From: tomes@iastate.edu ()
Subject: Re: PHYSICS OF STEERING (long) (was Re: What would you ride on a long...)
Organization: Iowa State University, Ames, IA
Date: Fri, 20 Nov 1992 23:45:54 GMT

In article <ZOWIE.92Nov18210725@daedalus.stanford.edu> zowie@daedalus.stanford.edu (Craig "Powderkeg" DeForest) writes:
>
>It's about time a trained physicist posted something about this...
>Here's rec.moto Steering Argument #597.
>

(great stuff that makes sense and almost makes up for the previous post
 which sounds like a pompous pissed off perfessor with no riding experience)


>
>Good call -- what went on here is exactly that: the force of gravity couldn't
>accelerate the c.g. *down* fast enough to match the roll of the bike.  In a
>weightless environment, the c.r. (1/2 way between the contact patch and the
>c.g.) would be stationary during the roll; gravity (almost) always pulls the
>c.g. down fast enough to maintain the contact with the ground; when it
>doesn't, you can't very well roll out of the turn, can you?

Say what???  Why would the center of roll in a weightless environment be 1/2
way from the contact patch (wait a minute: I thought that a contact patch
was bullshit?) to the cm?   Given no other forces, a force acting on a body
some distance from the body's cm and not pointing directly towards or away
from the cm will cause it to rotate about the cm.

>Yep -- bikes, and engines (most of the mass of the bike :-) that cluster
>around the c.g., make it easier to steer, than engines that spread out far
>above/below it.  Though I believe that, in most cases, steering geometry
>(wheel size, fork angle, offset between forks and axle, length of wheelbase)
>affects handling *much more* than does mass distribution in the bike.  The
>reason I think so is twofold: (a) unless you're screaming along around the
>twisties, you're not rolling the bike very much compared to the acceleration
>due to gravity -- you just don't lean very fast; and (b) *you* don't have
>to apply the force to turn the bike -- the tires do that for you.  Almost 
>any steering force you have to apply, was designed in by the engineers who
>drew the plans for your bike.  If they wanted effortless turns, they'd 
>place the contact patch closer to the (fork-line, ground) intersection;
>if they wanted a bike that liked to cruise in a straight line, they'd place
>it far behind that intersection.

Which is why the Wing handles more like a big standard with a low cm than
the majority of people will believe.  The designers wanted it that way.

>BTW, in a very tight turn the top half of you (the rider) is significantly
>closer to the center of rotation of the yaw, than is the bottom of you.
>This means that `down' is tilted more, relative to the horizon, for your bike
>and your feet, than it is for your head, so you might notice your back swaying
>a little while your balance tells you your straight!  [Don't tell novice
>riders/passengers this though, or they'll use it to justify keeping their
>bodies oriented to the horizon :-(]
Y
aw is referenced to ground for the bike, but to the bike for the rider?
I don't worry about it when I ride, and it isn't a problem here.  It just
adds another complication to the discussion.  My passengers do what I instruct
them to before they ride: look over the inside shoulder in a turn.  Or else
they very quickly become pedestrians.

>
>--
>Craig DeForest -- astrophysicist for hire.  

Bone up on your classical physics before you go jobhunting.

CP Tomes

Yaw?  I don need no steenkin yaw!!!
Newsgroups: rec.motorcycles