Napkin, what Andy is trying to describe is that there are two dimensions to this problem.
Dimension 1 is explained very well with the linkage sketches. It is the relationship of each part with respect to each other throughout the entire range of motion.
Dimension 2 is the force diagrams, the forces will change as the linkage goes through its range.
Next time, I would hesitate to call :bs: on his explanations just because you do not understand it.
This is how I would explain it.
Take your wrist.
Put it straight with your forearm.
Try to bend it.
It takes a lot of force to bend it ... right?
Take your wrist and bring it in as much as it will bend.
Now notice how little force it takes to move that wrist to its fully locked position.
Dimensional relationships look identical for range of motion ... right?
Visualizing the forces on the other hand (pun is fully engaged) cannot be seen, they can only be felt.
That is the hard part of visualizing with Andy's diagram.
One can start "seeing" these forces when you break them down in x-y vectors.
That is where the study of dynamics and a little bit of kinematics come into the equation.
Here are the laymans terms for these areas of study:
Statics = chit don't move - bridges, framing, trusses and all that stuff using action-reaction and torque to measure forces.
Dynamics = chit moves - introducing the different frictions, forces that change according to conditions
Kinematics = chit moves in all kinds of directions - regardless of what is causing it to move. You just take the part that is under focus and analyze from there. In this case we have two rigid bodies, the wheel swingarm assembly and the frame. The two make a system joined by mechanical joints.
To measure, you need dimensional relationships.
From there, you can now measure forces by using vector coordinates or cartesian coordinates. For me, the cartesian coordinates made things a lot simpler cause it broke the forces down into x-y coordinates and now it becomes easier to visualize.
