Everyone who has driven a motorcycle
has experienced it, and motorcyclists discuss it all the time. But what
is it, really? How does it work? Why does it work? All questions I will
try to deal with in this discussion.
At very slow speeds we steer a motorcycle
by turning the handlebar in the direction we wish to go. We can only
do that at speeds of less than about 5 MPH. At any higher speed we do
the exact opposite, whether we realize it or not. For example, assuming
we want to turn to the right, we actually TRY to turn the handlebar
left. This results in the front wheel leaning to the right and, as a
result of the lean of the wheel, a turn to the right. This is countersteering.
Why is it that we don't get confused
regardless of our speed? Because we have learned that steering a motorcycle
is an effortless chore. That attempt to turn the handlebar to the left
FEELS like we are pushing the right grip rather than pulling on the
left one. It feels like that because the harder we push it, the more
the motorcycle turns to the right and, thus, it feels like the right
grip is pushing back at you that much harder. In other words, we quickly
learn to associate countersteering feedback with the hand closest to
the direction in which we wish to turn. Further, even a little bit of
experience shows that countersteering is essentially effortless
while trying to turn the handlebar in the direction you want to go is
virtually impossible. Humans are relatively fast studies,
after all.
It takes only a modest familiarity
with a gyroscope to understand countersteering - at least to understand
how most people believe it starts to work. The phenomenon is called
Gyroscopic Precession. This is what happens when a lateral
force is applied to the axis of a spinning gyroscope. The spinning gyroscope
translates the force vector ninety degrees off the direction of spin.
Thus, if we try to turn our front wheel to the left, the force we use
appears as a lateral force forward against the axle on the right side
and this is translated into a force that trys to lean the wheel to the
right. Similarly, trying to turn the wheel to the right results in the
wheel trying to lean to the left.
But gyroscopic precession is not
a necessary component of countersteering. No matter how slight,
if your front wheel deviates from a straight path your motorcycle will
begin to lean in the opposite direction. It is entirely accurate to
assume that even without gyroscopic precession, the act of steering
the front wheel out from under the bike would start countersteering
in the opposite direction. This is a result of steering geometry - rake.
You can observe it at a complete stop. Just turn your handlebars in
one direction and you will see that your bike leans in the opposite
direction as a result.
In the case of a motorcycle, your
handlebar input is immediately translated by gyroscopic precession into
a lean in the opposite direction. Since your front wheel is attached
to the bike's frame, the body of the bike also attempts to lean. It
is the lean of the BIKE that overwhelms the handlebar effort and drags
the front wheel over with it - gyroscopic precession merely starts the
process and soon becomes inconsequential in the outcome.
If, for example, you had a ski rather
than a front wheel, the front would actually begin to turn in the direction
of handlebar input (just like it does with a wheel instead of a ski)
and body lean in the opposite direction would then overwhelm that ski
making countersteering still effective.
The ONLY WAY to turn a motorcycle
that is moving faster than you can walk is by leaning it (if it only
has two wheels). We have talked only about what starts that lean to
take place. Indeed, all we have talked about is the directional change
of the front wheel along with the simultaneous lean of the bike, both
in the opposite direction signaled by handlebar input. So then what
happens?
Before getting into what is actually
somewhat complicated let me say that if you were to let go of your handlebars
and provide no steering information whatever (or you were to get knocked
off your motorcycle), after some wildly exciting swings from side to
side your motorcycle would 'find' a straight course to travel in and
would stabilize itself on that course, straight up! That's right, your
motorcycle has a self-correcting design built into it - known as its
Steering Geometry - that causes it to automatically compensate
for all forms of leaning and speed changes and end up standing straight
up, going in a straight line, whether you are on the bike or not - until
it is traveling so slowly that it will fall down.
This diagram shows a typical motorcycle frontend. The handlebars are
connected to the steering column, which is connected to the knee bone, which
is... Oops, wrong discussion. The steering column (actually called the 'steering
stem') does not connect to the knee bone, nor does it connect directly to
your forks! Instead, it connects to what is known as the triple-tree
(shown as D in the diagr
am.) This is merely where both forks are
tied, along with the steering stem, to the bike's frame. You will notice that
the triple-tree extends towards the front and that as a result the forks are
offset forward some distance from the steering stem. (Notice the red diagonal
lines marked C and C'.) This is known as the offset.
Now please notice that the forks
are not pointing straight down from the triple-tree, but are instead
at an angle. This angle is known as the rake. Were it
not for that rake (and modest offset) the front tire would touch the
ground at point A. (Most rake angles are approximately 30 degrees.)
What the rake does for you is profoundly
important. For one thing, it causes any lean of the wheel to be translated
into a turn of the wheel towards that lean. For another, it slows down
your steering. That is, if you turn your handlebar 20 degrees at slow
speed your course will change something less than 20 degrees. [At higher
speeds you NEVER would turn your handlebars 20 degrees - the front wheel
is always pointing virtually straight ahead.] Rake, in
the case of higher speed turning then really does SLOW DOWN the realization
of the turn. (We will see why soon.)
Looking at the diagram, imagine that
instead of pointing to the right the wheel is pointing straight at you.
(The body of the motorcycle remains pointing to the right.) You will
now recognize that the contact patch which was B
before the wheel turned has now got to be near where C' is at.
In other words, the fact that your wheel is on a rake results in the
consumption of part of your steering input into a displacement of the
contact patch of the wheel. (This is why steering is 'slower' - and
the greater the rake, the slower it is. Note that 'slow steering' is
NOT the same as 'understeer'.)
Notice also that where the red diagonal
line marked C' touches the tire is higher than where B
touches the tire. This demonstrates that a consequence of turning is
that the front-end of your motorcycle actually lowers based on rake
geometry. The distance between where B and C (not C')
touch the ground is called trail. The more extreme the
rake angle, and the shorter the offset, the longer the trail is. Some
motorcycles will have the hub of the front wheel either above or below
the forks rather than directly in the middle of them. In effect, these
placements are designed to reduce or increase the effect of the offset
in order to increase or reduce trail.
The stability of your motorcycle
at speed is a function of how long its trail is. However, have you ever
noticed that the front wheel on bikes that have excessive rakes (and
therefore long trail) have a tendency to flop over (at low speeds) when
they are not aligned perfectly straight ahead? This is the phenomena
that explains just one of the reasons why your wheel actually turns
in the direction you want to go after it begins to lean in that direction.
Any lean whatever of the wheel, because gravity tries to lower the front-end,
receives an assist from gravity in its efforts to move the contact patch
forward along the trail. Further, notice that the pivot axis of your
forks is along C, not C' and that this is behind the bulk
of the front-end. Thus, gravity plays an even bigger role in causing
the wheel to turn than at first glance it would appear. (And now you
see why you have steering dampers - so that a little lean doesn't result
in a FAST tank-slapping fall of the wheel in the direction of the lean.)
But there is another, more powerful,
reason that the lean is translated into a turn - Camber Thrust.
Unlike automobile tires, your motorcycle rides on tires that are rounded
instead of flat from side to side. When you are riding vertically your
contact patch is right in the middle of the tire, at its farthest point
from the hub of the wheel. When you are leaning you are riding on a
part of the tire that is closer to the hub of the wheel. The farthest
parts of the tire from the hub of the wheel are TURNING FASTER than
any part closer to that hub. Thus, when you are leaning the outside
edge of the contact patch is moving faster than is the inside edge.
Imagine taking two tapered drinking glasses and putting them together
as in the next diagram. Does this not bear a striking resemblance to
the profile of your tires when looking at them head on?
Now imagine placing one of those
glasses on its side
on the table and
giving it a push. Note that the glass MUST move in a circle because
the lip of the glass is moving faster than any other part of it. The
same is true of your tires. This camber thrust forces your wheel to
turn in response to a lean.
Thus, both the rake geometry and
camber thrust conspire to cause a leaning front wheel to become a turn
in the direction of the lean. Then, of course, the motorcycle body follows
the wheel and it, too, leans in the direction of the turn.
So, now you know what countersteering
is, how it works, and why. What might just now be occuring to you is
with all of these forces conspiring to cause the wheel to lean and then
turn in the direction you want to go, what stops that wheel from going
all the way to a stop every time a little countersteer is used? And,
as I earlier mentioned, how does a pilotless motorcycle automatically
right itself?
The answer to both of those questions
is centrifugal force and, again, rake geometry. For any given speed
and lean combination there is only one diameter of a circle that can
be maintained. This is a natural balance point at which gravity is trying
to pull the bike down and centrifugal force is trying to stand it up,
both with equal results. (If you have Excel on your system you might
want to click on this link for a model that demonstrates this
concept.)
If the speed is increased without
a corresponding decrease in the diameter of the turn being made, centrifugal
force will try to stand the bike more vertically - ie, decreases the
lean angle. This, in turn, decreases the camber thrust and the bike
will, of its own accord, increase the diameter of the turn being made.
If the speed had been held constant
but the bike attempts to shorten the diameter of the turn beyond that
natural balance point then centrifugal forces are greater than gravity
and it stands taller, again lengthening the diameter of the turn as
described earlier.
Once your bike is stable in a curve
(constant speed and constant lean) then it will stay that way until
it receives some steering input. ie, you again use some countersteering
or the road surface changes or the wind changes or you shift your weight
in some way or you change speed.
As soon as any form of steering input
occurs the stability of the bike is diminished. Momentum, camber forces
and rake geometry will then engage in mortal combat with each other
which will, eventually, cause the motorcycle to find a way to straighten
itself out. That momentum will try to keep the motorcycle going in a
straight line is obvious, but it also works with traction in an interesting
way. That is, because the front tire's contact patch has traction the
momentum of the entire motorcycle is applied to the task of trying to
'scrub' the rubber off that tire. If the body of the motorcycle is aligned
with the front tire (only possible if traveling in a straight line)
then there is essentially no 'scrubbing' going on. But if the bike is
not in perfect alignment with the front tire, then momentum will try
to straighten the wheel by pushing against the edge of that contact
patch which is on the outside of the curve. As the contact patch touches
the ground somewhere near point B, and because that is significantly
behind the pivot axis of the frontend (red-dashed line C), the
wheel is forced to pivot away from the curve.
I believe you now see why if the bike were
to become pilotless it would wildly gyrate for a few moments as all of these
conflicting forces battled each other and the bike became stable by seeking
a straight path and being vertical. Clever, these motorcycle front-end designers.
No?
This an excellent graphic demonstrating
what is happening. The key is that moving your butt moves the cg (y
and x are the location in the image.) Basically what has to happen is
for the moments created from the weight (mg) and the centrifugal force
(mv^2/r) to cancel out. (A moment is a force times a distance, in this
case the distance to the contact patch in the direction parallel to
the force - mg*y for weight, (mv^2/r) * x for centrifugal force.) Then
the bike goes around the corner and sparks fly from your titanium knee
sliders. You can speed up and lean over (or drop your butt) until the
centrifugal force equals the maximum force that the tires/road combination
can handle. This is of course assuming that you're not doing anything
else like braking or accelerating which takes away from the grip available
for cornering. If you want the equations to play with: (mv^2/r) * y
= mg * x -> m = mass, cancels out! It doesn't matter how fat your ass
is for computing the speed/turn radius, just where it is and the ratio
of ass fat to bike fat (it does matter for maximum speed though) ->
v = velocity -> r = radius of turn -> y = vertical distance of cg from
center of rotation (contact patch) -> g = acceleration due to gravity
(constant 9.81 m/s^2) -> x = horizontal distance of cg from center of
rotation The two forces are trying to rotate the bike in opposite directions
which is why the are on opposite sides of the equation. so: v = square
root(rgx/2y) OR r = 2yv^2/gx You can see that as y drops or x gets bigger
(lean over more or hang off more for both, basically) either r has to
go down (tighter turn) or v has to go up! Don't you just love physics?
