Newsgroups: rec.motorcycles
Subject: rec.motorcycles FAQ of 93.01.01    (Part 2 of 5)
From: dave.williams@chaos.lrk.ar.us (Dave Williams) 
Date: 10 Jan 93 04:42:00 GMT
Reply-To: dave.williams@chaos.lrk.ar.us (Dave Williams) 
Distribution: world
Organization: The Courts of Chaos * 501-985-0059 * Public Access Usenet


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

Q: What is countersteering, and why would I want to do it?
*
A: That's a complex subject.  Ed Green (egreen@laser.East.Sun.COM)
   has submitted a mini-FAQ on the subject:

   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 countersteering)
      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 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 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
    cranks.  In most cases special motorcycle oils are probably a
    waste of money.

    Some motorcycles never seem to shift right, usually because they
    have clutches that run in oil and automotive oil doesn't always
    let them properly disengage.  Various rec.moto participants have
    reported greatly improved shifting and clutch action with special
    motorcycle oils.  Even expensive oil is cheaper than a clutch or
    transmission.

    Ed Hackett maintains a detailed FAQ on oil.  You can annoy him at
    edh@wheeler.wrc.unr.edu.

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

Q:  Can I use a synthetic oil in my motorcycle's engine?

A:  Motorcycles in general tend to be highly stressed, run hot, are
    abused when ridden, and tend to be stored for extended periods.
    These are virtually tailor-made circumstances where synthetic oils
    can shine.

    Unfortunately, most modern motorcycles use a "wet" clutch, that is,
    the clutch runs in oil.  (despite how wacko this sounds, it's true)
    Some synthetics can cause your clutch to slip.