Temperature Differential
The difference between the radiator average core temperature and the temperature of the cooling air is the driving force behind the transfer of heat from the coolant to the cooling air.  When an engine starts and is run up to rated load, the coolant begins to heat up.  When there is no thermostat in the system as in an MX bike, the coolant flows from the engine through the radiator and back to the engine.  Initially, the coolant and metal in the engine absorb the heat being produced and continue to do so until the temperature of these parts exceeds the cooling air temperature.  At this point, heat transfer to the cooling air commences.  The coolant temperature continues to rise until it reaches a temperature at which the difference between the radiator average core temperature and the incoming cooling air is great enough to transfer the entire heat load to the air. This then becomes a “steady-state” condition.