# Re: My streetfighter

On Wed, 3 Sep 2008 13:57:14 -0600, "Bob Myers"

"Andrzej Rosa" <bakters@xxxxxxxxx> wrote in message
news:df63p5-stm.ln1@xxxxxxxxxxxxxxxxxxxxxx

Actually it isn't. For a bike to lean (let's make it usually, to avoid
controversies) it needs to countersteer, which involves moving its
front wheel to the side.

No, it doesn't. As you say, ignore the forward motion;
consider only how the contact patch of the tire is moving
in the sideways direction with respect to the ground. To
be sure, the location of the patch on the tire changes,
but only a very slight distance compared with the side-to
-side motion of the rest of the bike above it. The "axis
of rotation" - that imaginary line which would be remaining
"fixed in space" while the bike rotates (leans) around it -
is clearly MUCH closer to the ground than the CG.

I think so too. If the frame of reference is the CG, then the bike by
definition does rotate about the CG, but then the tires and earth are
also rotating. about the CG axis.

Choosing the frame of reference to be the earth, and having that frame
of reference move in the same direction and speed as the bike, yields
a better picture. In the case where the frame of reference is on the
earth moving along with the bike, the tires move off to the side,
gravity latches onto the CG and pulls it toward the earth, with the
net result being that the bike rotates about the tire to earth contact
point. The CG is rotating as this occurs, but its axis of rotation is
also falling toward the earth. The CG is falling straight down so long
as the tire contact points are moving to the side at the same rate the
CG is falling.

So...
If the CG was a mile off the ground, it would have to fall half a mile
while the tires move half a mile off to the side for the bike to
achieve a 45 degree lean. If the CG were only a foot off the ground,
it would have to fall only half a foot while the tires only have to
move half a foot. That makes it seem like a low CG bike would lean
quicker than a high CG bike.

I'm not sure how inertia would affect that above, but doesn't seem
like a high or low CG would affect things either way as it looks like
gravity is doing all the real work. If the tires move off to the side
faster than the CG falls, then they will leave the earth. The CG will
fall at 32.2 ft per second squared.

It may be the flickable feel of a high CG is due to the fact that the
tires move farther to the side in the same amount of time than a low
CG given the same lean angle.

That's my gut feel. High or low, the amount of rotation will be the
same given the same lean angle. While the tires have more leverage
with a high CG, the tires have to travel a greater distance for a
given lean. Assuming the CG falls straight down, you're not really
moving any mass to the side, you're just rotating it.

A low CG means quicker leans than a high CG?
Robert
.

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