Tyres are constantly subjected to different forces and loads such as gravity, air resistance, surface adhesion, vehicle weight and many others. A car which is on the road needs to overcome these forces. Rolling resistance affects not only the performance of your car, but your wallet as well.
What is tyre rolling resistance?
Ideally, a smooth tyre rolling on a perfectly level surface wouldn’t be slowed down by any external factors. Unfortunately, the reality is quite different. Your car is subject to physical forces that resist its movement. Despite this, you can still move forward.
All the factors acting against this motion are called rolling resistance, which is the calculated ratio between the energy needed for the movement and the forces of external factors acting against it. Basically, the rolling resistance coefficient (RRC) is the value of the rolling resistance force divided by the wheel load.
How does rolling resistance impact your fuel consumption?
While driving, the tyre continually stretches in contact with the road, and returns to its original shape. The resulting momentary deformations are the cause of energy loss, recovering of which generates additional costs.
Rolling resistance accounts for 70% of the forces acting on the tyres. Bad aerodynamics require more fuel to drive along the same distance.
Low rolling resistance may reduce fuel consumption even by 0.25 litre per 100 km. This amount may not seem impressive, but if we compare this with the fact that the average annual mileage of a driver in our country is approximately 12,000 km, then this number becomes quite significant.
Watch our video and see how tyres affect fuel economy!
Good to know!
Low rolling resistance is not only lower fuel consumption.
Properly optimised rolling resistance also reduces the consumption of operating fluids.
It also has a positive impact on the environment by reducing CO2 emissions.
What affects the level of rolling resistance?
Rolling resistance is influenced by several key factors. The most important include:
- Aerodynamic resistance — the resistance of the surrounding air. The value of the aerodynamic resistance of the tyre increases with increased car speed and may represent 0% to 15% of the total rolling resistance value.
- Weight — the lower the weight of the vehicle with tyres, the lower the rolling resistance. In general, low-profile, narrow tyres are lighter. The height of the tyre also influences its rigidity (lower tyres have lower rolling resistance).
- Construction and shape of the tread — the rubber compound and optimised tread pattern may reduce the rolling resistance by even 60%.
- Micro-skidding — it is a loss of grip caused by deformations occurring within the tyre. Individual parts of the tyre are subject to minimal skidding, losing energy. The share of this factor in rolling resistance is relatively small. When driving straight, its value is below 5%. It may increase during sudden acceleration, braking, or sudden changes of driving direction.
- Pressure in the tyre — insufficient tyre pressure increases the contact of the tyre with the road. When the pressure level is too low, rolling resistance may increase even by 30%.
The influence of various factors on rolling resistance is shown in this simple diagram:
Reduced rolling resistance = decreased tyre grip. Is this true?
It is generally thought that by reducing rolling resistance, the grip of the tyre is also reduced and this was true for older models. However, modern technologies and the advanced rubber compounds used in low rolling resistance tyres compensate for those negative effects, providing a stronger grip and increased durability of the tyre.
The following graph shows the relationship between grip and rolling resistance for different types of rubber compounds.
How to read rolling resistance markings
Until 2012, checking the rolling resistance of a tyre model required reading product descriptions, driver opinions and the results of tests organised by ADAC or automotive magazines. The situation changed after the introduction of obligatory labelling, providing information about fuel efficiency, grip and noise emission. Now, you can easily check the level of the rolling resistance of a new tyre (marked with a red box in the illustration). The higher the fuel efficiency class of a tyre, the lower the Rolling Resistance Coefficient.
Tyre rolling resistance table
Fuel efficiency is determined on the basis of the Rolling Resistance Coefficient (RRC). The higher the coefficient, the lower the fuel efficiency class and higher fuel consumption. Determinants for each category are shown in the following table.