The
Dynamics Of A Highside
More often than
not, making a mistake while riding a motorcycle leads to
misfortune, usually not serious, but sometimes fatal. One of the
most deadly mistakes you can make is called doing a highside.
When a
bike is 'dumped', or 'laid down', it falls DOWN, gravity assisted,
all the way to the ground and ends up on its side. At slow speeds
this usually results in little or no damage to the bike or the
rider. Even at higher speeds, given that the rider is wearing
appropriate protective clothing, most damage is restricted to the
bike. In either case, these are known as doing a lowside
- meaning that the rider exits the bike by going in the direction
of the fall: down.
Obviously, doing a highside means that you exit the
bike by being thrown up and over the high side of the bike. That,
in itself, is not particularly deadly, but it happens that the
bike usually follows the rider into the air and then it comes back
down, often on top of him. Not too many people survive such an
encounter.
So how does a
highside happen? What causes it and what can you do to prevent it
from happening?
To begin
with, a highside starts when you use so much rear brake pressure
that you lock your rear wheel. If you are in a curve, (or if you
have also applied your front brake while going in a straight line,
or if there is substantial road camber, or severely unbalanced
loading of the motorcycle), this starts the rear end
sliding/skewing away from the direction the bike had been moving
because traction is diminished on the rear tire (it has become
'sliding friction' - about 80% of what it was just prior to the
skid) and that tire has begun to MOVE FASTER (in the direction of
bike movement) than the front tire (centrifugal force, among
others, is having its way.) The automatic, and correct, driver
response to this situation is to turn the front wheel in the
direction of the slide. [Actually, the front wheel will turn in
the direction of the slide by itself - your job is merely to let
it.] But now he can make a mistake that can cost him his life - he
can release the rear brake.
Let's look
at what is happening at the instant his rear brake locks up
causing his rear wheel to begin to slide and the instant that he
releases pressure on the rear brake. Let's assume a rider is in a
gentle turn at the time. (Riding in a straight line is exactly the
same as soon as the rear wheel starts to skew to one side or the
other of the front wheel track.) The bike is moving in the
direction pointed to by the front tire at this instant. Note that
the back tire is always 'scuffing' a little as it tries to get
into the same direction pointed to by the front tire.
Now at this
instant the rear brake locks and the rear wheel loses a
significant amount of its traction (at least 20%). It begins to
skew outward from the center of the curve.
The driver
now allows the front wheel to turn in the direction of the slide.
The direction of bike travel has thus changed. Meanwhile, the rear
end continues to slide and is still moving FASTER than the front
end at this instant. The bike is trying to 'lay down' [because
with the rear-wheel no longer spinning you have lost its
gyroscopic effect and, thus, attitude stability for about 80% of
the bike] and will do so if nothing else happens quickly.
But the
rider, realizing that his rear end is sliding completely out of
control, decides to release the pressure on the rear brake to try
to drive out of the situation. When he does so the rear tire,
which is being dragged forward as well as to the side, is suddenly
able to start turning again. This allows it to move in the forward
direction much more easily than a moment before, and just as
suddenly it regains traction (mind you, it lost only about 20% of
its traction when it began to slide and it is picking up only that
20% or so of traction at this point.)
Whether the
engine is driving the rear tire or not, because the bike is not
simply 'dragging/scuffing' the rear tire forward with it (because
the tire is now rotating), the bike begins to move faster
(actually, is slowing more slowly) in the direction pointed to by
the front tire. At the same time, because full traction has been
regained, the sliding movement of the rear end of the bike comes
to an abrupt end. And what next happens is the highside!
 Whether
the slide movement of the rear end is abruptly stopped because the
rear wheel hits a curb, or because the tire has regained traction,
the results are the same: centrifugal force, coupled with inertia,
try to keep the center of gravity of the bike moving in the
direction it was last traveling. Since the bottom of the rear
wheel has stopped sliding, (all stopping forces are at the contact
patch), clearly a torque is developed. The result is that the bike
is violently twisted in the direction of the earlier slide. The
front wheel actually helps this twisting action because it has a
bearing in its axle and the bike merely rotates using that bearing
as an axis.
Naturally,
the driver will be thrown in the same direction as the bike is
twisted.
The mistake,
of course, was releasing the pressure on the rear brake. Said
differently, if you are in a situation where the rear wheel is
sliding out from under you, despite having turned the front
wheel in the direction of the slide, then the safest
course of action is to RIDE THE BIKE INTO THE GROUND - do a
lowside. (i.e., do NOT release the pressure on the rear brake.)
Let me also
add that there is one more thing that could have been done to
avoid the highside described here: always straighten the bike
BEFORE you aggressively use your brakes when in a
curve!
If the bike
is moving in a straight line, particularly if the bike has any
form of integrated braking, and the rear wheel brake locks
resulting in a skid, it is still possible to do a highside, but
the odds of doing so are far less than when in a curve [the faster
you are moving, and the greater the camber (slope) of the road,
the higher the odds.] Still, the best decision the rider can make
is to NOT RELEASE the rear brake if it is locked to try to insure
that a highside does not result.
Abruptly
releasing the front brake when the rear wheel is locked and
skidding can also cause a highside because it will increase rear
wheel weight and, therefore, traction. Nevertheless, the
only possible way to 'ride out' of this situation is to get the
front end of the bike to go faster than the rear in the direction
of the skid. Thus, a gentle relaxation of the front brake
is a reasonable action to take. (Note, however, that with any form
of integrated braking, this is virtually hopeless because so long
as the rear brake is applied the front brake is also being
applied.) Increasing front brake pressure, on the other hand, will
almost certainly result in immediately laying the bike down on the
low side.
Can a
highside occur if you do not release the rear brake pressure at
all? You bet! If you have ever witnessed a 'straight line'
highside accident you will remember that the skid mark was a
straight line until the very end at which point it became a 'J'.
What that shows is that the rider successfully managed to keep his
front wheel pointed in the direction of the skid until he had
turned his wheel to its limit (a 'stop' was reached.) When that
happens, of course, he can no longer continue to turn into the
skid and the direction the bike travels begins to abruptly change
- the skid increases until it presents a 90 degree tire face in
the direction the bike is moving, which happens to present the
largest contact patch 'face' perpendicular to direction of travel
and, thus, maximizes the odds that traction can be reestablished.
This, then, is approximately when the bike stops its skid and
violently snaps into the air.
Having seen
that a rear end skid requires that you gently relax front brake
pressure and maintain rear brake pressure in hopes that the front
wheel can be coaxed into catching up with the rear one (slow more
slowly), what should you do if the front wheel begins to skid
instead of the rear one? EXACTLY THE SAME THING! Gently release
the front brake and maintain the rear one! Thus, you do not have
to make a decision based on which tire is skidding. The reaction
is the same.
So, above I
said that if you have a choice you should ride the bike into the
ground rather than do a highside. I also said that the dynamics
will almost certainly result in a highside even if you do what is
corrective - turning into the slide and feathering the front
brake. Is it hopeless? Must you do the highside? Not at all. It
means that as soon as you know the attempt you are making is not
going to work, CLIMB ON THE FRONT BRAKE! This will FORCE a lowside!!!
(If you have any form of interlocked brakes you can also force a
lowside by INCREASING rear-brake pressure because that increases
front-brake pressure as well.)
Please, I do
not want to get flamed for suggesting that you actively lowside
your bike! If you have ever seen the results of a highside, you
should kiss the ground that you have the ability to stop it by
laying your bike down. If you can do it, do it. If not, good luck
to you anyway.
[I have been
asked why aggressively using the front brake will cause a lowside
rather than making a highside happen sooner. This is because by
applying front brake you cause weight transfer that further
relieves the rear wheel traction which, in turn, both reduces the
odds of a highside and slows the bike faster. i.e., it falls over
(lowsides) sooner.]
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Whether
the slide movement of the rear end is abruptly stopped because the
rear wheel hits a curb, or because the tire has regained traction,
the results are the same: centrifugal force, coupled with inertia,
try to keep the center of gravity of the bike moving in the
direction it was last traveling. Since the bottom of the rear
wheel has stopped sliding, (all stopping forces are at the contact
patch), clearly a torque is developed. The result is that the bike
is violently twisted in the direction of the earlier slide. The
front wheel actually helps this twisting action because it has a
bearing in its axle and the bike merely rotates using that bearing
as an axis.
