<VV> Fan Belt Physics

kevin nash wrokit at hotmail.com
Mon Jul 1 23:44:27 EDT 2013

> Message: 7
> Date: Mon, 1 Jul 2013 16:51:36 -0700
> From: "Ron" <ronh at owt.com>
> Subject: Re: <VV> Fan Belt Physics
> To: "Bill & Chris Strickland" <lechevrier at q.com>,
> 	<virtualvairs at corvair.org>
> Message-ID: <FEA72EDCB1CE42D79F3BC8F82488E373 at YOUR76500D519C>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> 	reply-type=original
> Is there any slippage that anyone has ever documented or is it just another 
> theory?
> RonH
> Ron- The way most of us set our belts, (light finger pressure to turn the alternator), yes there quite a bit of slippage at 4000
rpm- a guy posted this on the fastvair board. He used the tach off of his sds (digital tach to measure engine rpm) and measured
the fan rpm with a optical tach- there was no slippage with the damper doors shut, but with the damper doors blocked open,
he was reading 600 rpm lower. Pressumably, he did this with the engine cold and not hot (theres  less pressure for the fan to push
against when the engine is hot)- however, he did not try doing the test by setting the belt as tight as GM wanted, which was
75LBS(!) belt tension set with a belt tensometer. Set that way, I seriously doubt there's any slippage  ( although I intend to 
do that test once I get finished with another project that I have to get done). Once, many moons ago, I did probably come close
to setting the belt that tight- I know it was just about so tight so as to turn the engine over with the fan (oops- I didn't know any better!)- anyways, the belt came off on one shift at 6500 rpm- never set it that way again. Anyways, I have measured shroud pressure with the engine good and hot, and found ZERO difference in pressure when the belt is set normally vs scary tight.
Also, I actually did verify what JVH said about the belt expanding more than the engine does hot vs cold, the belt really does
get significantly looser when the head temps get to 300 degrees vs dead cold. I measured the belt deflection at 1/2" when cold
and then when the head temps got to 300 degrees, measured the deflection at "very close" to 1"- I do have aluminum pulleys
on my car, which might have something to do with it (probably very little, as the pulleys didn't feel very hot when I touched them).
I also bothered to re-check the deflection of the belt after the engine was back to dead cold again, just to make sure that the
the idler hadn't slipped, and... the belt droop was back, as best I could tell, where it was when I initially set it to 1/2"
Kevin Nash


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