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Old 01-16-2013, 02:55 PM   #27
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Joined: Jun 2007
Location: San Mateo County Coast, CA
Oddometer: 636
Originally Posted by geometrician View Post
BillyD the lower temp (200f) fan switch works by beginning to shed heat sooner than the 220f switch.

In heat exchangers the temperature difference (delta) of the source (our radiator in this case) & the sink (atmospheric air) does indeed change the time required to shed heat to come down to (whatever) target temperature. By not allowing the coolant to reach that higher temp you're already 20 degrees closer to your target temp.

Wattman did extensive testing on cooling systems & found that having a fan on at 60+mph lowered the coolant temp 11f- albeit on a "lowly" KLR. Seems our bikes would fare better as the back area of the radiator is quite cramped

I've turned fans on before I rode into deep sand to pre-cool, which will allow you a few more minutes of run time before the bike heats up- sometimes it's all you need in a tight section before you can make some speed to bleed heat off. I'm talking about the +90f/80% humidity of America's South, don't know how it would work in the dry Southwest
Agreed. Pre-cooling (by switching on the fan) might help make it through a transitory situation without boiling over.

My comments assumed a more steady state situation, such as when I'm usually stuck in those tight technical sections for extended periods of time...alone...dying of thirst...crying for mama.

Here's a somewhat relevant graph. Switching on a fan increases air flow (MPH) and resulting in increased rate of heat rejection.

Per geometrician's comment, increasing the differential between air and coolant temperatures, would also increase available cooling, albeit temporarily in his example.

Alternately, if you were to substitute inlet-outlet temperature differentials (delta degrees C) for coolant flow (GPM) and air flow (MPH) rates on the graph, the graph would look largely the same. In other words, the higher the coolant temperature drop through the radiator or air temperature increase across the radiator, the better heat transfer performance at any given flow rate.

KTM '08 690 RFR, '08 990Adv S (carbs), '07 640Adv, '03 540EXC, Ducati 851, Moto Guzzi 850 Lemans, Ercole & 2VT and Yamaha RD350.

BillyD screwed with this post 01-16-2013 at 03:46 PM
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