cooling fan for heavy traffic?

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Jul 31, 2006
so i have been running a CHT gauge and have noticed that it doesn't take much slow puttering to get the cylinders to a "red-line" of 425 deg. F on a hot summer commute, even with an oil cooler. (i don't know if 425 really is an appropriate red-line, but 425-450 deg. F seems to be the suggested redline for airplane engines...)

i also noticed that it is pretty easy to slip a fan in between the heads and the frame down-tubes (ugly as it is).

has anybody experimented with this sort of thing? i am trying to run the fan with the least power draw since it will be running at low rpm where i am not charging the battery, but am trying to ballpark what CFM i would need to keep the cylinder heads happy...

as an example, 85 CFM seems to cost about .32 Amp / 3.84 watts, whereas 190 CFM costs 2 Amp / 24 watts...
I work for a US motor & blower manufacturer and even though 190cfm seems quite a bit of air it is not a lot though it would help. At cruising speeds you have the equivalent of thousands of cfm flowing over the engine. You would need to direct the flow or at least have the blower as close to the head as possible. I guess you're looking at a brushless design? If so you need to watch the vibes as these little devices cannot take lots of vibration. Generally the electronics which handle the commutation are encapsulated but are quite delicate.
I used a small computer fan (1.5" dia) to cool the stator (beltdrive) which was slowly cooking itself. That got shelved when it lasted an hour from engine vibes. I settled for removing all inspection caps. You may get away with the isolated frame mounting though. Oh and you'd need the 180w 3 ph stator setup for low speed charging.
thanks for the reply! you're right about that CFM thing -- I tried a cheapo 85 CFM 120mmx38mm computer fan right in front of the heads (mounted on the front frame tubes), and it is not enough to keep the engine from heading right to 400F at the heads, although the fan does help slow the temperature rise down a bit...

the good news is that after about 4 hours of riding (between 2-4k rpm) the fan has not self-destructed, and so now that i know i am not completely throwing my money away, i am going to try the 'fancy' 190 CFM fan... hopefully that will do the trick!

i am running a 3-phase 220 system... with the headlamp on, it still takes like 2k rpm to reach about 12.6 volts, but my system is probably not running perfectly because the alternator shims don't really center the rotor magnets on the alternator (the alternator is out a little too far)...

If I was facing 400F head temps that lasted more than ten minuets I would be running nothing but Red Line heavy duty Harley Vee Twin oil.A cooler without a stat is a bad idea.
Oil temp write-up

Hi HC,
You may want to read the write-up at

The guy installed heat sensors in various places in his engine and ran under various conditions.

Regarding the use of a fan, when you think of all the air cooled bikes out there over the years, Nortons, Triumphs, BSA, Harley, and yes 99% of the Japanese stuff up through the 80s, under normal circumstances cooling has never been a problem.

thanks for the article!!

i know that i might be a bit paranoid about all of this... i have a friend with an oil cooler-less BSA who rides under the same conditions and he has never heat seized (neither have i for that matter... ), and all of the area mechanics seem to think that i am crazy to add the fan..., but since my bike is a daily rider which will see a good amount of city commuter mileage, i am trying to be kind to it... shrug!
Buel has a fan on their newer models. you may need to take a look at the set up they have. It's thermostatically controlled and runs even when the engine is off during cool down, thats when you will see the astronomical rise in heat (believe it or not the incoming fuel has a cooling effect).

well, after about 3 hours the plastic 120mm 190 CFM computer fan seems to still be working... in 85F weather, the fan can (slowly) settle the heads to roughly 325F.

not exactly sure how long the plastic bodied fan will survive all of the heat radiating from the engine since the fan will be off most of the time... now the trick will be hiding my horrible wiring job (i also added some turn signals in the process) before any of my friends see what has become of my bike!
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