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RE: Sportster and Center of Gravity Debate


From: "Frank LeGeros" <flegeros@austin.rr.com>
Subject: RE: Sportster and Center of Gravity Debate
Reply-To: [Author] - [XL-List] - [All]
Date: Fri, 29 Dec 2006 21:58:25 -0600

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In a previous post I wrote........

>> So if you have two bikes with the SAME COG and the SAME moment of
inertia, the bike with the higher seat and bars will seem to lean from side
to side more easily. <<

And Ken replied........

>> This assumes that you are leaning the bike over with your weight.  <<

Yes.  Such as when the bike is standing still and you try to wiggle it from
side to side.  As I said, I primarily considered a static analysis.

Ken goes on.......

>> If you are counter steering ( at over 20 mph or so), the forces arising
from the gyroscopic action of the front wheel as it is forcibly twisted
about its axis will lean the bike much more abruptly than
just using your weight to lean the bike over. <<

Yes.  And those gyroscopic forces will only "see" the moment of inertia.
The overall height of the bars and/or seat will not affect how quickly the
bike responds to the gyro action.

But I think that when the bike leans over it is responding to the SUM of the
gyro forces generated by countersteering and the forces resulting from the
rider shifting his weight.  And the response to weight shifting will be
influenced by the "leverage" resulting from seat & bar height - assuming
that the COG is the same.

But the above is only presented to illustrate the principle involved.  In
reality, raising the seat will also raise the COG because it is the COG of
the bike and rider TOGETHER that matters in a dynamic turn, (i.e. whenever
your feet are off the ground) and with a higher seat the rider's weight sits
higher  - causing both the COG and the moment of inertia (of bike + rider)
to be higher/larger as well.

>> And if you slide the tubes up in the triple clamps an inch or so,
changing the front end geometry, it will roll into a turn even faster. Been
there, done that. <<

By sliding the tubes up in the trees, I assume this lowers the front of the
bike.  If so, I agree.  This will lower both the COG and moment of inertia,
and so increase the responsiveness of the bike.

Frankie
2005 XL883, taxes paid

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