Definition
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Rotation adds inertia, but it only resists motion out of plane. A mounted tire spinning at 10,000 rpm takes NO extra force to control vertically (90° to axis of rotation, such as caster change or suspension travel). It will resist attempts to incline (“tilt”) it, such as camber change, very strongly, and resistance increases as the square of the rotational speed.
Since the total camber change is only 10° or less in most cases, the improvement is only felt as improved steering response, and less compliance/deflection in steering linkage (tie rod, etc.).
I'm trying to figure out how much of an advantage the aluminum parts of a 727 are, since they turn FASTER than engine speed - are therefore are some of the MOST valuable (not to mention the safety factor). I just don't know enough about AT to do it yet.
As a rough guide to added power from reduced inertia:
The faster it spins, the more power lost, but this also varies as the square of the radius of gyration (think of this as the "center of gravity" of a rotating object - about which the forces act as if they were focussed).
Therefore: alloy wheels are good because they're very large, but not that good because they don't rotate very fast.
A driveshaft turns much faster, but not so good because the radius is so small.
If trying to substitute aluminum parts for steel, look for things that rotate first, especially in the power train - the faster the better, and pick the largest diameters as being the most effective. Too bad an aluminum ring gear on a converter wouldn't hold up very long! This is another reason why smaller converters are faster: smaller internal volume of fluid, and rotating on a smaller radius works the same way as a light flywheel.
BTW: a "steady-state" dyno will generally NOT show a power increase from lighter crank, rods, pistons etc. because the extra energy (that wasn't used to speed them up) doesn't register unless the engine speed changes.
The dynos that accelerate the motor from speed A to speed B in X seconds will show power increases.
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