From: egreen@east.sun.com (Ed Green - Pixel Cruncher)
Newsgroups: rec.motorcycles
Subject: Re: counter-steering?
Date: 15 Oct 1992 14:20:48 GMT
Organization: Sun Microsystems, RTP, NC
Distribution: world
Reply-To: egreen@east.sun.com
NNTP-Posting-Host: laser.east.sun.com

Don, mail to you bounces.  

Ignore the 413 smartasses who are guaranteed to reply to this.

 Here's what I tried to send...

   ----- Transcript of session follows -----
Connected to oclc.org:
>>> RCPT To:<hur@oclc.org>
<<< 550 <hur@oclc.org>... User unknown
550 <hur@oclc.org>... User unknown

   ----- Unsent message follows -----
Date: Thu, 15 Oct 92 10:15:28 EDT
From: egreen@sunpix.East (Ed Green - Sun NC Development Center)
Message-Id: <9210151415.AA28426@sunpix.East.Sun.COM>
To: hur@oclc.org
Subject: Re: counter-steering?

>I overheard some bikers at a local shop talking about "counter-steering".
>Can someone tell me what that is?  I just steer my bike by turning
>the bars.

But which way do you turn them?  Most people think it works like a
tricycle, pull the right grip in, push the left grip out, to turn to
the right.  On a motorcycle, it is the opposite:  push on the right
grip to lean right to turn right,  push on the left grip to lean left
to turn left.  That is called countersteering.  It's been debated to
death here, and then some.  I composed the following summary after one
long thread.



How do motorcycles turn?
         by Ed Green, a recognized non-expert in the field

A frequent topic of discussion, and the source of much confusion, on
rec.motorcycles concerns the mechanics, and underlying physics, of how
motorcycles turn. It would seem that such a basic maneuver should be
well understood by all motorcyclists. Surprisingly, many experienced
riders have no idea how they turn, they probably never think about it.
Many others have very definite ideas about how they turn, which are
likely incorrect. Even among the elite and erudite Denizens, there is
considerable disagreement as to the mechanics involved with both
leaning a motorcycle, and actually turning one. Outlined here are the
four major theories of motorcycle turning which came out of a recent
flame war, er, discussion, on the subject (hopefully presented somewhat
objectively).

To begin with, motorcycles, at greater than parking lot speeds, must
lean to turn. This is part of why they are so fun to ride. The four
predominate theories of turning are evenly divided between leaning and
turning, thusly:

1. Leaning (including counter steering) 
   1.1 Gyroscopic Precession 
   1.2 Steering-induced Roll 
2. Turning 
   2.1 Camber Thrust 
   2.2 Steering Geometry 

1. Leaning 
   Many people assume one leans a motorcycle by shifting body weight.
   While it is possible to induce lean in this manner, the quickest,
   easiest, and most controllable method of leaning a motorcycle is by
   counter steering. Counter steering is a technique whereby the rider
   exerts pressure on the *inside* handlebar (left for a left-hand
   turn, right for a right-hand turn), to lean the motorcycle in the
   desired direction. This is in effect steering the motorcycle the
   "wrong" way, from the point of view of operating a tricycle, thus
   the term "counter steering." THAT it works is undeniable, and can be
   proven easily with a short ride. Both the rate of transitioning to
   the lean (how quickly the bike leans) and the degree of the lean
   (how far over it leans) can be controlled by the rate and degree of
   pressure applied to the inside grip.

   HOW counter steering works is debatable. The two leading theories
   are gyroscopic precession and steering-induced roll.

1.1 Gyroscopic Precession 
   When riding at any appreciable speed, both wheels of the motorcycle
   exhibit the properties of a gyroscope. This contributes to the
   stability of a moving motorcycle. One such property, called
   gyroscopic precession, is that when a lateral force is applied to
   the axis of a free spinning gyroscope, the axis moves in a direction
   90 degrees from the applied force, in the direction of the spin. For
   a motorcycle front wheel, this means that a force applied which
   attempts to move the axis on one side of the wheel back, toward the
   rear of the bike, will result in an effort by that axis to move up.
   Similarly, a force applied to move the hub forward results in a
   force trying to push it down.  Thus to execute, say, a right hand
   turn, the rider pushes on the right grip, seemingly to steer the
   front wheel to the left. The forks translate this input to hub of
   the wheel as a force pushing forward on the right side of the wheel,
   and backward on the left side.  Gyroscopic precession will generate
   a force directed down on the right side, and up on the left, which
   forces the bike to lean to the right.

   This gyroscopic business can most easily be demonstrated with a
   bicycle. Position the bike in such a way that the front tire is in
   the air and can rotate freely, and spin it up to a good speed.
   Holding the handlebars, attempt to turn the wheel to the left, by
   pushing on the right grip, and pulling on the left. You will feel
   the wheel trying to push the whole assembly over to the right.

1.2 Steering-induced Roll 
   The point of contact between the motorcycle and the road is called
   the contact patch, and is that small area of the front and rear tire
   beneath the hubs (on level ground) which is resting on the road
   surface. All actions which depend on frictional forces between the
   tires and the road (such as acceleration, braking, and cornering)
   depend on this contact patch, and the contact patch is the point of
   application of the frictional forces. As long as one accepts that
   the center of mass of the motorcycle+rider system is somewhere above
   this contact patch, then any frictional force which displaces the
   tires to the side acts at the point of contact with the road, and
   will induce a pivot about the center of mass (with a moment arm of
   the distance from the contact patch to the center of mass). This
   pivot about the longitudinal axis of the motorcycle is what is
   referred to in aviation as roll (when one wing of the plane is
   higher than the other).

   This steering-induced roll is much easier to visualize than
   gyroscopic forces, and is most simply described as "steering the
   wheels out from under the bike, to make it lean the other way."

2. Turning 
   Through one or the other of these techniques, or some combination of
   the two, the motorcycle is leaned over by counter steering. The
   motorcycle can also be leaned through weight shift, in which case
   neither of the above theories hold. That is only half the answer,
   however. Once the motorcycle is leaned, what makes it turn? Assuming
   for a moment it doesn't just fall over, why shouldn't it continue
   leaned over in a straight line until it runs out of gas? The good
   folks of rec.motorcycles have been hard at work on that one, too.
   The two leading theories for cornering of a leaned-over bike are
   camber thrust and steering geometry.

2.1 Camber Thrust 
   Unlike the tires for cages and lawn mowers, motorcycle tires have a
   round profile. The diameter of the tire changes from a maximum at
   the center (the contact patch if going straight), decreasing to a
   minimum at the rim. When the bike (and thus the tire) are leaned
   over, during a single revolution of the wheel, the center portion of
   the tire must travel further than the part near the edge. This
   induces what is called camber thrust, and it forces the tire to
   trace out an arc in the direction of the smaller diameter.

   Camber thrust can be demonstrated by placing an empty ice cream cone
   on its side, on a flat surface, and giving it a push. The cone will
   not roll in the direction it was pushed, but will constantly turn
   from that path, toward the smaller end.

2.2 Steering Geometry 
   The steering stem and handlebars of a motorcycle are not positioned
   directly above the hub of the front wheel, but are angled back to
   varying degrees, referred to as rake. Thus, the weight of the
   motorcycle+rider is supported by the front wheel, not straight up
   and down, but at an angle which points through to a point ahead of
   the contact patch. The distance from that point to the actual
   contact patch is called trail. During straight-up and straight-ahead
   riding, this has no effect on steering, because the force is
   transmitted along the center axis of the motorcycle, which is the
   same as the center axis of the front wheel. When leaned over,
   however, this configuration causes a force to be applied to the rear
   part of the front tire, causing it to lift, turning the wheel into
   the curve. The motorcycle then naturally follows the front wheel.
   This view of turning is essentially the same as that for other
   vehicles, using lean only to counter the centripetal acceleration
   caused by the bike's linear momentum (the same force that pushes one
   to the side in a cage when turning).

   This aspect of steering geometry can be demonstrated by holding a
   bicycle (or motorcycle) upright, with the handlebars and front tire
   free to move. Lean the bike to the right, and the front tire will
   "fall" (actually, be pushed by the steering geometry) to the right,
   and similarly for a lean to the left. Thus, the lean of the
   motorcycle, even if initiated by a countersteer, forces the front
   tire to point to the inside of the curve, carving out a turn, and
   the lean also balances the centripetal force which would otherwise
   push the bike over to the outside of the turn.

So, in summary, a motorcycle must lean to turn. The quickest way to
lean a motorcycle is to use counter steering. Counter steering leans
the motorcycle through some combination of gyroscopic precession and
steering-induced roll. Once leaned over, the motorcycle turns by some
combination of camber thrust and steering geometry. What is the exact
combination of each of these effects in actually causing leaning and
turning?  Pick two, three, or all four of them, come up with your own
mix, go find a biker, and argue about it.


---
Ed Green, former Ninjaite |I was drinking last night with a biker,
  Ed.Green@East.Sun.COM   |and I showed him a picture of you.  I said,
DoD #0111  (919)460-8302  |"Go on, get to know her, you'll like her!"
 (The Grateful Dead) -->  |It seemed like the least I could do...

Newsgroups: rec.motorcycles