Tyres are the only point of contact between a heavy goods vehicle and the road, and their construction has a direct bearing on fuel consumption, payload and ride comfort. Two technologies in particular dominate the specification of a modern long-haul tractor: tyres designed to roll with as little resistance as possible, and the light-alloy wheels on which they are mounted.
Rolling resistance is the energy a tyre dissipates per unit of distance travelled as it deforms and recovers shape while rotating under load. It manifests as a force opposing the direction of motion. At motorway speed it accounts for a major share of the total energy a tractor has to provide, so it is a primary target for fuel saving on long-distance work.
The rolling resistance coefficient (RRC, also written CRR) is the standard metric. It is the rolling resistance force divided by the vertical load carried by the tyre:
RRC = F_RR / Z
The coefficient is dimensionless. Force is expressed in newtons and load in kilonewtons, so RRC is quoted in N/kN, which is numerically identical to kg per tonne. Normalising by load is what makes the figure useful: a steer tyre and a trailer tyre carrying very different loads can be compared directly.
The origin is the visco-elasticity of rubber compounds. Rubber is elastic, so it springs back, but also viscous, so it does not return all the energy it absorbed. The difference leaves as heat. This loss mechanism is called hysteresis.
The familiar analogy is a rubber ball: drop it and it never rebounds to the height it was released from. The missing energy went into deforming the rubber. A truck tyre does the same thing several hundred times a minute at motorway speed.
Because the tyre recovers more slowly than it compresses, the contact patch carries more reaction force at its leading edge than at its trailing edge. That asymmetry creates a moment opposing rotation, and that moment is rolling resistance.
Three deformation mechanisms dominate:
Two secondary mechanisms contribute less: micro-slip at the contact interface, and aerodynamic drag from the rotation of the tyre itself.
Rolling resistance is one of five forces opposing forward motion. Total driving resistance is their sum: