From: zowie@daedalus.stanford.edu (Craig "Powderkeg" DeForest)
Subject: Re: Slip-sliding away (was Re: What would you ride...)
Followup-To: sci.physics
Organization: Stanford Center for Space Science and Astrophysics
	<ZOWIE.92Nov18195114@daedalus.stanford.edu>
	<By1FMv.LK8@news.iastate.edu>
Date: 22 Nov 92 22:11:14

In article <bar> tomes@iastate.edu () writes:
   In article <foo> zowie@daedalus.stanford.edu (ME!) writes:
   >The front wheel is rotating about the hub, because there is an
   >inertial frame (moving with the bike) in which the wheel's motion is
   >constant and zero along one line -- the center of the axle.  There is
   >no inertial frame in which any other part of the wheel is stationary.
   >...
   >And don't give me any of this `contact patch' bullshit, because `the
   >contact patch' doesn't describe any particular part of the tire!  
   
   page 21, figure 11.7,"University Physics",  Harris Benson, 1991
      `When a wheel rolls without slipping
      the point of contact with the floor is instantaneously
      at rest and acts as an instantaneous center of rotation.'

   This is only one of several references that I have seen to 'instantaneous 
   centers of rotation' or 'instantaneous roll centers', the rest of the books
   are in storage or at home.  

Well -- the problem with `instantaneous centers of rotation' is that
there's nothing special about 'em.  That is, in some other reference
frame a different part of the wheel will appear to be stationary.
That part isn't necessarily on the rim, either.  If you tell me any
point on the wheel, I'll hand you back a reference frame in which the
wheel is instantaneously rotating about it.  Yet, for most points on
the wheel, you would think it absurd to say the wheel was rotating
about that point.

If the wheel can be said to be rotating about a particular axis at all
(rather than `instantaneously rotating'), it is rotating about its
axle. 

Sure, you can imagine the motion as an infinite series of cycloids,
with the wheel hopping from cycloid to cycloid as it rotates.  This is
a valid way to analyze it, and when you're trying to explain rolling
action to freshmen, it's probably the most convenient.  But it's much
simpler, and, in the sense mentioned in my 50 words, more correct, to 
imagine the motion as a simple rotation about a moving axis.

There -- do we feel better now? 

   BTW, Craig, if you need help on learning about contact patches, take
   the MSF Experienced RiderCourse: there are some supplemental materials 
   handed out that do a very good job of explaining to 
   classical-physics-impaired people what they are and why they are a valid 
   concept.

I have the materials in question.  I don't dispute that contact
patches are important and etc.  I just claim that the contact patch
isn't a well- defined part of the tire.  You can't hold up a part of
the tire, and say, `This is the contact patch', because different parts
of the tire are the contact patch at different times.  

BTW, tomes, if you need help on learning about netiquette, please read
net.announce.newusers.  There are some supplemental materials posted
there that do a very good job of explaining to politeness-impaired
people what it is and why it is a valid way of posting.

Followups to sci.physics; flames to email or alt.flame, please.
--
Craig DeForest -- `Before hitting "F", ENGAGE BRAIN'
From: gharriso@hpcc01.corp.hp.com (Graeme Harrison)
Date: Thu, 19 Nov 1992 17:53:25 GMT
Subject: Caught by CHiP Moto = No Ticket!
Organization: the HP Corporate notes server