On the 2008 Honda CBR600RR, the upper tube from the coolant reservoir tank is connected to the overflow port on the radiator. The purpose of this tube is to allow excess coolant to flow from the radiator into the reservoir tank when the system is under pressure.
During normal operation, as the engine heats up, coolant expands and increases pressure within the cooling system. If the pressure exceeds the designated limit, the excess coolant flows into the reservoir tank through the upper tube. As the engine cools down, the coolant contracts, creating a vacuum in the cooling system. The vacuum then draws coolant from the reservoir tank back into the radiator through the lower tube.
This continuous circulation of coolant between the radiator and the reservoir tank helps maintain the proper coolant level in the system and prevents overheating. It also allows for expansion and contraction of the coolant as the engine temperature changes.
If you suspect any issues with the coolant system or are experiencing coolant leaks, it's advisable to consult a qualified mechanic or refer to the Honda service manual for detailed information and proper troubleshooting steps specific to your motorcycle's cooling system.