Introduction

It is a known fact that tyres play a significant role in the vehicle’s performance. There has been tremendous pressure on automotive OEMs to improve the performance of the vehicles. For the last few years, performance of tyres has played a significant role because of the change in requirements of automotive OEMS and end users. Tyre ratings & labeling programs are steps towards improvement of tyre efficiency. This gives option to the customers to select the brand of their choice. Day by day, the Indian road infrastructure is getting improved and the vehicle used in such improved infrastructure demands high-performance tyres with more safety features. Rolling resistance (RR) is one of the important parameters of the tyres under optimum conditions of use to rate the performance of the Tyre/Vehicle for fuel economy. Rolling resistance test has been directly correlated to fuel economy of the vehicles.
In this paper, the importance of rolling resistance (RR) of tyres in the fuel economy of the vehicles is highlighted. This paper also covers the Indian and world scenarios on RR, various test methods and factors affecting the RR. The bench-marking study on RR done as per ISO 28580 : 2009 shows that there is a significant difference in the RR values from one brand to another for the same size of passenger car tyre. For example, 175/65 R14 82T, which may be attributed to difference in material composition of the tread and other related components of the tyre.

What is Rolling Resistance?

Rolling resistance (RR) is one of the forces acting on the vehicle. RR is the force acting opposite to the travel direction when a tyre is rolling. Due to the vehicle load, the tyreis deformed in the contact area with the road surface leading to increased resistance to movement. In simple terms, rolling resistance is the energy consumed per unit travel of the tyre during the vehicle movement.

Why is Rolling Resistance important?

It is a known fact that energy in vehicles is created by the burning of fuels. Energy is lost in the form of heat, which is produced by the friction between the road and the tyre, by the flexing of tyre sidewalls, and between the rim and tyre, etc. It is reported that 5 to 15 per cent of the fuel used by a typical vehicle goes into overcoming rolling resistance which can be easily managed by suitable rubber compound design and optimization through material Research and Development.

RR of tyres: Global scenario

Many of you must be aware that, in Europe, tyre labeling which covers rolling resistance of tyres, tyre noise and wet grip under EC regulation 1222/2009 has got implemented w.e.f. November 2012. EC regulation 1222/2009 –labeling rolling resistance covers rolling resistance coefficient values for passenger car tyres (C1-tyres), light commercial vehicles (LCV) tyres (C2-tyres) and commercial vehicles tyres (C3-tyres).
Other countries in the world such as South Korea, Japan, Brazil, the US etc have also started tyre labeling under mandatory/voluntary certification. Tyre labeling helps in regulating tyres used in vehicles for fuel economy and tyre manufacturers are always expected to high level research to face the challenges of the global performance requirement of tyres. The Indian tyre manufacturers are upgrading their technology to meet such challenges.

RR: Indian scenario

The Regulatory agencies in India such as BIS, AIS and ITTAC have already started the study on implanting RR in tyre specification and accordingly the discussion and the deliberation under different forum have been made on RR.
The Committee Members of Transport Engineering Department-7 (TED-7) BIS have proposed simultaneous study on wet grip along with RR to avoid any “trade-off” in properties resulting in “unsafe” tyres.
The RR test facility, uniquely available with the Indian Rubber Manufacturers Research Association (IRMRA), is an independent testing laboratory in India. This facility is utilized by most of the leading Indian & international tyre manufacturers and automotive OEMs. IRMRA has been involved in testing of RR and generating data for many brands of passenger and commercial vehicle tyres which are available with IRMRA for reference purposes. Having administrative control of the Department of Industrial Policy & Promotion, DIPP, Government of India, IRMRA is closely working with standardization body and also with other regulatory agencies such as ITTAC, Bureau of Energy Efficiency on RR-related regulations / policy matters.
IRMRA is well-equipped for tyre testing research as well as material research facilities and supports consumers to carry out their testing and analysis needs. Being an NABL-Accredited laboratory, many times referred as Referral Lab for testing and investigation of disputed cases with respect to rubber products which includes tyres, hoses, belts, etc., IRMRA is in the process of getting the status of the Candidate laboratory – Alignment for the measurement of tyre rolling resistance in accordance with Regulation (EC) 1235/2011 and 1222/2009 following the test procedure described in ECE Regulation No 117 – to become the first Indian nodal testing laboratory for measurement of rolling resistance of the tyres.

Test methods for RR measurement

The following are three types of methods for measuring rolling resistance of tyres
1. Force method: Measures the reaction force at tire spindle & converts it to rolling resistance.
2. Power method: Measures the power input to test machine & converts it to rolling resistance
3. Torque method: Measures the torque input to test machine & converts it to rolling resistance.
In this study, torque method has been used for rolling resistance measurement. In this method, we measure the torque input to test machine & converts it to rolling resistance.
Rr = T / R, Where Rr: Rolling Resistance
T: Net input torque
R: Test wheel Radius
Cr = Rr /FL, Where Cr: Rolling Resistance coefficient
Rr : Rolling Resistance
FL: Tyre Load

International standards for RR measurements

The test standards for the measurement of RR for two/three wheeler, passenger car and commercial vehicle tyres are ISO 18164, ISO 28580, SAE J 1269 and SAE J 2452. These standards are being referred by global and leading tyre manufacturers and also OEMs. Sooner or later, India also will adopt or prepare its own standards. The standardization body in India, such as BIS/TED-7 & AIS, has already taken up the matter with its committee members. The draft AIS 142 standard aligned with UN R117 for evaluation of tyres with regard to rolling sound emissions and/or to adhesion on wet surfaces and/or to rolling resistance is under circulation. AIS committee members are of the view of notifying the rolling resistance and wet grip of the tyres simultaneously in the first stage. Rolling sound would be taken be up in next stage once RR and wet grip of tyres are notified.

RR test facility at IRMRA

The Center of Excellence for Tyre Research, Testing and Certification at IRMRA, established with the support of of the DIPP, Government of India, under the 11th Plan, is a world-class testing facility with reliability. It has the capacity to test two/three, passenger car and commercial vehicles tyres by Torque method. The Center has compliance with ISO/IEC 17025 : 2005. The Center has also done Inter laboratory comparison on RR with all the leading Indian tyre manufacturers

Parameters/factors affecting tyre RR

The factors, other than material composition of the different tyre components, RR is dependent on are the tyre inflation pressure, tyre speed, tyre load and tyre temperature. Hence consumers using vehicles are supposed to make themselves aware of these facts so that they do not abuse the tyres fitted in vehicles.
HOW THESE FACTORS AFFECT TYRE PERFORMANCE
Effect of tyre inflation pressure on RR
The tyre 155/65R13 of four different make was tested as per ISO 28580 under regulated pressure condition to study the effect of inflation pressure on rolling resistance of tyre. From the study, it was found that there was an increase of RR by 8-11% if the tyre was under-inflated by 30 Kpa and there was an increase of RR by 19-23%, if the tyre was under-inflated by 60 Kpa. The under-inflated tyres get higher deflection leading to more frictional force between tyre and the road surface resulting in higher value
of RR.
8.1 Effect of speed on RR
The tyre 235/65 R17 104 H was tested as per ISO 18164 at four different speed (30, 60, 90 & 120 km/h) under regulated pressure condition to study the effect of speed on RR. From the study, it was found that there was an increase of CRR by 5% if the speed increased from 90 to 120 km/h.

Effect of load on RR

The tyre 185/65 R15 88S was tested as per ISO 28580 under regulated pressure condition. From the test observation, it was found that there was an increase of RR by 21% if the load was increased by 20%

Case Study

To understand the benefits of RR in the vehicle fuel economy, IRMRA has done case one study at its Tire Research Center. The tyre 175/65 R14 82T of four different brands were tested as per ISO 28580 : 2009. From the test observation, it was seen that there was approximately 40% difference in RR from one brand to another which might cost around 10% greater fuel consumption.
The said difference is RR values is attributed towards the difference in material composition of the different tyre components, mainly the tyre tread compound and the tread pattern.

Conclusion

The study clearly indicates the importance of the RR of tyres with respect to fuel economy of vehicles in addition to reduction in the
CO2 emission by way of reducing the RR values of tyres. To get the best performance of fuel economy
using RR measurements, the user must maintain the correct/recommended tyre inflation pressure and avoid over-speeding and over-loading.
Such awareness will also help consumers to take such precaution for the benefit of fuel economy, and such precaution will also help avoid road accident and tyre-burst during service condition.
The RR study gives an opportunity to tyre manufacturers to upgrade their existing technology on a par with technology of the leading tyremanufacturers which the global market demands.