW and is coupled with a planetary gear reduction, the maximum deliverable output torque is around 1250 Nm. Thanks to this arrangement, the steering axis falls exactly in the middle plane of the wheel, removing any tire scrub effect during the steering maneuver. Moreover, being the steering system mounted on the moving frame, this condition holds also for any value of imposed camber angle.

The main innovation introduced by the MoLAS system is the possibility of testing tires both in braking and traction slip conditions, thanks to its complex driveline, which is composed by a V6 2.9 liters ICE coupled with a 8-speed automatic transmission, providing the driving torque; an electromagnetic retarder, applying and controlling the braking torque; an electro-magnetic clutch able to decouple the wheel from the transmission; two angular drives, transferring the rotating motion between perpendicular axes; three torsional joints, connecting the shafts.

2.1 Forces and Moments Acquisition

Contact forces and moments acting at the wheel center are measured by means of the six-axis dynamometric wheel sensor shown in Fig. 2, right.

Fig. 2.
figure 2

Six-axis force sensor telemetry system (left), six-axis hub sensor for tire forces and moments measurement (right).

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The sensor is constituted by a three-spoked structure connected to an outer ring by means of specifically designed laminae. The proposed structure realizes a “quasi”-statically determined structure, which allows to obtain the highest measuring accuracy [2, 7, 9]. A set of strain gauges is able to compute the three forces and three moments acting at the hub by measuring the bending strain on the three spokes. The proposed structure has been successfully employed for the realization of six-axis load cells [2, 3, 7, 9] and smart wheels for road vehicles [6] and motorcycles [5]. A wireless communication system has been specifically set up to transmit the signals related to the six-forces/moments and to the wheel rotation angle to the telemetry system located on board of MoLAS. The computed signals are sent to an on-board receiver via Bluetooth connection; the receiver is connected to a Controlled Area Network (CAN) and transmits the signals to the MoLAS datalogger. The scheme of the wheel telemetry system is depicted in Fig. 2, left.

3 Indoor Tire Characterization

A complete characterization of a 245/45 R18 100 Y radial tire was performed on the MoLAS test bench. The characterization was done by placing the MoLAS semi-trailer over the RuotaVia drum at the Laboratory for the Safety of Transportation (LaST) of Politecnico di Milano as shown in Fig. 3.

Fig. 3.
figure 3

MoLAS semi-trailer on the RuotaVia drum.

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The tire was inflated at \(210 \ \textrm{kPa}\), the drum tangential speed was set to \(22.2 \ \mathrm {m/s}\) and a series of tests in pure lateral, pure longitudinal and combined lateral-longitudinal conditions have been conducted. The drum surface was covered with a specific sandpaper tape (see Fig. 3) to replicate high-grip dry asphalt conditions. Three different vertical loads were set for the tests, namely \(2900 \ \textrm{N}\), \(4000 \ \textrm{N}\) and \(5200 \ \textrm{N}\). The lateral force was applied by imposing a steering motion law in the range \(\pm 10^\circ \) following a triangular waveform. The longitudinal force was applied by setting different levels of the retarder brake intensity. Signals related to the tire forces, actuators position and angular speeds were synchronously sampled at a frequency of 500 Hz and filtered down to 25 Hz in post-processing stage. Experimental data were used for characterizing the tire behavior under pure cornering, pure longitudinal and combined slip conditions. A curve fitting procedure was implemented to identify the MF parameters. Post-processed experimental data, coming from the tests, were collected in different TYDEX files [13] which were used as input for the Adams tire data fitting toolkit (TDFT) [1]. The PAC 2002 MF tire model [1, 10] was selected for interpolating the experimental data. The selected model was proven to be applicable for tires with camber angles not exceeding 15\(^\circ \), travelling on smooth roads up to frequencies of 12 Hz [8]. The Adams TDFT outputs the coefficients of the MF in a tire property file (*.tir), containing all the information regarding the tire behavior under the possible operative conditions. In Fig. 4, the experimental data (circles) and the MF model (solid lines) are reported. On the left, the identified curves of the lateral force \(F_y\) for pure cornering slip condition under three different vertical loads and zero camber angle. On the right side, the longitudinal force \(F_x\) as function of the longitudinal slip is shown for pure braking conditions. The comparison shows that the identified model is well fitting the experimental data for the three considered vertical loads, showing a good agreement both in the linear and saturation regions.

Fig. 4.
figure 4

Characteristic curves of 245/45 R18 tire. Circles represent experimental data, solid lines represent PAC2002 fitted curve.

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4 Conclusions

The MoLAS is a cutting edge testing laboratory, as it allows to obtain a complete characterization of road vehicles tires under a large variety of working conditions in a single unit. The main innovation introduced by this test rig relies in the ability to test the tire under both braking and traction slip conditions, thanks to the combined action of a controlled electromagnetic brake retarder and an ICE installed on the driveline. Tire-terrain contact forces and moments are measured by means of a specifically designed 6-axes force sensor mounted on the rotating hub. In this paper, the MoLAS trailer was placed on on the RuotaVia drum of the LaST laboratory of Politecnico di Milano for a complete indoor characterization of a 245/45 R18 100 Y radial tire. A PAC 2002 MF tire model was selected to characterize the tire behaviour under lateral, longitudinal and combined steady-state conditions. Experimental data were employed to identify the required set of coefficients of the MF model, by means of the TDFT implemented in Adams. Results confirmed the ability of the MoLAS to provide reliable experimental data to be used for tire characterization and modelling, which are essential for accurately create vehicle dynamics simulations.