Posted on 30 August, 2017

Why Does Rolling Resistance Matter?

In 2016 GB-registered heavy goods vehicles (HVGs) drove 19.2 billion kilometres[1]. With an average of 8.5 mpg[1], UK logistics companies spent around £5.4 billion on fuel and released around 18.7 million tonnes carbon dioxide equivalent into the atmosphere.

Whilst we may think of this as simply the cost of moving cargo, in fact 20-40% of this fuel is consumed by lorry tyres in the bending of rubber. By improving the efficiency of this rubber bending by a very conservative 5%, the UK logistics industry could save £80 million a year and avoid releasing 280 thousand tonnes of carbon dioxide equivalent into our atmosphere. This article seeks to:

•   Explain a little about what rolling resistance is and where it comes from.

•   Shed some light on the EU tyre labelling system.

•   Demonstrate the potential large fuel savings made possible by analysing the exact coefficient of rolling resistance, instead of relying on EU bandings.

For the equivalent article on aerodynamic drag, follow this link [2].

What is Rolling Resistance?

If you set a tyre rolling on a perfectly flat surface it will not roll forever; it will slow down and stop. This slowing is caused by the complex interplay of many forces. These forces are usually treated as a single force opposing the tyres motion, known as the rolling resistance.

Where Does Rolling Resistance Come From?

The rolling resistance has three main contributions:

•   Internal friction of the tyre material (85-92%).

•   Friction between the tyre and the road (7%-10%).

•   Tyre aerodynamic drag (1%-5%).

There are at least 8 physical mechanisms responsible for the above resistance contributions[3], these are shown in figure 1.

 

Figure 1. Eight mechanisms which contribute to the rolling resistance of standard tyres.

The majority of the of the energy lost to rolling resistance is dissipated as heat created by internal friction in the rubber. As a tyre rolls the portion of the tyre in contact with the road is deformed before returning to its original shape[4]. During the deformation process, the polymer chains making up the rubber must move past each other. Much like the friction we can observe by rubbing larger objects together, this process generates heat. Any energy lost as heat in the tyres has to come from energy generated by the engine, and so ultimately is wasted fuel.

The factors affecting rolling resistance

The mechanisms shown in figure 1. are influenced by the ‘real-world’ factors shown in figure 2.

Figure 2. Everyday influences on rolling resistance.

These factors typically have an influence on more than one of the physical origins. For example, an increase in tyre load will increase the rolling resistance from internal tyre deformation and friction between the tyre and the road, but not from tyre aerodynamic drag. Whilst some of these factors are difficult to control, such as road surface and ambient temperature, others, such as inflation pressure and tyre material should be carefully considered and optimised for maximum fuel efficiency.

The Rolling Resistance Coefficient

The magnitude of the rolling resistance force is described by the rolling resistance coefficient (Crr). This coefficient describes how the rolling resistance force (Frr) increases with the normal force (N) (the normal force is equivalent to the vehicle weight on a flat surface). The rolling resistance force is given by Frr = Crr × N. In simple terms the larger the rolling resistance coefficient or vehicle weight, the larger the rolling resistance force. As heavy goods vehicles have, by definition, a large weight, the minimisation of Crr is key to fuel efficiency.

The European tyre rating system

Under EU tyre labelling regulations, the rolling resistance, wet grip and noise levels of tyres must be specified (see figure 3.). Tyres are ranked and colour coded from A-G depending on which band their Crr and wet grip coefficients fall.

Figure 3. Standard EU tyre labelling.

It is worth noting that these bandings are divided into passenger car (C1 tyres), light truck (C2 tyres) and truck and bus (C3 tyres) classes. The rolling resistance coefficient bandings and wet grip bandings are not comparable between these classes i.e. a B rated passenger tyre will have a rolling resistance coefficient between 6.6 and 7.7 whereas a B rated truck and bus tyre will have a rolling resistance coefficient between 4.1 and 5.0. Detailed information about the band boundaries can be found in reference [5].

Rolling Resistance – The rolling resistance coefficient is grouped in bandings from A-G, with A being the highest fuel efficiency performance and G being the worst. Each band corresponds to a range of rolling resistance coefficients (measured in kg/Tonne).

The rolling resistance coefficient is measured using the drum test method according to the ISO 28580:2009 standard[6]. The tyre is mounted on in a drum and pressed against it with a defined load. To rotate the drum without a tyre requires a certain torque. Once the tyre is brought into contact with the drum, the torque required to rotate the drum is increased. The difference in torque with and without the tyre in contact is used to calculate the rolling resistance.

Wet grip – The better a tyres wet grip rating the shorter the breaking distance on a slippery road. Tyres are tested according to the ISO 15222:2011[7] standard which measures the distance travelled by a vehicle after breaking at 50 mph in wet conditions. Tyres with the best wet grip rating will typically exhibit a 30% shorter braking distance than those with the worst rating.

It is worth noting that high performance in the wet grip braking test does not necessarily translate into other safety parameters such as road handling, aquaplaning behaviour, directional control and performance at higher speeds.

Noise levels – The final aspect of the EU rating system is the exterior noise rating (in dB) shown as one, two or three sound waves. A tyre rated at one wave is half as noisy as a tyre rated at two waves and a tyre rated at three waves is twice as noisy as two waves. The test procedure is performed by measuring the sound at the side of the road as a vehicle with its engine switched off passes at 50 mph.

The Importance of Analytics for Fuel Efficiency – Distribution of rolling resistance coefficients within bandings

Choosing the correct tyres can have a significant impact on fuel consumption. This choice of which tyre to buy is a balancing act between efficiency, rate of wear, safety, cost, reliability and practicality. Logistics is a competitive industry with tight margins which makes it especially critical to fully utilise the available data to inform purchasing decisions. It is not the aim of this article to analyse every aspect which must be taken into consideration, but instead to demonstrate the potential large fuel savings made possible by analysing the exact coefficient of rolling resistance, instead of relying on EU bandings.

Figure 4. is a heatmap showing how the exact rolling resistance coefficients of 362 tyres are distributed within the EU labelling bands. The dark blue colour shows a high density of tyres and the lighter colour shows a low density of tyres.

Figure 4. Distribution of tires within each EU band rating. Each band has been individually normalised to the same colour value for ease of comparison. Data was acquired from 362 tyres[8] and determined using drum tests according to ISO 28580 procedures[6].

It would be reasonable to have assumed the spread of Crr was evenly distributed within the bands. Figure 4. reveals that this is not the case, instead tyres are typically tightly grouped within the bands. Tyres with good to mid-range efficiencies (bands B-D) typically cluster with Crr close to the worst end of their band. This is likely a by-product of the manufacturers awareness of the band system, once a tyre is in a certain band there is no reason for a manufacturer to improve its rolling resistance unless it can be improved enough to just get in to the next band up. A slightly different trend is seen for the poor efficiencies (bands E-F). In these bands Crr is far more likely to lie towards the top of a band.
How should these data affect purchasing decisions?

Let us assume a lorry with an annual fuel cost of £50,000 with a moderate 30% of its fuel used to overcome rolling resistance. First let us assume the lorry currently has D rated tyres and the decision has been made to upgrade to a C rating. If we assume all other features (wet grip, rate of wear etc) are equal we may expect a cost difference of ~£80 a tyre and may be fitting 12 tyres (here truck and trailer must be considered as the fuel cost includes the trailer) so a total cost difference of £960 between D and C.

So, is this additional cost worth it? The answer is that the expected return on investment (ROI) depends on the exact Crr of the old and new tyres.

•   The worst-case scenario

◊   Crr of our current tyres sits at the good end of the D band.

◊   Crr of the new tyres sits at the bad end of the C band.

◊   Fuel saving – £246 per year.

•   The best-case scenario

◊   Crr of our current tyres sits at the bad end of the D band.

◊   Crr of the new tyres sits at the good end of the C band.

◊   Fuel saving – £4225 per year.

This is a significant difference in saving and will likely make the difference between a positive or negative ROI. The correct answer requires accurate tyre data followed by careful analysis. The take home message from this one aspect of the process is that, in many cases it is important to base the purchasing decision on the exact rolling resistance coefficient, not just tyre banding.

What if you cannot get access to the exact Crr? Based on this limited study of 362 tyres, we can draw some general rules of thumb

•   Upgrades from C-B or D-C have a high probability of giving a ‘mid-case’ scenario ROI.

•   Upgrades from E-D have a high probability of giving a ‘worst-case’ ROI.

•   Upgrades from F-E have a high probability of giving a ‘best-case’ ROI.

It should be noted here that the worst cases in the F band are typically retreads. This is another example of a situation which requires careful analysis. Whilst a retread may be cost effective and environmentally friendly in the short term, if Crr of retread tyres is high they will consume considerably more fuel in the long term. This process may then have a negative fuel cost and environmental impact. This will be discussed in depth in a future article.

Summary

This article aims to help readers understand more about the complex factors effecting rolling resistance. Due to the significant effect of tyre performance on fleet running costs and environmental impact, this article has highlighted the benefits of performing a careful and detailed analysis when making purchasing decisions.

Questions about improving efficiency? Ideas you would like to see discussed? Please get in touch!

Dr Chris Durrant
Head of Analytics
Dynamon Ltd
chris.durrant@dynamon.co.uk

References

[1] Domestic Road Freight Statistics, United Kingdom 2016, Department for Transport (2017). https://www.gov.uk/government/statistics/road-freight-statistics-2016

[2] Spoiler Alert! What Makes a Good Roof Spoiler, A. Webb (2017). http://dynamon.co.uk/spoiler-alert-what-makes-a-good-spoiler/

[3] Fundamentals of Vehicle Dynamics, Gillespie, T.D (1992).

[4] Transportation Research Board of the National Academies of Science. 2006. “Tires and passenger vehicle fuel economy: informing consumers, improving performance, TRB Special Report 286 (2006).

[5] EU Tyre Labelling Regulation 1222/2009 Industry Guideline on tyre labelling to promote the use of fuel-efficient and safe tyres with low noise levels – Version 3, European tyre and rubber manufacturers association (2011). http://www.etrma.org/uploads/Modules/Documentsmanager/2011-11-30–industry-guideline-on-tyre-labelling-(vers3).pdf

[6] ISO 28580:2009 Passenger car, truck and bus tyres — Methods of measuring rolling resistance — Single point test and correlation of measurement results International Organization for Standardization, Geneva, Switzerland (2009). https://www.iso.org/standard/44770.html

[7] ISO 15222:2011 Truck and bus tyres — Method for measuring relative wet grip performance — Loaded new tyres, International Organization for Standardization, Geneva, Switzerland (2011). https://www.iso.org/standard/55388.html

[8] Rolling Resistance Comparison, Michelin (2017) http://www.michelintruck.com/tools/rolling-resistance-comparison/