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(Of
course you can)
By
James R. Davis
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I received an interesting
message the other day from a rider who had just completed his first
MSF class. The man commutes to work on his bike and in that letter
he described a situation that he confronts every day which seems to
defy what he was told in his MSF class - a long curve on a steeply
declining slope. The man remembers that his instructor told him that
he should establish his entry speed before he enters the curve and
that he should gently accelerate all the way through the curve. He
finds it impossible to do - safely - and asked for advice. He is right!
It is impossible to safely accelerate (using the throttle) all the
way through a long steeply declining curve, and that is NOT what the
MSF has tried to teach. There are several intents of the MSF range
exercise that the man refers to.
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You should establish your entry speed BEFORE you enter the curve.
That is, you are to eliminate all excess approach speed, with
braking if necessary, while still traveling in a straight line
and while the bike is vertical. |
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You should 'set' your suspension BEFORE you enter the curve.
That is, you should NOT have to deal with a changing center
of gravity that results from weight shifts that are caused by
changes of acceleration or braking while in a curve. You should
have already established your entry speed at this point so your
springs/shocks are resting at normal riding positions. But because
you want maximum control of your bike through the turn, you
want your front tire to be able to handle modest bumps and surface
distortions without destabilizing your bike so you want to shift
some weight to the rear tire. That increases rear tire traction,
loads the rear shocks/springs somewhat more than the front,
and increases over-steer. And you want that attitude all the
way through the curve so you maintain a modest acceleration
all the way through it.
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All of which sounds like
you should accelerate all the way through a curve, I know. But, NOT
IF YOU ARE ON A DECLINE! To begin with, you know that you must lean
the bike in order to make a turn. That the faster you go through a
given turn, the greater the lean that must be used. Clearly you can
accelerate to a speed that is beyond your ability to negotiate a turn.
Thus, if you modestly accelerate all the way through a turn it must
be that you established a low enough entry speed to allow it, and
that you did not use excessive acceleration through the turn. Next,
by virtue of being on a decline, you will accelerate without any throttle
at all. If the rate of acceleration is high enough, there is no safe
entry speed that would allow you to complete the turn safely (without
some braking.) And, because you are on a decline, there is already
more weight on the front tire than you can safely shift to the rear
via acceleration to give you the handling stability that is sought
from acceleration without exceeding your ability to negotiate the
turn. Finally, because you are in a turn you are, by definition, already
accelerating! (Delta V - any change in velocity is acceleration -
even if you are slowing down!!!) Your shocks receive increased loading
just because you are in a curve - from centrifugal force. So, it sounds
like if you are on a decline while in a curve you should NOT use your
throttle to accelerate. CORRECT! Instead, you should use your brakes
and/or engine braking to either maintain your entry speed or to allow
only modest increases of speed. At the same time, you should have
moved back as far as possible in your seat to shift weight to the
rear tire. But, this gentleman reminds me that his MSF instructor
told him not to use brakes in a curve. WRONG! What his instructor
told him was that if he used his brakes while in a curve to STOP
he should gradually straighten the bike up as his speed decreases
and gradually increase brake pressure until the bike is vertical.
That remains completely true. But if you are not slowing down, merely
maintaining your speed, then there is no straightening of the bike
required. The MSF instructor also told him that if he needed to make
the fastest stop while in a curve he should straighten the bike up
first, then apply his brakes. But we are not talking about slowing
or stopping the bike here. Of course you can use your brakes in
a curve. Brakes are not used merely to stop your bike, they are
used to offset acceleration (i.e., to either slow the bike or to prevent
it from accelerating as fast as it otherwise would go from gravity
assist.) If you do have to use your brakes in a curve for modest deceleration
then the question arises as to which? Front, rear, or both?
You will recall that front brake usage tends to shorten the wheelbase
and, thus, both quicken your steering and straighten the turn (make
it wider.) The rear brake, by itself, tends to lengthen the
wheelbase. The clear choice, then, is use of the rear brake
by itself unless you need to aggressively slow down (in which case
you would use both together.) Finally, if the decline/curve is long
enough you should rely heavily on engine braking rather than just
your brakes in order to keep your brakes functional (not overheated)
should you need them. |
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