Re: Gyroscopes - Usenet Physics FAQ - Reference frames

From: greywolf42 (mingstb_at_marssim-ss.com)
Date: 08/04/04 

Date: Wed, 4 Aug 2004 12:45:37 -0700

"Tom Roberts" <tjroberts@lucent.com> wrote in message
news:vXVPc.3$Y94.2@newssvr33.news.prodigy.com...
> greywolf42 wrote:
> > "Myxococcus xanthus" <mold-guardian@comcast.net> wrote in message
> > news:ce5e7813.0408030119.77bb3b25@posting.google.com...
> >>Please do a google search for references to
> >>Jones, David E.H., "The Stability of the Bicycle", Physics Today (April
> >> 1970): 34-40
> >>The gyroscopic contribution to bicycle stability is relatively minor.
> >
> > As a teenager -- purely as a scientific experiment ;) -- I repeatedly
> > released an old bicycle to roll down a local hill. Except for the times
> > it didn't make the turn, it stayed rolling for several hundred yards.
> > That's hardly minor.
>
> But, of course, the fact that an unoccupied bicycle can roll on its own
> has nothing to do with gyroscopic action of its wheels. It can do that
> because the geometry of its front fork provides feedback to make it
> steer the wheels under its center of gravity.

LOL! And the geometry, of course, simply acts on it's own. ;)

The question is *how* the feedback is provided.

> The couple formed by the
> lean of the bike, the camber of the front wheel[#], and the weight of
> the bike is MUCH larger than the gyroscopic force. And unlike the
> latter, it is independent of speed,

Then the bike would have only one stable speed. And yet, my bicycle stayed
up only at higher speeds. It fell over immediately, if I did not get it
moving fast enough (a short run). And once going, it was stable from about
5 mph to what I later estimated at 30 mph. (The bike took some damage when
it hit a ditch.)

I presume that it is the gyroscopic effect that gives rise to the 'couple'
to which you are referring.

> and works at low speeds (but not at
> zero speed, so it does fall when friction brings it to a halt)

How about at 0.1 mph? Nope it falls. Even though friction is overcome, and
the handlebars turn.

> [#] extend the axis of the front fork to the ground, and note
> it is in front of the wheel's point on the ground. That
> distance is called the camber. The front fork is usually
> bent forward to reduce the camber to the desired value for
> the inclination of the fork. Compare a racing bike and a
> touring bike....

To a "chopper.".....

I believe you mean "caster", not "camber." "Camber" is the angle of the
wheel from vertical, not the displacement from the turning axis.
http://www.team3s.com/FAQcastcamb.htm

> If the effect were gyroscopic action then the bicycle would presumably
> be stable going backwards also. Try it -- it isn't stable at all and
> falls immediately -- the geometry of the front fork (at the rear) causes
> oversteer.

I've also driven forklifts -- which have their steering wheels in the
rear -- and don't lean. And they are unstable in steering going forwards.
In other words, the instability of steering in the following wheel(s) due to
the dynamics of inertia, and is not just dependent upon caster. Examine the
dynamics of a turn with steering in the lead wheel(s) and with steering in
the following wheel(s):

Lead: The wheels initiate a turn to the right. They force the center of
mass to the right, by moving the front of the vehicle to the right. The
inertia of the vehicle will oppose the turning tendency. (Negative
feedback.)

Following: The wheels initiate a turn to the right, by moving the rear of
the vehicle to the left. They force the center of mass to the left. The
inertia of the vehicle will enhance the turning tendency. (Positive
feedback.)

Now, one can engineer a car or bicycle with 'oversteer' by fiddling with the
caster. But this relates to the force needed -- not the balance points. 

--
greywolf42
ubi dubium ibi libertas
{remove planet for e-mail}

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