Yeah, I don’t really understand, either. He said that the factory torque values were specific to the rubber bushings. That the aluminum ones didn’t need nearly that much.
Thinking out loud here, but I’m trying to visualize the forces that are seen at those bushings. Also, I’m not an ME, so please don’t laugh if I say something stupid.
The springs carry the weight of the car, so the up/down motions (z axis) of the wheel probably doesn’t really translate a whole lot to the bushings. That leaves the x-axis (side-to-side) and y-axis (accel/braking).
Looking at the acceleration (y-axis), that’s going induce a forward force on the control arm, which translates to the subframe, to the bushing, to the bolt. So… effectively… the wheels are pushing the car by pushing the 2 subframe bolts…?
Same deal with braking, but because the fronts are probably braking harder than the rear, probably less actual force applied to the bushing here.
Lateral (x-axis) forces would twist the control arms with one arm being the pivot point and either pushing down or up on the edge of the subframe (depends on the direction the wheel is going).
Don’t think I’ve really revealed anything, though. The only thing I can really think of is that the bushing is really only there to transmit lateral subframe forces to the bolt and back. There’s not really much vertical force to contend with. Maybe that changes with rubber – maybe the slight deflection induces more vertical force on the nut that could cause it to come loose?
One other thing to consider is that with 2 rubber bushings, you could have the whole subframe twist around one of the bolts – which could contribute to the bolt twisting and being undone. Maybe this concern doesn’t existing when you have solid bushings as there’s no twist?
I guess this would go back to why bolts are tightened to begin with. Can’t say I know all the physical/mechanical nuances to answer that question.
Som