Quote:
Originally Posted by swamp2
In short when you change a wheel weight (or any drivetrain inertia) you affect performance (acceleration) but you do not affect the power.
To understand this fully it pays to look at a simple case. Let's consider the case of a car moving in a straight line first. If you add a passenger to your car does it change the cars power output (crank or wheels)? No, however, it does change it's acceleration. This is simply changing one side or the other of Newton's Law, force = mass x acceleration, F = ma. Change the engine and make more output and you change the left side, change the mass and you change the right side. Both will affect acceleration.
Now when we add the effects of rotating bodies we simply realize that rotating bodies translate along with all of the sprung mass of the car and they spin up relative to the translating car. All changes to rotating masses or moments of inertia change the sides of the equations representing the mass (or intertia) and thus bodies resistance to forces or torques. They do not change the "source" the force or torque terms (and thus power as well).
However, if you use the "wrong" type of dyno (say Dynojet - inertial dyno) to measure a before and after where a wheel (or drivetrain component) changes its mass or inertia you will get a different number because an inertial dyno simply assumes the same inertial mass for every car. A hub (brake) dyno will not show a power gain from modified drivetrain masses or inertias.
These subtleties are certainly worth understanding in detail. Hope that helps.
P.S. I see you are in San Diego as well. Ever come to the San Diego e90post.com meets?
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It does make sense and I understand the example of adding a passenger vs. being a lonely driver...but...if you change the mass of an object (wheel) that you want to rotate (we'll say decrease it) against a surface, then it becomes easier to rotate it (less force). So, doesn't it stand to reason that if less force is needed to rotate that object ,the translation of applying the same input power is that the object (wheel) will spin faster (increased acceleration)? If so, then if we draw a line on the ground, the power measured when the vehicle crosses over that line with the heavier rotating object (wheel) will be less than the power measured when the vehicle crosses over that line with the lighter object (wheel). Essentially, you are at a higher RPM when you cross the line with the lighter wheel (screw the object reference) then you were with the heavier wheel.
Are the e90 meets in Clairemont on Wednesdays? If so, I never know which Wednesday is a meet day (I believe it's every other Wednesday).