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Why Counter-Steering Begins At
About 6 MPH

By: James R. Davis

I can't remember how many times I've asserted that the only way you can steer a motorcycle moving faster than you can run is by using counter-steering. Asked just how fast that is, I have said about 6 MPH. Many, many people have sent me e-mails challenging that assertion. So, I will try to explain why it happens. [Note that this discussion assumes your bike has a normal rake angle.]

When you park your bike your front wheel will not be pointing straight ahead. It will be pointing off to the left and the body of your bike will be leaning to the left onto its side-stand.

Because the front-end forks are at an angle other than straight down (called a rake angle), whenever the front wheel is turned in a direction other than straight ahead the wheel will not only point in that new direction, it will lean in that direction as well. Because your front-end is linked to the body of the bike through your triple tree, and because of that rake angle, whichever direction the front-end moves the body of the bike tries to move in the opposite direction.

Because there is far more mass, and therefore inertia, it is far easier to move the front-end than it is to move the body of the bike. At slow speeds virtually all of the movement when you turn the handlebars will be of the front-end while the body of the bike tries to remain in place. If you turn left, for example, the front-end will move to the left while the relatively massive body of the bike remains where it was and that moves the Center of Gravity slightly to the left. At very slow speeds gravity is stronger than centrifugal force and it will prevail as it tries to pull the bike down on its left side. The bike body then leans to the left.

You will remember that centrifugal force is a function of the square of the velocity and the radius of the turn. In other words, it grows VERY FAST as compared to speed. As speed increases from zero to about 6 MPH that centrifugal force is simply not as great as the force of gravity. It is for that reason that direct steering works - when you turn the handlebars to the left the bike slowly falls to the left.
But once you get to about 6 MPH centrifugal force is equal to gravity and at that point the bike body will neither attempt to fall to the left or move to the right. You will turn gently to the left.

And, of course, any speed greater than about 6 MPH will result in ever increasing centrifugal force (gravity, of course, is constant) and that will cause the bike body to lean to the right as the front wheel moves to the left - and THAT's called counter-steering.

Forget everything you've ever heard about gyroscopic precession CAUSING counter-steering. It's all a result of centrifugal force and rake. If your front tire was replaced with a ski counter-steering would still work. Gyroscopics merely helps it out, but it does not cause counter-steering.



Copyright © 1992 - 2018 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.)

     
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