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nathanTX said:
Yup, agreed. Most bikes' (not just Ducs) fans come on somewhere between 200 and 225 F. Why? 212F = 100C = boiling point of water at atmospheric pressure and in an unaltered, untreated state. That is, boiling is when the pressure of the vapor equals/exceeds the pressure of the atmosphere and therefore is released. Put water under pressure and it now has to be heated above 212F/100C before the vapor pressure exceeds the surrounding pressure and liquid turns to gas. A closed coolant system such as is found in cars and motorcycles significantly raises the pressure and therefore significantly raises the boiling point. So, even before you factor in the chemical properties changed by mixing in coolant or other additives, a closed coolant system will allow you to safely exceed 212F/100C by quite a bit.

Engineers will calculate exact boiling points based on the system's operating pressure and the type of coolant/additives they will be using, calculate a margin of safety and depreciating cooling abilities, and come up with a "safe operation" number at which to activate additional cooling measures to keep things in check. If you must know, boiling point in a 15psi system as is found in most cars, using a 50/50 mix of water and ethylene glycol antifreeze will be 265F (I looked it up on an automotive site). But the layperson just needs to know that once you get much above the "natural" boiling point of 212, you should start keeping a close eye on things. 220-225 is still fine and some bikes don't kick on fans until right around there. If you are seeing 230 or 240 I'd definitely be wondering if everything is working correctly, but you are probably still OK (at least for short intervals). If it continues to rise towards 250 or up, I'd be shutting her down since coolant in some areas might be approaching the vaporization point, or less than optimal pressure or mix ratios might give your system a slightly lower boiling point.
Nice write-up! Thanks!
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