Motorcycle Tips & Techniques

Motorcycle Safety/Dynamics

Skip Repetitive Navigational Links
Safety TipsBraking   Tip128   Print article Print

Braking In A Curve
(Of course you can)

By: James R. Davis



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.
  • 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.


  • 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.

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.

Copyright © 1992 - 2019 by The Master Strategy Group, all rights reserved.
http://www.msgroup.org

(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

     
Visitors
48,660,546
A plea for your help
Views
117,325,849