James R. DavisPeer Review:
Posted - 02/07/2004 : 7:05 PM
Rake Angle And Steering
Trail Causes Self-correction
By: James R. Davis
We will look at two things that result from the fact that your front forks are not pointing straight down - that is, there is a rake angle to those forks: how counter-steering is initiated and how weave and wobble are diminished.
The diagram above represents your front tire pointing to the left. The diagonal dashed line represents your steering stem as if it were extended to the ground. Please note how this defines ground trail. (The diagram exaggerates how far forward of the contact patch the steering axis point is for clarity.)
When you turn the handlebar you are attempting to turn the tire about the steering axis at ground level, not about the contact patch. For example, if you turn the handlebar to the right you are trying to get the tire to turn as shown above. However, as the contact patch is touching the ground and the axis is not, the contact patch CANNOT simply slide off to the left as shown. Instead, the body of the motorcycle moves in that direction via force at the triple-tree. You have, in effect, steered the front tire out from under the bike by steering the bike away from the tire. [At a dead stop turn your handlebars all the way to the right and observe how the top of the bike has moved to the right.]
As a result, gravity now tries to pull the bike down towards the left and that drags the front wheel with it and our travel direction has begun moving to the LEFT. (For the purists out there this is not arguing that gyroscopic precession didn't play a part - only that because there is a rake angle counter-steering would work even without gyroscopic precession.) [Note that because of the huge difference in mass between the relatively light front-end and the rest of the bike, when traveling at less than about 6 MPH you actually CAN make a significant turn of the handlebars and there is not enough centrifugal force to push the top of the bike away from the direction you are pointing to. Instead, the bike falls INTO the turn at these slow speeds and THAT is why counter-steering does not work at such slow speeds.]
Wobble and weave are diminished because when the wheel is pointing at an angle other than straight ahead the contact patch is not in alignment with the direction of travel of the bike - that is, a slip angle is created. A restoring force is applied to the contact patch by the ground which attempts to force that alignment. Thus, because of trail, the front wheel tries to go in a straight line. [This restoring force, sometimes called a 'righting moment' or 'castor effect', is a function of the length of trail. The longer the trail, the stronger it is. It is also a function of traction. The higher the traction, the stronger it is. Thus, braking increases the restoring force. This is primarily what 'dumps' a bike when the front brake is applied during a slow speed turn.]
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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)