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Originally Posted by Nutsy
Why is there a performance hit with 19s?
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It has to do with moment of inertia and it's not just that the 19" combo is heavier. It's that it's heavier farther away from the center of the wheel. As weight moves farther from the center it has a exponentially worse effect both on acceleration on and on stopping.
Think of a lever and the saying "give me a lever long enough and I will move the world"..
Here, for braniacs (unlike myself) this site should give headache.. lol
http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html
For people more like myself..
http://en.wikipedia.org/wiki/Moment_of_inertia
&
http://www.britannica.com/eb/article-9042379
Ok, so they are talking about bike wheels.. same principal..
http://www.canecreek.com/168.html
So to sum it up:
Polar moment of inertia
The resistance of an object to rotational acceleration. When the mass of an object is distributed far from its axis of rotation, the object is said to have a high polar moment of inertia. When the mass distribution is close to the axis of rotation, it has a low polar moment of inertia
Wheel weight is bad.. wheel weight farther from the axis of rotation is worse. That why a 20-inch wheel that weights 30lbs is far worse in regards to acceleration and braking when compared to a 17" wheel.. even if they both weighed the same (which I doubt they would) since most of the weight of the 20" combo is so far from the center point of the wheel.
Did any of that make sense? lol
It's not that you shouldn't run 19's or 20's or whatever. It's just that you should know the physics behind what you decide to do and understand what trade offs some decisions entail.