Explain me the cooling system :)

So I have been playing with my cooling system lately. Shortly - new thermoswitch, new radiator cap, coolant system flush and new coolant in and air out lifting the front. If you want longer read its there: viewtopic.php?f=5&t=19232&start=50#p267063 The problem were, I found out my radiator empty and bike overheating at idle and at the same time expansion tank were full (so I presumed when its full there is enough coolant in the system; overheating at idle indicated the problem at the end).

After flushing the system and filling cooling system with fresh Motorex 5.0 coolant, I adjusted overflow tank exactly between min and max mark. And it was again full one day after a few rides, over the max mark. So I drained out the coolant from overflow tank between min and max mark again. Has been done again few drives since, and the level has been stayed steady, and radiator full when checked under the cap. Today before the blast with cold engine and bike has been not used for two weeks I opened radiator cap, to check is there any air in radiator and need to fill up. Surprisingly at opening the radiator cap, it filled the overflow tank over the max mark and significant amount of coolant also slurped out from the radiator cap as well. So, I drained overflow tank exactly between min and max mark again. There were no need to add coolant to radiator as it was full. But I lifted the front and there were possible to add significant amount of coolant, may be 100ml. So went for a 10km blast and level in overflow tank were exactly between min and max mark.

After it I went for good 2 hour drive on the same roads where 1290 SD press release were done in Andalucia. And after return the overflow tank is again full over "max" mark. How does this fooking cooling system works Should it suck back the coolant from expansion tank when cold again Do I need to check coolant level from expansion tank Or should I check coolant behind radiator cap Can I keep the expansion tank full Or between min and max mark

I know these are probably way too simple (read: stupid) questions to ask - but I really do not know the answers, and I don't really care much to look a bit stupid to ask them

? air in the system, could drain and refill, lift front then fill very very slowly,stoping every 30 secs to let any air surface, then when full, unso drain plug and run until only coolant comes out, do up, top up coolant and expansion chamber, level bike run , then re check. (you've prob done all this) then if still have probs, its something else. sorry thats all i know. unless you have a pipe leaking and its also sucking air in ? good luck.

Sounds like the radaitor cap is not sealing correctly, I would suggest replacing the cap and checking the sealing faces are clean,

OK, first things first. Expansion tank is for coolant expansion while engine is hot, right? Should the engine & radiator suck expanded coolant back when engine is cold again??? Cause mine does'nt do that.

Radiator hose nr. 28 is very-very slightly leaking through cotton reinforcement, at the of connection with 27. New Motorex 5.0 coolant leaves nice greenish-blue marks and makes possible easily trace any leaks. But WTF is going on, I am interested purely from standpoint of simple physics? Why such an expansion?

Stupid question but did you bleed properly?
How high did you get the front wheel up to do this?

The coolant is at least 50% water (or greater percentage ...), and the remainder is mostly glycol (usually ethylene).

When heated (by the engine), these liquids (the "coolant") expand in volume (some perecentage of which may reach a vapor state at high enough temperature). When these substances cool, be that by passing through the radiator (which transfers - "radiates" - heat from the water/coolant into the air), or after the engine is shut down, they reduce in volume (condense back to liquid, if in vapor phase).

When you ask about the "physics" - how far do want the subject taken? Molecular properties and behavior from the heat (the thermodynamics, if you like), or more fundamentally, the statistical mechanics?

Noted elsewhere, I've had to replace nearly every hose/tube clamp on my SuperDuke due to tiny leaks - including exactly where you describe. Gets pretty warm down here (much like parts of Spain or Italy). Those OE "tongue and groove gap-free pinch clamps" are so thin-walled, combined with the gradual loss of elasticity in the hoses, vibration, and thermal cycling (including effects of thermal expansivity of the engine's various aluminium alloy components to which the hoses are fitted), that the clamps become less effective over time.

In replacing the OE clamps, I use stainless steel (including the screw) "worm clamps" for soft tube (so as to not cut into the hose material from tightening, expansion/contraction, and vibration) or what are known as "bolt clamps".

I don't know if this addresses your question(s) ...

I'd look at doing the flush process again, paying attention to the level that the front is lifted and SLOWLY adding coolant until full (you did remove the bleed screw when filling, right?).

I flushed mine over the winter and was riding it for at least a month in the spring before all of a sudden the level in the tank dropped to almost dry. I freaked; thinking that I had a worn water pump impeller and coolant was going into the crankcase. Full oil and filter change revealed no sign of coolant in the oil. Topped up the coolant and have had no issues since. I'm thinking that the cooling system "burped".

Good it's fixed.