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464 Posts
Discussion Starter · #1 ·
Those of you who have read my posts know that I pay a lot of attention to weight of components on my motorcycles, and particularly weights, shapes, and locations, of components that ROTATE when the motorcycle is in motion.

The chart below, from "Motorcycle Design and Technology" by Gaetano Cocco, gives a glimpse as to "why". I found the chart tonight while reading Cooco's book, which by the way is fantastic, as it makes even difficult Physics concepts, that affect performance of any motorcycle, understandable.

As you can see in that chart, even when the weights of rotating components appear roughly similar, their shape and location have a profund effetc on their moment of inertia.

For those not well versed in Physics, moment of inertia (MOI) is to rotational motion what mass is to straight line motion. The higher the MOI, the more resistance to rotation, and to CHANGES in rotation (speed of rotation, angle of rotation, etc), which means the higher the moment of inertia, the more resistance to linear acceleration, rotational acceleration, and changes in direction ("handling"). i.e. the higher the MOI, the "slower' a bike becomes - in multiple dimensions.

In the chart, the tire for the example motorcycle is pretty similar in raw weight to the wheel rim and to the dual brake disks, but its MOI is 4 to 9 times higher, because it (a) has a large radius of rotation in use, and (b) almost all its weight is concentrated very close to its outer radius.

So, when you take a pound of weight off a tire, it's a big deal for the performance of the motorcycle.

Jim G


91 Posts
I would imagine the effects of weight reduction are much more noticeable on these little bikes due to the
percentage of reduction vs overall weight. I have noticed you ARE very focused on the physics of motorcycle
operations, which initially I thought was kind of a different approach to what most riders think when riding or
customizing. But after more thought and research on my behalf, I get your reasoning.
With the limited output of these little thumpers, proper tuning with the right components and using the laws of
physics can make a difference that is noticeable.
Losing a few pounds here and there on a 500 lb bike is probably not noticeable, but on the Z it could be
much more significant.
Back in my early riding days, I would have said, so what. Get on it and pin it.
But, guessing you may be retired and coming from a background of sciences, I get where you find building
this little bike entertaining.
My technique, to make mine faster......lose the spare tire.....on my belly. 😂
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