I'm sick of seeing people not understanding this concept so I'm going to lay it out in this thread as clearly as possible.
First off, unsprung (rotating) mass is: weight that is NOT
held up by the suspension, and is SPINNING as the bike is moving.
The "weight" of unsprung mass is irrelevant. The moment of inertia (or weight distribution) is the unit of measure that really counts, and those numbers are hard to extract meaning from. It's not gonna be like "Oh man, I can REALLY feel that last couple inches^4 that we got off there". Rims do make a HUGE difference though.
The reason why brake horsepower numbers are always higher than wheel horsepower numbers is because there's extra weight to rotate before the power can get to the ground. As such, rotating mass directly affects the acceleration, deceleration and horsepower on a motorcycle. You're going to see more power on the dyno with less rotating mass (like lighter rims/rotors/chain/sprockets), because you've got less parasitic weight in the drive train to eat up power.
What I'm saying is that if you weighed a stock rim and a forged magnesium (for instance) rim on a scale and they both turned out to be the same WEIGHT, that wouldn't mean that they would perform the same. It's all about weight DISTRIBUTION
, hence why the units for moment of inertia are in length^4. It's determined by cross-sectional area times the distance to the center of rotation (the axle) squared
. This is why cutting a couple pounds of rotating mass from the tires has a much more dramatic effect than cutting a couple pounds of rotating mass from a rotor. The tires are farther from the axle, and that distance is squared when computing moment of inertia. This is why doing the 520 conversion for a lighter chain and sprocket than stock while leaving the gearing alone won't really do much for you. It all helps, but you aren't going to feel that little of a difference in moment of inertia. Same with rotors..
3 pounds lost off the weight of the bike (like from a lightweight battery) does NOT yield the same performance gains from mounting rims that are 3 pounds lighter, necessarily. You can't even determine the effectiveness of your lighter rims with just a scale. Yes, lighter parts are almost always better, but that's not what counts. What I'm saying is that you can't say that mounting a tire that's 1 pound lighter is going to give the same performance benefits that losing 1 pound from the rotors, because they're not the same distance from the axis of rotation (the axle in this case).
Now does everyone understand why saying "these rims are 3 pounds lighter than stock" means absolutely nothing in terms of actual performance gains?
Another member explains this concept with a great analogy:
Originally Posted by icepic
perhaps i can assist in the understanding of this argued concept. imagine you have two baseball bats of equal weight but unequal size. one bat has most it's weight near the grip (imagine holding the bat backwards) and the other bat has more weight near the end.
which bat will be harder to swing? the bat with the weight farthest from your body.
here's why: although your are technically swinging the same amount of weight, your moving a different amount of mass a different distance. the bat with the mass closest to the grip only has to move a short distance through space. however the bat with the weight farthest from the grip is traveling a much greater distance. so instead of moving 3lbs across a two foot distance, your moving 3lbs across a 6 foot distance, thus more effort is required.
did this clear it up for anyone?