Rolling resistance has become one of the key parameters that the vehicle industry is focusing on in their efforts to make vehicles more energy efficient. Rolling resistance is generally measured in steady state on a test drum that results in a higher rolling resistance than flat track measurements for the same test settings due to the curvature of the drum, which deforms the tire more. Therefore, the drum steady-state rolling resistance is commonly converted with Clark’s formula, as suggested in the rolling resistance measurement standards. Freudenmann et al. suggest an adjustment of Clark’s formula, claiming that it would improve the accuracy for steady-state conversions. The aim of this work is to compare non-steady-state drum and flat track measurements, performed at the same inflation pressure and tire temperature, to investigate whether Clark’s or Freudenmann’s formula can be used to convert the drum measurement to a corresponding flat track level when not in steady state. Non-steady-state measurements have been performed on both a test drum and a flat track. As expected, Freudenmann’s formula is not good for the conversion at non-steady-state settings because it was empirically developed for steady state. Clark’s formula works for non-steady-state conversions of measurements performed at the same tire temperature and inflation pressure. However, the dependency of rolling resistance on temperature is not the same in the drum and flat track measurements, causing a difference between the results that increases as the tire temperature decreases. Further research to improve Clark’s formula for non-steady-state measurements by including the effects of tire temperature would be beneficial.  

The rolling resistance of car tires is one of the most important parameters characterizing tires today. This resistance has a very significant contribution to the energy consumption of wheeled vehicles. The climate crisis has forced tire and car manufacturers to place great emphasis on the environmental impact of their products. Paradoxically, the development of electric vehicles has led to an even greater importance of rolling resistance, because in electric vehicles, a large part of the influence of grade resistance and inertial resistance has been eliminated due to re-generative braking, which resulted in rolling resistance and air resistance remain as the most important factors. What is more, electric and hybrid vehicles are usually heavier, so the rolling resistance is increased accordingly. To optimize tires for rolling resistance, representative test methods must exist. Unfortunately, the current standards for measuring rolling resistance assume that tests are carried out in conditions that are far from real road conditions. This article compares the results of rolling resistance tests conducted in road conditions with the results of laboratory tests conducted on roadwheel facilities. The overview of results shows that the results of tests conducted in accordance with ISO and SAE standards on steel drums are very poorly correlated with more objective results of road tests. Significant differences occur both in the Coefficients of Rolling Resistance (CRR) and in the tire ranking. Only covering the drums with replicas of road surfaces leads to a significant improvement in the results obtained. For investigations of rolling resistance in non-steady-state conditions, the flat track testing machine (TTF), equipped with asphalt cassettes, is shown to provide measurement data in agreement with the road test data.   

In the strive towards more energy efficient vehicles, efforts are made to reduce rolling resistance as one of the main resistive forces. Within the European Union (EU) tyres are labelled to guide consumers to choose a tyre with low rolling resistance. The tyres are labelled based on a standardised rolling resistance test performed at 25 ⁰C on a test drum, where the tyre is run until it reaches steady-state conditions. However, due to the high ambient air temperature, steady-state conditions and the curved surface of the drum, the rolling resistance in a standardised test is commonly measured at a higher tyre temperature compared with many real driving situations. The overall aim with this work was to experimentally measure the rolling resistance at low operating temperatures on a flat surface, which better correlates to realistic operating conditions compared to the EU standard measurement method. The investigated tyre temperature range, 0 ⁰C to 35 ⁰C, was determined based on tyre temperature measurements on a car in traffic during springin Sweden. Rolling resistance measurements were performed on a flat track test equipment for four car tyres with the same dimensions but different rolling resistance labelling. For the tyre with the largest temperature influence, the rolling resistance increased by almost 80 % for a temperature reduction of 20 ⁰C. These results emphasise the importance of the tyre temperature influence on rolling resistance. Further research is needed to conclude whether, and then how, the standardised measurements should be updated to address this temperature influence.

A rolling resistance model (RRM) has been created and parametrised with the purpose of modelling tyre rolling resistance within complete vehicle dynamics simulations. The RRM is based on a combination of the Masing and Zener models to simulate the Payne effect and the viscoelastic properties of rubber. The parametrised model is able to recreate the relationship between the rolling resistance and the tyre deformation well and it has a low computational power requirement. Today the model is limited to simulation of free-rolling tyres on a flat surface, but it can be extended to also include the effects of changes in operating conditions such as wheel angles or road surface.

Road transport accounts for nearly a fifth of the global greenhouse gas emissions. Despite significant reductions, achieved by e.g. the introduction of electric vehicles, more can be done. One way is to reduce the rolling resistance, which is one of the resistive forces acting on a moving vehicle.

The overall goal of this work is to fill part of the knowledge gap regarding tyre rolling resistance to support the development of more energy efficient vehicles. Focus is on the influence of the operating conditions, with emphasis on tyre temperature and non-steady-state measurements. Today, rolling resistance of new tyres is rated to guide consumers to choose an energy-efficient tyre. However, this rating is based on standardised steady-state drum measurements at 25°C which often results in the measured rolling resistance being evaluated at higher operating temperature than during normal driving. To account for the drum’s curvature, Clark’s formula is used to convert the drum measurement to represent the tyre’s rolling resistance on a flat surface.

In this work, a new method for non-steady-state rolling resistance measurements on a flat surface has been developed and used to show that rolling resistance has a large and non-linear dependence on tyre temperature. This influence varied across the tested tyres, which indicates that the rolling resistance rating could be affected by the measurement temperature. It was shown that the influence of the tyre temperature on rolling resistance was more profound on a drum compared to a flat track. Therefore, it would be beneficial to include a temperature dependency in Clark’s formula to increase its accuracy.

Furthermore, a rolling resistance model has been developed to describe the relationship between tyre deformation and rolling resistance. The model is parametrised with measurement data and simple enough to be used in complete vehicle dynamic simulations.

Vägtransporter står för nästan en femtedel av de globala utsläppen av växthusgaser. En stor minskning har åstadkommits genom bl.a. introduktionen av elektriska fordon, men det räcker inte. Ett sätt är att minska rullmotståndet, som är en del av det färdmotstånd som verkar på ett fordon i rörelse. 

Det övergripande målet för den här avhandlingen är att fylla en del av kunskapsluckan om rullmotstånd för att bidra till utvecklingen av mer energieffektiva fordon. Fokus ligger på driftsvillkorens påverkan, framförallt däcktemperatur och icke-steady-state. Idag klassificeras däcks rullmotstånd för att hjälpa konsumenter att välja energieffektiva däck. Klassificeringen baseras dock på en standardiserad trumprovning under steady-state i 25°C, vilket ofta ger betydligt högre däcktemperatur än vid verkliga körförhållanden. För att ta hänsyn till trummans kurvatur används Clarks formel för att omvandla trummätningen till ett motsvarande rullmotstånd på en plan yta.

I detta arbete presenteras en ny metod för rullmotståndsmätningar vid icke-steady-state på plant underlag, vilken har utvecklats och använts för att påvisa att rullmotstånd har ett starkt och olinjärt beroende av däcktemperaturen. Däcktemperaturens inverkan på rullmotståndet varierade mellan de provade däcken, vilket indikerar att rullmotståndsklassificeringen kan påverkas av mättemperaturen. Vidare visades att däcktemperaturens påverkan på rullmotståndet är större på trumma jämfört med plant underlag. Därmed skulle det vara fördelaktigt att inkludera ett temperaturberoende i Clarks formel för att öka dess noggrannhet.

Vidare har en rullmotståndsmodell utvecklats som beskriver sambandet mellan rullmotstånd och däckdeformation. Modellen är parametriserad med mätdata och tillräckligt enkel för att användas i fordonsdynamiska simuleringar av ett komplett fordon.

Measurement methods to determine the rolling resistance of tyres during different operation conditions are essential in the work towards more energy efficient vehicles. One of the influential parameters is the tyre temperature distribution, which has a large impact on the rolling resistance. Today, the standardised test procedure to measure rolling resistance is steady-state measurement on drums. However, the steady-state temperature on a drum is not the same as the temperature during ordinary driving conditions. The aim of this work is to develop a measuring method that enables to set a desired measurement temperature, which would create the possibility to study the relationship between tyre temperature and rolling resistance in more detail. The measurement method was developed by the use of a flat track equipment but should be applicable to other rolling resistance measurement equipment such as drums. The resulting method gives a repeatable tyre temperature and rolling resistance and can be used for measurements on tyres heated to a chosen measurement temperature. 

Efforts to reduce greenhouse gas emissions from today’s increasing number of cars and trucks, are crucial in counteracting global warming. These efforts include the intent to reduce the effects of the resistive forces acting on the vehicle. Rolling resistance is one of these forces. A reduction in rolling resistance would aid in reducing greenhouse gas emissions, while also reducing the driving costs and increasing the driving range per charge for electric vehicles. This PhD research contributes to these efforts by the development of a rolling resistance measurement method on a flat track test equipment that avoids the curvature effects present in the standardised drum test. Another contribution is the development of a rolling resistance model that can describe the relationship between the tyre deformation and the forces acting on the tyre. The model is parametrised by results from the developed measurement method and is simple enough to be included in complete vehicle dynamicssimulations. In this thesis, the effects of different operational conditions, such as inflation pressure, tyre temperature, speed, load, road surface or tyre angles, are investigated and presented. The results from this investigation were used for the development of the measurement method for flat track test equipment. Tyre temperature is an important operating condition influencing rolling resistance and the proposed measurement method can be used to investigate rolling resistance at different tyre temperatures. The results obtained with the proposedmeasurement method, which are comparable to drum measurements performed under the same operating conditions, are used to parameterise the developed rolling resistance model. The model gives a good fit for the relationship between rolling resistance and tyre deformation. The measurement method and the model build a good platform for deeper investigations of rolling resistance and its connection to tyre temperature.

Arbete för att minska utsläppen av växthusgaser från det ökande antalet bilar och lastbilar är en viktig del i att motverka den globala uppvärmningen. Detta kan göras genom att reducera påverkan från de resistiva krafter som påverkar fordonet, med fokus på rullmotståndet. En minskning av fordonens rullmotstånd skulle medverka till att minska växthusgasutsläppen samt bidra till att reducera körkostnaderna och öka räckvidden per laddning för elbilar.Denna licentiatuppsats bidrar till detta genom att skapa en metod för rullmotståndsmätningar på plant underlag, för att kunna undvikakrökningseffekterna i den standardiserade trummätningen. Ett annatbidrag är en rullmotståndsmodell som beskriver växelverkan mellan däckdeformationer och däckkrafter. Modellen parametriseras med resultat från den framtagna mätmetoden och är tillräckligt enkel för att vara användbar i en komplett fordonsdynamiksimulering. I denna uppsats presenteras påverkan av olika driftsvillkor som däcktryck, däcktemperatur, hastighet, last, underlag och kurvatur. Dessa resultat nyttjades i utvecklandet av nämnda mätmetod för rullmotståndsmätningar på plant underlag. Däcktemperatur är ett viktigt driftsförhållande med stor påverkan på rullmotståndet och den föreslagna mätmetoden kan användas för att mäta rullmotstånd vid olika däcktemperaturer. Denna mätmetod användes sedan för att parametrisera indata till den utvecklade rullmotståndsmodellen. Det visade sig att modelldata avviker från uppmätt data för förhållandet mellan hjullast och däckdeformation på grund av modellgeometrin. Men modellen ger en god överenstämmelse för förhållandet mellan däckdeformation och rullmotstånd. Mätmetoden är, tillsammans med den föreslagna modellen, en bra bas för mer genomgående undersökningar av rullmotstånd och dess korrelation med däcktemperatur.

Reducing the rolling resistance for future vehicle designs creates a possibility to reduce the fuel consumption and make the future vehicles more economical and ecological. For electric vehicles it is also an enabler to increase their driving range per charge. When optimising for reduced rolling resistance, contradictory requirements such as force generation for maintaining safety and performance need to be considered. Furthermore, it is important to include both the effects of road surface and vehicle, to avoid sub-optimisation regarding only the tyres. A cross-functional conflict on the component level is well known, in form of energy consumption versus wet grip (traffic safety). On the system level, different wheel settings to optimise energy consumption conflicts with vehicle dynamical properties related to traffic safety, such as stability or steer response. The long term vision of the work presented is to create tools for more energy efficient vehicles by reducing the rolling resistance during driving. The first part is to establish a credible measurement method for rolling resistance on road under controlled conditions (lab environment). Today’s existing measurement methods on rolling resistance under laboratory conditions commonly utilise a rotating drum, whose curved surface affects the results. Therefore, rolling resistance influence of vehicle settings such as camber or toe angles is difficult to assess using standard methods, and there is a need for measurements using a more realistic contact patch, which would need a flat surface. The existing unique tyre testing facility at the Swedish National Road and Transport Research Institute, VTI, is used as a base for developing the new rolling resistance set-up. The tyre test facility is today used to determine tyre characteristics such as brake and steering forces. The method to measure rolling resistance with this equipment under highly controlled conditions is under development, and some preliminary results are presented.

For at least 50 years, the interest in understanding and reducing the rolling resistance of pneumatic tyres has been growing. This interest is driven by the need to reduce vehicle fuel consumption and CO2-emissions, for environmental and economic reasons. The amount of rolling resistance generated depends on the vehicle type, tyre properties and operating conditions. The main objective of this literature review is to provide an overview of the most influential operating conditions with respect to rolling resistance, their effects and their connection to different measurement techniques. The examined operating conditions are the inflation pressure, the temperature, the curvature of the test surface, the load, road surface, speed, torque, slip angle and camber angle. In addition, the definition of rolling resistance is investigated, which shows lack of harmony in the literature. There are important areas where little research can be found and where further research would be valuable. Examples of such areas are effects of the torque, slip angle and camber angle on rolling resistance, thorough comparison between flat-surface and drum measurements, effects of temperature difference between laboratory measurements and actual driving on rolling resistance and evaluation of Unrau’s formula for temperature correction of rolling resistance measurements.