to really understand why the angle affects it look at it from a slightly different perspective.

look at just the axle and swingarm, make the axle center a point of origin, and the swingarm rotates around it(instead of axle rotating in the swingarm.) it requires a specific ammount of torque to rotate the swingarm that changes depending on its angle from horizontal... at the momemt the swingarm is horizontal, 100% of the force to rotate it must be in the upward (Y) direction. once the swingarm is no longer horizontal less force is required to rotate it because you are no longer opposing gravity with 100% of the force, some of it is converted to a horizontal force(X-direction).
the steeper the swingarm angle, the easier it is to make this rotation.
add to that the fact that the forces imposed by the rest of the frame at the swingarm pivot are mostly downward(same direction as gravity) the steeper the swingarm angle, the easier it is to make the swingarm rotate & lock out the suspension.

also, the shorter the swingarm the easier it is to cause this same problem.

best fix, lower preload or zero-preload converion. lower the swingarm angle.

 

[img]i/expressions/face-icon-small-mad.gif[/img] i have the same problem with my Z-400 chain adjustment. i doubled up on the wrench and it will hold good untill take a hard landing. i was thinking of making a stop inside the swingarm to correct the problem

 

yep