Press Release
Reducing rolling resistance is one of the main challenges in the further development
of tires.
Working towards better solutions
One property always comes hand in hand with the great comfort of pneumatic tires. They
deform under pressure and thus roll less easily. Reducing rolling resistance is therefore one
of the main challenges that has been faced by tire engineers since the invention of the
pneumatic tire. The rolling resistance of tires costs money and material.
The advantages of air-filled vehicle carriers are great, however: Ride comfort, suspension
comfort, protection of vehicle components and the comparatively easy maintenance and
replacement of pneumatic tires are unbeatable in everyday use.
The problem occurs above all with the task of moving loads on a surface. The combination of
driving force (horizontal) and the weight (vertical) acting on the tires also causes the
deformation that results in rolling resistance.
Due to the deflection under load, the tires are “elastically deformed” and counteract the
propulsion in the form of rolling resistance. In addition to the two basic forces (weight and
propulsion), there are other forces that can have more or less influence depending on the
situation. Varying material properties in different tire designs also play a part as does the
vehicle speed (it influences the form of the elastic deformation), the temperature of the tires
(warm tires have a different rolling resistance value) and the type of movement environment
(gravel, rain and ice drastically change the rolling resistance).
The measurement for the rolling resistance is called the “rolling resistance coefficient”, which
corresponds with the relationship of wheel load to resistance force. It describes the
resistance of a body to rolling as a comparative variable. There is no need to distinguish the
deformation of the rolling body and that of the surface it is traveling on, which both contribute
to the rolling resistance coefficient. This allows the combination “tires on sand”, in which the
tire (elastic) is deformed less and the sand more (plastic), to be measured equally clearly for
comparison purposes as the combination “tires on cold asphalt”, in which the asphalt
deforms less and the tires more.
Working against the resistance
While the task for the tire engineers is clearly outlined, it is still complicated. The favorable
material properties of the tires should be preserved while the resulting interfering factors are
-2to be removed. Finally the natural phenomenon of rolling resistance on vehicle tires is
nothing other than a traveling resistance that has to be balanced against fuel costs, tire wear
and higher exhaust gas and noise emissions.
The basis for the work of the tire developers is the tire itself, or to be precise, the deformation
of the tire and of the rubber, which occurs whenever the tire rolls, and the specific properties
of different rubber compounds. Both are the actual cause of the tire rolling resistance. This
importantly includes the so-called “dissipation”. This term describes the conversion of the
deformation energy produced during the cyclical deformation of rolling tires due to “inner
friction” of the polymer chains in the rubber. The energy that is converted into heat through
dissipation is lost for the propulsion and thus causes the typical additional costs.
Tires with improved rolling resistance can therefore reduce costs considerably for bus and
truck companies in view of the mileages they cover. The potential fuel savings, for example,
depend on the application. The more the vehicle fuel consumption is affected by rolling
resistance in the respective area, the greater the savings potential will be with optimized
rolling resistance. The rolling resistance in particular is responsible for a substantial amount
of extra consumption if the route does not involve much acceleration like, for example, on
freeways. Research carried out by Continental shows bandwidths of up to 40% for the
contribution of rolling resistance to fuel consumption in long-distance transport while the
average of the truck tested is 25%. The fuel savings potential is thus up to 4% for longdistance transport and around 2.5% for regional transport when tires with improved rolling
resistance of “just” 10% are used.
The long way through tire technology
The road to this aim is complicated, however, because a low tire rolling resistance can only
be achieved through a combination of many individual measures. It is only possible if
optimization is carried out in all areas of tire development.
-3According to the engineers, the most effective means is the rubber compound. Since the
rolling resistance is caused by deformation of rubber compounds, optimizing them is
particularly important. Continental has come up with two approaches for this.
The first is reducing the internal friction in the rubber compound. The compound of rubber,
soot and other fillers as well as crosslinking agents (vulcanizing agents, aging protection and
others) can be improved specifically. The internal heating through dissipation results from the
friction of the polymer chains between themselves and with the filler particles. Through the
controlled coordination of the filler types, quantities and rubber combinations, the internal
friction can be reduced and thus good rolling resistance with good wear, grip and tear
properties achieved. For this reason, in addition to the material research department that
deals exclusively with the basic optimization of the rubber compound, Continental has other
development departments that develop special rubber compounds configured for the
respective application.
The next step is optimizing the energy flow in the tire. A tire consists of many different
components with different properties that work with each other. Compounds optimized for
rolling resistance are used preferably for components and material mixtures with great
deformation potential. They absorb the energy transferred, release it again while rolling and
thus counteract the rolling resistance.
The variety of tire components leads to the next step — optimizing the tire design. At
Continental, tires are designed with the aid of computer simulations (FEA/Finite Element
Analysis). This allows material to be used specifically and safely where it is needed (e.g. for
reasons of strength) and easily omitted where it is not needed. The engineers have thus
managed over many years of development work, to improve the material weight and obtain
lighter tires. This benefits the fuel consumption for two reasons: less material converts less
energy into lost performance and lighter tires have lower masses that can then be
accelerated with less effort and fuel.
-4There is also great potential in process technology. This is also why there is a specialist
department within research and development at Continental that exclusively looks into the
improvement of tire heating. If the green tire is “baked” (mild heating) at particularly low
temperatures when the tire is heated during vulcanization (crosslinking of polymer chains),
the tire chemicals (crosslinking agents) have a longer time to dissolve into the rubber and be
distributed evenly. This reduces the internal friction of the rubber compound and allows lower
rolling resistance in later use. The longer time for the heating process does lead to higher
process costs, but Continental still employs this method to reduce the rolling resistance.
Producing the rubber compound is still another large development field. The performance
capability of a rubber compound does not just result from the recipe for the components. The
mixing procedure in which the compound ingredients are brought together also affects the
quality of the rubber produced.
The work of the development department for this manufacturing step therefore also has
complex consequences. Filler particles, which are added to the compound to improve the
wear and tear behavior, need to be distributed particularly evenly to allow a low rolling
resistance. Since, in contrast to crosslinking agents, they cannot be dissolved in rubber, but
only distributed mechanically, particular attention is paid to the mixing of recipe components
in Continental’s “rubber kitchen”. This does further increase the manufacturing costs because
of the longer periods in the mixers and the increased energy requirement, but it is very
advantageous for lower wear and low rolling resistance.
Decades of research
The intensive networking of research, development and production at Continental did not
come by chance. At the start of the eighties, the Hanover-based tire manufacturer was
intensively involved in a project by the then German Ministry for Research and Development
with the aim of reducing tire rolling resistance considerably. The motivation back then was —
no surprise — fast rising fuel prices. Based on the developments from the German Ministry
for Research and Technology project, the first technology for tires with improved rolling
resistance was introduced by Continental in 1986/87. Conti was thus the first manufacturer to
-5supply tires for considerably reduced fuel consumption. The experiences made since then
are still influencing the latest products from Continental.
For example, the L range of low-rolling resistance tread geometries and tread compounds
are also used for tires specially developed for national and international transport. Further
new tires can be expected for the product segments where low rolling resistance is
particularly important, i.e. the regional and long-distance segment, between the end of 2008
and start of 2010 depending on the tire size and tread application.
Continental Group is one of the world’s leading automobile component suppliers with an expected
turnover of over €26.4bn. in 2008. The company contributes to greater driving safety and climate
protection as a supplier of brake systems, systems, drive components, powertrain components,
instrumentation, infotainment solutions, vehicle electronics, tires and technical elastomer products.
Furthermore Continental is a specialist partner for networked, automobile communication. The
company currently employs around 150,000 people at almost 200 sites in 36 countries.
The Truck Tires division, which develops, produces and globally distributes truck, bus and industrial
tires, recorded a turnover of around €1.5bn. in 2007 with over 8,000 employees.
The tire divisions are an official sponsor of the FIFA World Cup 2010TM, which will be staged in South
Africa in 2010. You will find information on this at www.ContiSoccerWorld.de,
www.ContiFanWorld.com and www.continental-corporation.com