Experimental Investigations on Tire/Road Friction Dependence from
Thermal Conditions Carried out with Real Tread Compounds in Sliding
Contact with Asphalt Specimens
ARRICALE Vincenzo Maria1,a, CARPUTO Francesco1,b, FARRONI Flavio1,c*,
SAKHNEVYCH Aleksandr1,d and TIMPONE Francesco1,e
1Dept. of Industrial Engineering, University of Naples “Federico II”, Naples, Italy
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
Keywords: test bench, friction, grip, British pendulum, tire, compound
Abstract. The understanding of tire’s adherence with a rough surface is a common goal for several
fields in the automotive sector. In fact, grip is synonym of safety and performance, playing a
decisive role for braking distance and vehicle stability, fuel consumption, wear rate , noise
generation and for the vehicle dynamics control system (e.g. ABS, TCS, AYC and other) . This
paper deals with tire tread grip experimental investigation and evaluation under different conditions
that influence it during the sliding contact . In this regard the test campaign involves the use of
different tire compounds (in terms of viscoelastic characteristics), tested in several conditions:
different contact pressure, sliding speed, temperature, sliding contact length and road surface. The
test bench employed by the UniNa Vehicle Dynamic Research Group is an upgrade of the British
Pendulum, an instrument for outdoor tribological tests on road sections. The principal sensors
installed on the test bench are an encoder, for the evaluation of the sliding speed of the tire
specimen, and a load cell, for the measurement of the force arising at tire/road interface in the
longitudinal and vertical directions . In fact, the grip shall be determined as the ratio of the
longitudinal force and corresponding load on the tire. The paper's aim is the description of the
experimental campaign after an accurate introduction on the test setup and an illustration of the
equipment. Finally, the preliminary results and the methodologies used to process the acquired data
Nowadays every vehicle is equipped by many driving assistance systems useful to control vehicle
safety, stability, and performance, e.g. ABS, TCS, EBD, PGS . All of them require the
knowledge of the friction coefficient to describe tire dynamic behaviour; indeed, knowledge of the
current maximum coefficient of friction would allow an anti-lock brake system (ABS) controller to
start braking with the optimal brake pressure . The described applications make it
necessary to develop a model of local friction , under different conditions, and this can
be achieved subsequent of an experimental acquisition campaign. With regard to the tire grip
testing, several methods have been already developed by many authors in literature, as shown in Fig.
1. Such tests can be carried out both indoor and outdoor, using proper tire and tread specimens. The
outdoor tests are usually executed with an instrumented vehicle/trailer on track to experiment
different boundary conditions on the whole tire . To evaluate the grip in the indoor tests on the
whole tire, specific test benches are used, as the rotating drum and flat track . Other indoor
tests can be executed on a tread rubber specimen using pin on disk, in which the rubber specimen
under investigation is approached to a rotating disc coated with different surfaces , and using a
tribological test device called “British Pendulum”, in which rubber specimen mounted at the end of
a pendulum, slides on the testing asphalt when the pendulum is left free to oscillate from a given
Key Engineering Materials Submitted: 2019-04-08
ISSN: 1662-9795, Vol. 813, pp 261-266 Revised: 2019-05-21
doi:10.4028/www.scientific.net/KEM.813.261 Accepted: 2019-05-27
© 2019 Trans Tech Publications Ltd, Switzerland Online: 2019-07-22
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
Tech Publications Ltd, www.scientific.net. (#506895299-25/07/19,00:06:53)
The analysed results in this paper have been obtained by an acquisition campaign on an evolved
version of the British Pendulum, described in detail in the following paragraphs.
Fig. 1. Different methods to experimentally investigate the friction at tire-road interface
Description of the Test Bench
The developed tribological test bench is an evolved version called BP-Evo , of the classical
British Pendulum Tester, an instrument used for the asphalt characterization , shown in Fig.
2. Indeed, differently from the traditional BP, it is equipped with two sensors: a tri-axial load cell
(1) and an encoder (2).
The load cell (with only two channels of interest, tangential and normal force, equipped) is a
“HBM K-MCS10-010-3C”, positioned at the end of the oscillating arm (3) long 682 mm, whose
purpose is to measure the normal and the tangential forces arising during the sliding between the
rubber specimen (4) and the road (5), with this last positioned in a tank (8). The encoder “Fritz
Kuebler type 8.5820.1442.1000” measures the angular speed of the arm, thus the sliding speed of
In order to investigate the influence on the friction force by contact pressure at tire/road interface,
the measurement equipment has been completed by a system composed by levers and a pre-loading
springs (6), whose stiffness can be changed, changing the spring. An endless screw (9) has been
employed to control the height of the oscillating arm from the road to set the contact length.
Furthermore, a graduate crown (7) has been located on the frame, in order to set the height of the
launch, thus the sliding velocity.
The measurement signals from the two instruments are acquired by an A/D board “Mantracourt
SGA/A & SGA/D” and processed in Matlab environment, in order to express forces and velocity.
Fig. 2. The British Pendulum Evo
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Description of Tread Specimen and Road Surface
The tire tread specimens, selected for the experimental campaign, belong to three tire compounds
with different optimal working conditions. Such compounds have been chosen with the aim to
evaluate the influence of the tire material properties on the friction with a road characterized by a
known roughness and wavelength. The thickness of specimens is equal to 5 mm, whereas height and
width are both equal to 25 mm. The rubber parallelepipeds are glued to a specimen holder by means
of an ethyl-cyanoacrylate glue being careful that the specimen leading edge is aligned to the
specimen holder one, in order to guarantee the identical geometrical conditions for each test.
Fig. 3. a) Storage Modulus of tire tread specimens vs frequency. b) tan( ) of tire tread specimens vs
The properties of the specimen have been obtained by means of dynamic mechanic analysis
(DMA) test, resulting in diagrams of the storage modulus and tan( ) versus frequency, as shown in
Before starting the test campaign, the load cell should be calibrated in order to remove the eventual
offsets. The calibration procedure consists of two steps. In the first step, the road is not placed in the
tank and the arm is positioned horizontally to calibrate the Fz (Fig. 4), in order to allow the load cell
to measure all the forces along the x direction, according to the local reference system in Fig. 4. In
the second step the arm is positioned vertically to calibrate the Fx (Fig. 4). The load cell therefore
measures all the forces in the z direction of the local reference system.
Fig. 4. Load cell reference system
Then, the road is positioned in the tank and the sliding length is set equal to the nominal value of
50 mm by the endless screw.
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During the sliding, the tread specimens deposited rubber particles on the road, due to the friction.
Furthermore, the wear of the tread specimens and the “rubberization” of the road were observed.
Such phenomena should be taken into account in order to keep both the boundary condition and the
working parameters as constant as possible. Therefore, the data were acquired for 50 oscillations
observed for each new specimen, making the results repeatable. Moreover, due to the
“rubberization”, the road surface has been periodically cleaned by means of a metal brush.
The test plan was structured according to the following table:
Table 1. Variables examined during the acquisition session
Sliding Length [mm]
Spring Stiffness [N]
As previously mentioned, the tests have been performed using springs with different stiffness in
order to investigate the influence of pressure acting on the tread specimens. Fig. 5 shows the
pressure distribution in the contact patch for a stiffness equal to 50 N. According to Fig. 5, the
nominal area of the specimen is shown in grey, whereas the contact patch is represented by colours.
Fig. 5. Pressure distribution in the contact patch for a stiffness equal to 50 N. Specimen sliding is in
x axis direction
Postprocessing and Results
In order to process the acquired data, it is worth understanding the involved forces, and which of
them are effectively read by the load cell. Thus, a lunch was made without positioning the road in
the thank, and in this case, the forces read by the cell were therefore only the inertial forces (the
friction forces are not involved). Such inertial forces should be subtracted by the forces acquired
during the sliding of the tire tread on the road. For the sake of illustration, the signal acquired for
only one specimen is shown in Fig. 6, in terms of Fx, Fz and sliding velocity.
Fig. 6. An example of acquired signals
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The following figure shows the friction coefficient (µ) obtained as the ratio between tangential
forces (Fx) and normal forces (Fz) in the contact zone.
25 30 35 40 45 50 55 60 65 70
P = 50 N v = 1.5 m/s
Fig. 7. Friction coefficient (µ) vs temperature (T) and velocity (v) for the compounds tested
According to Fig. 7, the results of the preliminary acquisitions show the influence of the contact
pressure on the friction coefficient. This effect may be due to both the saturation of the contact patch
and to percentage strain effect on the material behavior . Moreover, the friction coefficient
changes with the temperature and sliding velocity depending on the material mechanics properties
(Storage Modulus and Loss factor). More detailed analysis will object of following testing
An evolved version of the classic “British Pendulum” machine for the tribological analysis has been
developed to test different tires tread by varying the types of asphalt, sliding velocity, contact
pressure, and temperature. In this paper, the test procedure and the preliminary results of a test
campaign are described. The results show the influence of the variables (i.e., compound, pressure,
temperature, sliding velocity) on the friction coefficient. In conclusion, the “British Pendulum
EVO” can be used to investigate the phenomena involved in the interaction between two bodies,
finding large application in the automotive field.
The authors thank Mr. Gennaro Stingo and Mr. Giuseppe Iovino for their fundamental technical
support during the testing and bench development stages.
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