Posted: Fri Feb 03, 2006 5:03 am
Countersteering Explained
Everyone who has driven a motorcycle has experienced it, the MSF classes
mention (but don't explain) 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
counter-steering.
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 counter-steering feedback with the hand closest to
the direction in which we wish to turn. Further, even a little bit of
experience shows that counter-steering 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 counter-
steering - 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 tries 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 counter-steering. 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 counter-steering
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.
[Please note that though gyroscopic precession is not a necessary component
of counter-steering it GREATLY facilitates it. Indeed, it is the precession
of the REAR tire that results from the momentary change of direction of the
bike that 'pushes' about 80% of the bulk of the bike into a lean in the
direction you want to go.]
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 counter-steering 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 front-end. 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 diagram.) 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 'under-steer'.)
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. (Trail, as you can see, is determined by rake angle, offset
and tire radius.) 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 counter-steering is, how it works, and why. What might
just now be occurring 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 counter-steer
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 - i.e., 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. i.e., you again use
some counter-steering 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 front-end (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?
__________________
taken from - http://www.msgroup.org/TIP048.html
Everyone who has driven a motorcycle has experienced it, the MSF classes
mention (but don't explain) 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
counter-steering.
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 counter-steering feedback with the hand closest to
the direction in which we wish to turn. Further, even a little bit of
experience shows that counter-steering 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 counter-
steering - 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 tries 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 counter-steering. 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 counter-steering
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.
[Please note that though gyroscopic precession is not a necessary component
of counter-steering it GREATLY facilitates it. Indeed, it is the precession
of the REAR tire that results from the momentary change of direction of the
bike that 'pushes' about 80% of the bulk of the bike into a lean in the
direction you want to go.]
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 counter-steering 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 front-end. 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 diagram.) 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 'under-steer'.)
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. (Trail, as you can see, is determined by rake angle, offset
and tire radius.) 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 counter-steering is, how it works, and why. What might
just now be occurring 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 counter-steer
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 - i.e., 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. i.e., you again use
some counter-steering 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 front-end (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?
__________________
taken from - http://www.msgroup.org/TIP048.html
