Rake & Trail

 

Basic Principals

Rake Angle
1. Rake angle is the angle of steering axis as compared to vertical.
2. Given the relationship and interaction between rake, trail,
    load distribution and centers of gravity it would be very 
    hard to accurately define the function of rake alone.
  a. It is generally agreed that a vehicle with:
    1) "MORE" rake will have straight line stability and
        be somewhat harder to steer.
      a) The castor (self centering) effect on the front 
          wheel is dependant on force created by 
          "lever arm" action ("length" of lever arm).
        (1) Positive rake reduces length of "lever arm".
        (2) Positive rake also reduces the steering angle
             between tire and ground, as compared to angle
             to which the handlebar is turned. In theory, this
             would also reduce the "length" of lever arm.
          (a) Therefore we need more trail as rake angle
               is increased.
          (b) The castor effect can become negative with
               large steering angles. In this situation the front
               wheel is attempting to push itself to a
               point behind the steering axis.
            1) At smaller steering angles there is some reduction in trail.
                A larger amount of trail may be initially required to overcome
                the negative castor effect of additional rake.
            2) "Less" rake will have less straight-line stability and be somewhat
                easier to steer.

 

Steering Head Drop
1.The greater the rake angle, the greater the head drop.
2. Head drop tends to work against self-centering effects of castor.
  a. Must "lift" weight supported by steering head.
  b. Offset front wheel has no real effect on self-centering characteristics.
      Actually, offset can contribute to steering inertia.
  c. Common Misconception: A front wheel offset places the center of
      gravity ( of wheel and fork) ahead of steering axis.
    1) This produces a torque tending to steer the wheel into the corner
        when vehicle is leaned over.

    2) This is true only when bike is stationary . When moving through a 
        corner, a centrifugal force acts through the offset to steer the wheel "out" 
        of turn. This force, however, is balanced by the gravitational force trying 
        to "steer into" the turn.

 

Trail
1. The distance from the center of the tire contact to patch point where
    the steering axis meets the ground.
  a. Applies to both front and rear wheels.
2. Positive trail.
  a. The contact patch is behind the steering axis.
  b. Automatically counteracts the deflection providing a Degree
      of Directional Stability.
3. Negative trail.
  a. contact patch is in front of the steering axis.
  b. Automatically reinforces deflection making the machine directionally
      unstable.

 

Castor Effect
1. "Self- centering" effect
  a. Direct result of positive trail, therefore the measurement of this castor
      is called trail.

 

Steering Effect
1. The amount the front wheel trail affects the amount of steering torque
    the rider must apply to maintain the correct steering angle.
   a. The more trail, the more steering torque required.
  b. The less trail, the less steering torque required.
   c. AKA "Steering Feel".
2. This force tends to turn the steering to the position where the steering head is lowest.
  a. The phenomenon can be verified by leaning  and turning handlebar of a stationary 
      vehicle.
  b. The steering head rises or falls depending on the position of the handlebars.
3. With positive trail, the turning effect is into the corner.
4. With negative trail, the turning effect is the opposite.

 

Straight Line Feel
1. Straight-line steering is actually a series of corrections with the handlebars turning
    slightly from side to side all the time.
2. Straight-line steering feels lighter on wet or slippery surface than on a dry surface
    with normal traction.

 

Camber Thrust
1. Rolling surface of tire is smaller in circumference around outside of tire than 
    on middle of tire.
2. When leaned, it acts like a rolling cone.
  a. Edge with larger circumference travels farther than edge with smaller circumference.
3. This is the reason that when a vehicle is leaned, it must turn. Counter steering
    must be applied to maintain straight-line movement.

 

Slip Angle
1. Position of turned wheel as compared to direction of travel.
 2. A slight force is created by slip angle at a right angle to tire. 
     This force acts as a "lever arm" to provide a correcting torque to the
      angled  wheel.
3.Restorng torque created by slip angle is dependent on:
  a. Tire properties
  b. Surface adhesion
  c. Trail
4. Although slip angle is present at both front and rear wheels, the tire has little 
    restoring effect due to smaller slip angle.

 

Wheelbase
1. Generally, longer wheelbase will yield greater directional stability and require
    greater effort to complete turns.
    Lets see why...
  a. A long wheelbase vehicle needs the front wheel to be turned farther into the
     corner.
    1) This means that more effort is required for turning corner.
    2) Also, deflections, such as road bumps, will have less effect on 
        directional  stability.
        Since more effort is required to turn wheel.
  b. With front wheel turned, rear-wheel angle is reduced. This also
       improves directional stability.
  c. Weight transfer is reduced during braking and hard acceleration, 
      assuming that center of gravity remains constant.
  d. Increase in yaw- moment of inertia. This "slows" the response.

 

Summary
 Wheelbase dimensions are compromise. Dependant on use of vehicle
 and interaction between rake, trail, moments of inertia, centers of gravity,
 and load distribution.

 

Wheel Diameter
1. Wheel size effects gyroscopic forces.
  a. Bigger wheel start gyroscopic effect at lower speed.
2. Negative trail by placing the contact patch of the tire in front of the steering 
    axis would reinforce deflection.

 

Summary: Trail
1. Positive trail counteracts deflection, such as uneven road surfaces, and gives
    a measure of directional stability.
2. Negative trail by placing the contact patch of the tire in front of the steering
    axis would reinforce deflection.

 

Summary: Rake
1. We need a self-steering effect to give us just the right steering angle.
  a. Too much and rider must apply reverse effort to the handlebar.
  b. Too littler and rider need to steer into corner.
  c. It is not possible to build in self-steering effect that is perfect for all speeds 
      and turn radii. Design is always a compromise.

 

Castor Instability (Wobble)
1. Trail, while providing a degree of stability, can cause a wobbling 
    type of instability.
  a. The restoring force created by trail can be strong enough to overcorrect 
      for the initial disturbance when front is displaced by road surface irregularity.
  b. The wheel will swing beyond "center" and starts a pendulum effect.
 2. Natural tendency to wobble is determined for the most part by:
  a. Moment of inertia of front wheel and fork about steering axis.
    1) Can be worsened by heavier fender, tire, accessories on fender,
        or baggage on fender\fork. 
  b. Degree of restoring torque to a given steering angle.
    1) Determined by rake, trail, tire size, characteristics and stiffness of 
        frame and fork.
3. General rules:
  a. Higher the steered moment of inertia, lower the natural frequency 
      (tendency to wobble).
    1) Longer wheelbases tend to have a Higher moment of inertia.
  b. Higher the restoring torque (per degree of deflection), higher the natural
      frequency (tendency to wobble).
 4. Interactions (with wobbling front end).
  a. If the front end is shaking back and forth, rear end shakes side to side.
    1) Usually worse with: saddlebags, tour box, tall sissy bar, baggage 
        and/or passenger.
    2) Most movements takes place in flexing of rear tire. Important that tires are of 
        adequate load rating to prevent sidewall flex.
    3) The above items influence handling in steering movements are combined 
        with leaning movements. Cross-couples steering and banking effects.
5. Damping:
  a. Provided by: Fork, bearing friction, wiring, control cables, tire friction, internal
      tire damping (hysteresis)- slowing  of effect when forces acting on a body are
      changed and riders contact is handlebar.
 6. Wobble characteristics:
  a. Determined by: Natural wobble frequency, road induced forces and damping.
  b. Usually between 25 and 40 MPH, while slowing down through this range.
  c. Only way  to prevent wobble is to damp or tune it out of system. The
      fundamental mechanisms for causing wobble are part of design in
      conventional motorcycles.
  d. Possible helpful measures:
     1) Increase the frame and fork stiffness.
     2) Reduce trail.
     3) Reduce weight of front wheel and fork, this reduces their moment of 
         inertia about the steering axis.
        a) reduces energy in oscillating parts.
        b) Raises speed of wobbles natural frequency. At higher speeds,
            gyroscopic forces will be stronger and better resist wobble.

 

MOMENTS OF INERTIA
Measure of inertia effect about a particular axis. Value determines the ease with which we can accelerate the machine about the axis.

 "Distribution of weight about a given axis"

The more weight you add and the further you locate it from the axis, the greater the amount 
 of inertia will be.

 

Roll
Centers on tire contact patches at ground plane. Low roll moment of inertia is desirable for
 rapid banking changes (e.g. S-curves). Usually means a low center of gravity.

 

Yaw
(Steering Assembly) Centers on steering axis.
1. Low moment of inertia: Helps in rapid changes in direction, minimizes effects
    of slide.
2. Low moment considered best.
3. Concentrates mass close to longitudinal center.

 

Pitch
Axis can vary, depending on configuration of machine. e.g.:
1. If vehicle is sprung at front and not at rear. axis would be the rear wheel center.
2. On a conventional machine (sprung at both ends), axis depends on suspension geometry
    and spring rates.

 

About the Wheel Axis
Centers on wheel axle center.

 

Flexure
  Lateral displacement of fork.
  Lateral displacement of frame.
  Lateral displacement of rear swing arm.
  Fore and aft displacement of fork.
   - Shudder and wheel hop.

 

Fatigue
  Of frame, can lengthen wheelbase and induce lateral displacement.
  Of forks, bushings, can induce fore and aft displacement.
  Of rear shocks and bushings, can induce lateral displacement.
  Frozen swing arm bearings or stiffened shocks causing swing arm flex to 
  be the "suspension".
   -Also creates front-end problems due to forces directed forward from lack
    of suspension in rear.