Newsgroups: rec.motorcycles
Subject: rec.motorcycles FAQ of 92.09.01          Part 2 of 5
From: dave.williams@chaos.lrk.ar.us (Dave Williams) 
Date: 2 Sep 92 18:25:00 GMT
Reply-To: dave.williams@chaos.lrk.ar.us (Dave Williams) 
Distribution: world
Organization: The Courts of Chaos * Jacksonville AR USA * 501-985-0059


   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 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 bulldozers, 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.

      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.


*** Mechanical:

Q:  I pulled a wheelie the other day.  Now my steering has a "catch" in
    it, the the bike wants to veer to the side.

A:  You probably notched the steering head bearings.  Under a side load
    (like whopping the front wheel back on the ground) the races can
    get dented, balls flattened, or both.  The only fix is to replace
    the bearings.  Taper rollers are much less susceptible to this sort
    of damage, and conversion kits are sometimes available.

                          =====================


Q:  My 1978 Suzuki GS750 is supposed to use grade SE oil.  All I can
    find is SG.  Will it work?  What about special motorcycle oils
    like Golden Spectro?

A:  The SE and SG are Service Ratings.  SE has been superceded by SG,
    which is generally a superior grade of oil.  There should be no
    troubles running SG.

    The vendors of the specialty motorcycle oils claim their oils are
    better than ordinary automotive oils.  That could possibly be
    true, particularly with regard to motorcycles using roller bearing
Newsgroups: rec.motorcycles