Information Your fan is kicking on? So what?

yowsaboss

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It seems that a great deal of though, effort and money has been spent on trying to improve the RC’s “deficient” cooling system. SPAL fans, upgraded radiators, even auxiliary electric water pumps. My operating theory at the moment is that these might be solutions in search of a problem.

My Cup bike (I also own the street bike) has the fan kicking on as I exit the track, after about 30 seconds of not running at pace, even on a 72 degree day. So I cut back the radiator shroud hoping to get more air moving through the radiator. A free fix. This did seem to delay the fan’s activation- for about 75 yards. So my next move was going to be to remove the thermostat, as any thermostat is going to restrict flow, even when wide open.

I was considering installing a digital temperature gauge to better measure running temperature and to accurately measure how successful my efforts were. But after thinking about it, I considered the possibility that the fan kicking on isn’t cause for alarm at all. That it doesn’t mean anything significant, and it certainly doesn’t mean you’re on the verge of overheating, or even "running hot"..

The system was engineered to have the fan kick on at a given time. Some engineer made a decision what temperature that is, based on whatever the optimal running temperature should be. When you begin to exceed optimal, the fan is there to mitigate the rising temperature. Likewise the (horrible) gauge reacts with “bars” to indicate the coolant temperature. The gauge doesn’t tell you what the temperature is, it’s only a visual indicator of the coolant temperature. People have long assumed the fan kicking on is cause for alarm: “it’s running hot”

Is it?

Here’s what the KTM engine manual says:

The thermostat opens at 88° C and is fully open at 96° C. The waterpump pumps the coolant through the cylinder, cylinder head, the radiator and the thermostat. The pressure in the cooling system (max 1.4 bar) is regulated by a valve in the radiator cap. The coolant level in the compensating tank must be between the min and max mark when the engine is cold. The fan switches on at 96° C and off at 90° C.


Based on this I am making some assumptions:

The thermostat starts to open at 88C, (190F) degrees. I’m thus assuming this represents the lower range of the designed operating temperature. Let’s call it the minimum. It’s fully open at 96C (205F). My Google search seems to indicate that for cars, normal temperature range is 195-220, with 230F being boil over. So 207F would be the mid point of this 195-220F range. The behavior of the thermostat seems to be in agreement with this. It’s fully open (thus normal operating conditions have been achieved) at 205F, just a couple degrees short of that mid range point.

The following argument is going to be based on the premise that 205F represents optimum or normal operating temperature, because that’s the point where the thermostat is fully open, with 215-220 being the presumed safe maximum, the point where you start worrying. Any dispute about my argument is going to need to assert I have these numbers wrong.

The fan kicks on at 205F degrees. This is coincidental with the thermostat fully opening, when the bike archives optimum temperature, 15 degrees short of my presumed maximum safe 220F. The fan isn’t kicking on because you’re running too hot, but rather to prevent the temperature from rising above the optimum. That’s optimum mind you, not the maximum safe temperature, as has been assumed.

The fan-on temperature is obviously not some arbitrary number decided by the engineers, since it corresponds to the temperature where the thermostat is fully open.

So your fan kicks on at a traffic light. You’re at 96C, 205F. Optimum temperature. If the fan is effective at all, your the temperature isn’t rising more than a few degrees above that, well below any danger point. After you pull away, 1/4 mile later, the fan shuts off, you breathe a huge sigh of relief. You’re now at 90C/ 194F. At no time have you been “running hot”. In fact, the fan shutting off indicates the coolant temperature has dropped, not out of the “danger zone” as has been assumed, but to the point where the thermostat is just beginning to open! You're now 6c/ 43F below the temperature required to fully open the thermostat. Sounds like a long way from overheating to me.

So all your SPAL fan does is perhaps drop the temperature down to 194F a bit faster. Possibly holding your temperature at the light at 205, maybe even reducing the temperature slightly. No idea.

I don’t know what others’ experiences are in hot climates. A 95 degree day, maybe your fan stays on for extended periods while on the move. Without a digital gauge, it’s hard to know. But if you’re at 205F, who cares? Maybe it stays on constantly because you’re holding at 207F. What are you worried about?

As to any refutation of what I am saying-

First we have to have KTM confirm normal operating temperature range and optimal operating temperature, rather than me just guessing. If I am wrong about 205F, then the premise of my argument disappears. But I think the evidence suggests this number is pretty close. You’re not designing a thermostat that doesn’t open until after you’re already running too hot, but rather to maximize coolant flow to prevent you from running too hot. All the alarm seems to be centered on the idea that 205F is the Upper Safe Limit of temperature, because that’s when the fan turns on. This is incorrect, a false premise.

Fan On = Too Hot?
No, Fan On = Thermostat Fully Open!

How to verify this? Assuming that 205 number is correct, you’d have to install an accurate digital temperature gauge so you knew actual running temperature, and at what temperature the fan actually turns on and shuts off. What’s a normal number for 60mph on the freeway at 72 degrees air temperature?

I am not curious enough to put in the effort. I’ve satisfied myself that what I thought was a problem isn’t a problem at all. My contention is that any efforts to “improve” the cooling system are unnecessary. You haven’t done any harm, it simply wasn’t necessary.

Thoughts and insights welcome. Please don’t tell me I am an idiot, I knew that long before you figured it out.
 
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