Sorry for my ignorance, but what is a bottom axle preload bolt?
Here's the lowermost frame linkage axle. If I understand it right, the topmost part has a slot in outer threads, a conical recess and some thread inside underneath the recess, so you can screw
this bolt into it. The further the bolt is screwed in, the more outer axle thread diameter increases (it can because of outer thread slot, like two flower petals), which in turn restricts axle movement through inner bearing race. The whole setup looks similar to
this Shimano non-driveside crankarm preload cap.
