Brake or Swerve
Not really a viable choice
By: James R. Davis
The MSF Basic (beginner) Rider Course (BRC) and other such classes teach that if you are confronted with an obstacle in your path of travel you should either brake or swerve to avoid it. Instructors may have said that you should 'brake then swerve' or 'swerve then brake' and that instruction has left many students believing that they cannot brake and swerve at the same time - ever.
Clearly you can and sometimes must brake and swerve (turn) at the same time, so what is the real message being delivered to the students? It is that you cannot safely brake and swerve (turn) at the same time if either effort is aggressive. That message is gospel.
But the other implication in this teaching is that you have a choice when confronted with an unexpected obstacle in your path of travel: to brake or to swerve.
If that obstacle is small, and stationary, that, too, is gospel. By 'small', I mean less than three feet wide. And by 'stationary', I mean laterally; it is not moving or able to move to its side. A pot hole or DEAD animal in the road are examples of obstacles that you can usually avoid with a swerve. A live animal or a stalled car are not.
Otherwise, your only viable choice is to use your brakes, so long as your speed of travel is at least 10 MPH.
Huh?
Well, let's start this discussion by looking at an aggressive stop.
Assume that you are riding along at 40 MPH and an obstacle appears in your path. Let's say that you can attain a 0.8g rate of deceleration with your brakes. That means that you could come to a complete stop, on your wheels, in just 66.7 feet of braking with another 26.4 feet of reaction time for a total of 93.1 feet.
Now let's focus on a quick turn (often improperly called a swerve).
Counter-steering is required with speeds in excess of about 10 MPH. At slower speeds you use direct-steering. For that reason, alone, during your MSF training you were required to make 90 degree turns within the space outlined on the range, but ONLY at speeds less than 10 MPH. At any greater speed, you CANNOT accomplish such a maneuver.
Let's see why.
Let's say that you are traveling at 40 MPH and are in a turn. The radius of that turn determines how much centrifugal force you experience and, thus, what the lean angle of your motorcycle will be.
Following are representative travel paths and resulting lateral acceleration forces that your motorcycle would experience while riding on the road:
If you were already riding a circular path with a radius of 215 feet, then for every 66.7 feet of longitudinal distance traveled starting at point 'A', your motorcycle would move laterally by 10.6 feet as shown below.
But your motorcycle must go from a path of travel that is pointing straight ahead to a curve with a radius of 215 feet, and it can't do that instantaneously. And you know that you must actually turn in the opposite direction of your desired path of travel before the motorcycle then begins traveling along a decreasing radius turn until it reaches the desired path of travel.
The dotted black line in the graphic below is a reasonably close approximation of the path of travel of your front tire as you travel that 66.7 feet on the roadway after a reaction distance of 26.4 feet. Computations used by accident reconstructionists demonstrate that at 40 MPH, with an aggressive lateral acceleration rate of 0.5g's, your motorcycle will actually only travel 6.2 feet laterally if it travels 93.1 feet longitudinally.
Assume that you are riding along at 40 MPH and an obstacle appears in your path. Let's say that you can attain a 0.8g rate of deceleration with your brakes. That means that you could come to a complete stop, on your wheels, in just 66.7 feet of braking.
Okay, so let's say you have 66.7 feet between you and the object when you start to swerve and let's say you do an aggressive swerve with 0.5g's of lateral acceleration (your bike is leaned over at 30 degrees - the MAXIMUM that most Harley-Davidson cruisers are capable of attaining). The amount of lateral movement you can achieve in that 66.7 feet is only 6.2 feet! That's about half the width of a car.
If you still run into that obstacle despite your aggressive swerve, you will hit it at 40 MPH. Considering that your handlebar extends to the right and left of your bike's centerline by more than one foot, you can be sure in this scenario that you could NOT have avoided the collision.
Swerving is NOT a viable choice.
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http://www.msgroup.org
(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)
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