J
John Morgan
Guest
Why does a slack headtube angle make a bike easier to handle over rough terrain? I will try
to explain.
By changing the headtube angle, what you are really doing is changing the distance between the
contact patch of your tire and the extended steering axis (this is called "trail"). Imagine a
straight line going through your steerer tube, past your fork and hitting the ground. If you
lengthen the "trail" distance your steering will be slower and more self-centering, and if you
shorten this distance your steering will be quicker and less self-centering.
So what about fork rake? Increasing the rake has the same effect as steepening your headtube angle.
In all the suspension forks I see, the hub is offset in front of the slider tubes to make up for the
fork legs being straight. This allows for less distance between the tire contact patch and the
steering axis intersection.
To answer the original question, the reason the headtube angle makes such a big difference is
because when the trail is large, the centering force is large (the force that makes the wheel want
to point straight forward), when the trail is small, the centering force is small. Now if you hit a
rock that deflects your wheel to one side, a bike set up with a slack steering angle will be easier
to handle because the force of the rock is counteracted by a greater centering force on the wheel.
So now the question I put to you all is this: why does a steep headtube angle enable you to make
sharper turns?
-John Morgan research: http://www.phred.org/~josh/bike/trail.html
to explain.
By changing the headtube angle, what you are really doing is changing the distance between the
contact patch of your tire and the extended steering axis (this is called "trail"). Imagine a
straight line going through your steerer tube, past your fork and hitting the ground. If you
lengthen the "trail" distance your steering will be slower and more self-centering, and if you
shorten this distance your steering will be quicker and less self-centering.
So what about fork rake? Increasing the rake has the same effect as steepening your headtube angle.
In all the suspension forks I see, the hub is offset in front of the slider tubes to make up for the
fork legs being straight. This allows for less distance between the tire contact patch and the
steering axis intersection.
To answer the original question, the reason the headtube angle makes such a big difference is
because when the trail is large, the centering force is large (the force that makes the wheel want
to point straight forward), when the trail is small, the centering force is small. Now if you hit a
rock that deflects your wheel to one side, a bike set up with a slack steering angle will be easier
to handle because the force of the rock is counteracted by a greater centering force on the wheel.
So now the question I put to you all is this: why does a steep headtube angle enable you to make
sharper turns?
-John Morgan research: http://www.phred.org/~josh/bike/trail.html