[Shotimes] Re: Rotating mass and dyno tests

[MonsieurBoo@aol.com]

"What they determined is that the larger brakes caused the net 7 whp  loss, 
as these were the only changes."
 
I must be missing something here.  The logic is that there's a "force  
multiplier" effect with larger diameter rims/rotors/tires:  Even if the  total 
weight of the larger-diameter combo is less that the pieces it replaces,  more HP 
will be required to rotate the larger-diameter assembly, because the  weight is 
located farther from the center of rotation.
 
That certainly holds true for ET, provided we assume the car is  accelerating 
all the way to the traps.  Taken to an extreme it would make  the car bog off 
the line and when shifting.
 
But for HP at a constant RPM I can't figure where it would make the least  
bit difference, even if you are measuring it at the wheels.  Since the  engine 
isn't accelerating the spinning weight, no HP is being lost.   (Although, if 
you had rims with "turbine fan" spokes, the parasitic drag  would kill ya.  But 
that's not a weight issue.)
 
Maybe it's as simple as when they increased the rolling diameter, they took  
the engine out of the sweet spot of its power curve by changing the actual,  
final drive ratio.  It was running flat out but at a different RPM and just  
not on the sweet spot anymore.
 
Cheers
Mark LaBarre
94 atx 130k