Well you tell me! When I said "dangerous" I meant to the fingers/tools/fan blades of those who don't know the fan is not under their control after the bike is shut down.
I did notice that the fan turned on for a very limited time, but this was in temperatures in the 40s. I remain unconvinced that any cooling that might take place after the water pump has been turned off is significant to the longevity of the engine. There isn't any advantage to cooling an all-aluminum engine down more quickly that I can see. If anything, it seems like a rapid cool-down might be a disadvantage.
On the other hand, if you have a car with aluminum heads on an iron block, I can see the benefit of anything that keeps these two at close to the same temperature as long as possible, because of the differential expansion rates of the different materials. That difference puts a huge stress on the ability of the head gaskets to maintain the seal, and without that you of course risk the contamination of the oil with water, which is never good. And even in this case, the water circulation needs to be maintained (which it rarely is) in order to circulate the cooled water out of the radiator and through the block and heads.
I just don't see the point, but I'm always a sucker for a good argument!
Personally, I would not recommend that you deactivate the cooling fan operation following engine shut down. The Z1300 cooling system is typical of many engine cooling systems in the 70’s and 80’s in that it uses a sealed, fluid only engine circuit with a valved connection to an open expansion tank.
When the engine is in operation, the “hot” parts of the engine, generally the combustion chamber surfaces and cylinder bore surface run much hotter than the coolant temperature. When the engine is shut down, there is an element of what we call “heat soak” from these hot areas into the coolant which locally can raise the coolant temperature to the point where it will boil. Although the cooling pump is no longer pumping any fluid with the engine stopped, there is an element of what is called “thermo-syphon” flow if the coolant in the radiator is cooled below the temperature of that in the engine. The reason that the fan switches on after the engine stops is due to the heat soak mechanism. The fan then operates, cools the radiator coolant which sets up a flow through the engine – when this cooler fluid reaches the temperature switch, the fan switched off – often this process will occur two or three times after shut down.
If the fan does not run, there is a risk that local boiling will expel a large volume of coolant into the expansion tank which would then overflow – all of us have probably performed the “burp” process to expel any vapour from the coolant system after refilling – this is a similar process and needs to be managed carefully.