ncdogdoc[/i]
[B]I cannot honestly believe that larger rotors (which weigh less if I understand) would create a 12 hp loss. That is like 7-8%. The inertia effect is true said:
Are you losing the power or the ability to accelerate as quickly?
I think the power is still there, just the rolling diameter is larger
Right or wrong?
There are several factors that change when moving from smaller wheels to larger wheels, and these are getting confused here:
- weight
- distribution of weight
- rolling diameter
First, let's remember that what we're talking about is actually the wheel and tire, not just the wheel.
Weight is a performance factor, but that's not what we're talking about.
Rolling diameter is a factor - it's often referred to as "effective gearing" because changes have the same effect as changing gear ratios. Changing the size of the wheels and tires may or may not change the outer (rolling) diameter of the tires, depending on the size chosen. (Usually, you try to choose a tire size that
doesn't affect the diameter.) Anyway, this too is a tangent, because rolling diameter is not what we're talking about here, either.
The
distribution of the weight is what we're talking about here. You can have two different wheel/tire assemblies, with the same weight and the same rolling diameter. However, let's say one uses a 16" wheel, and the other uses a 19" wheel (which would be called a "plus 3" fitment). The statement above assumes (AFAIK correctly, but I'm not sure) that the 19" wheel/tire assembly would have its weight distributed further out towards the tire tread, whereas the distribution of the weight of the 16" wheel/tire assembly would be concentrated closer to the middle. If that is indeed the case, then the 19" assembly would require more force to accelerate, due to its greater "moment of inertia" (that's the physics term for the force being distributed towards the edges vs in the middle), thereby resulting in a greater difference between crank hp and wheel hp (although again, like Gary, I question how much of a difference we're talking about).
This concept is very familiar to all airplane pilots on the board. Before taking off, you perform a "weight and balance" calculation. You calculate the weight to make sure that the airplane is not overweight. You then calculate the balance, using the moment of inertia to calculate the moment arm of each component (the empty airplane as well as each of its contents that vary - the fuel, the front seat passengers, the rear seat passengers, the baggage, etc). You make sure that both calculated results - the weight and the balance - are within the performance envelope of the airplane. It is even required by law that the information needed to calculate the weight and balance is stored inside the airplane, to permit this calculation.