The Stovebolt Site Forums The Shop Area The HiPo Shop Mustang II Lower Ball Joint Failure

That is a really good question. Just thinking about the geometry and load directions, one would think they would be engineered differently.
The upper joint carries almost no load vertically, doing most of its work transferring lateral loads to the frame through the control arm.
The lower joint not only transfers lateral loads, but also holds up the vehicle under tension. That nut takes all the vertical loads and transfers them to the spring through the ball joint and the lower control arm.
It is possible that by the time the ball joint is built to handle all the cornering loads, it is strong enough to handle the weight loads as well with a good margin, but I would want an engineer to calculate that before I trusted an upper joint to a lower position.
Just me thinking out loud...