Generally, the rolling resistance of tire is as follows.

1. Influence of the load: Rolling resistance is approximately proportional to load. Since the deformation of the tire is approximately proportional to load, it can be claimed that rolling resistance is proportional to deformation. The value determined by dividing rolling resistance by load refers to the coefficient of rolling resistance (RRC) and is used as an index of tire performance. Technically, RRC decreases slightly with an increased load.



2. Influence of inflation pressure: Rolling resistance is inversely proportional to air pressure. Since the tire supports the load mainly with air pressure, inflation pressure is inversely proportional to deformation and to rolling resistance. This relationship is maintained to approximately the upper limit of the tolerance. But if more pressure is applied the reduction of the rolling resistance reaches a peak at a certain point. This is attributed to the following mechanism. At a very high pressure, tire deformation becomes extremely small and the influence of tire body decreases while contact pressure on the road surface becomes high and hysteresis loss increases due to the compression of tread rubber. Since at high internal pressure not only does ride comfort worsens with an increase in the spring constant but the road grip also decreases with a decrease in the grounding area, and internal pressure should be determined by taking into account the balance between rolling resistance and other characteristics.



3. Influence of vehicle speed: Rolling resistance increases gradually with vehicle speed up to approximately 110km/h and it increases sharply accompanied by the occurrence of a standing wave. At an extremely low speed i.e. less than several km/h it shows a lower value. Maximum speed of a tire is specified in the standard. For example, compared to the S range (180km/h) tire, the H range (210km/h) tire has a further reinforced construction. As a result, at a speed lower than 100km/h, the S range tire with a simpler construction is simpler shows a lower rolling resistance. While at a speed greater than 100km/h, the rolling resistance of the S range tire increases sharply whereas the rolling resistance of the H range tire shows a slower increase and becomes lower than that of the S range tire.



4. Influence of temperature: Since hysteresis loss of rubber decreases at high temperatures, rolling resistance also decreases with an increase in tire temperature. Tire temperature depends on ambient temperature, heat generated while driving and cooling by surrounding air and the wheel. It varies with the time that elapses from the start of running. Accordingly, when measuring rolling resistance, it is necessary to perform break-in driving and conduct measurements after the temperature stabilizes.



5. Influences of tire size (aspect ratio): Lower aspect ratio (height of cross sectional shape of the tire/tire width) is likely to be advantageous for rolling resistance. For example, if construction and materials are identical, a 60 series tire is less than a 70 series by about 5%. The tension of the belt (tread) section is higher on a lower aspect ratio tire because the belt section and side section share the tension. As a result deformation of tread decreases when loaded and the loss at tread section of which contribution is high also decreases. However, with a super flat tire equal to or flatter than a 55 series, the belt section is reinforced to withstand the tension. Accordingly, any advantage in rolling resistance can hardly be achieved. For most part, the balance of rolling resistance and the other performance is best around 65 series.

Reference
Book title: EV Handbook
Written by: EV Handbook Publisher's Group
Published by: Maruzen Co., Ltd. (URL http://www.maruzen.co.jp)