Strange fan behaviour: The hotter the slower!

[Hanky]

I still have a little 50x50 Y.S. Tech fan on my relatively cheap Coolermaster HS (for a TB 800). I want to buy a better HSF soon but meanwhile I found that the fan shows some strange behaviour. After a cold boot when the system and the CPU are still cold the fan spins at ~4800 rpm but after heating up the system it slows down to ~4300 rpm. Could this be some general problem with the fan or maybe it's not even a problem but has to happen or is rpm measuring as inaccurate as temperature measuring?

 

[KouklatheCat]

I would GUESS something like heat builds resistance or when the bearings in the fan get hot they expand. I dont have rpm monitoring on my fans, most of mine draw too much current for the MOBO headers.

 

[Hanky]

Maybe it has something to do with the kind of bearing...the Y.S. Tech fan is a two-ball bearing fan...on my modded GeForce I also have a 50x50 fan but it's a very high-quality sleeve bearing fan from Papst. This fan stays at the same speed (its "rpm-window" has a maximum size of 50 rpm) no matter how hot the GPU gets. Maybe Y.S. Tech just isn't as good as I always thought...

 

[Supradude]

hm... just a hunch. but depending on your fan setup, you may be causing some weird turbulence and pressure systems inside your case which may introduce drag on your fans...

 

[ericd]

How many watts is you PSU. Maybe the amount of power being supplied to the fan through the header is dropping off??? Just a thought.

Eric

 

[Hanky]

My PSU is the 330 W model from Enermax (modded to be more silent), so I don't think it can't supply enough power because I don't have anything power-hungry in my system besides the GeForce and the TB800 but it should be able to handle them!
The suggestion with the turbulences is hard to prove or reject...all I can say is that my cooling setup is more or less standard: one front intake fan and one back outtake fan.