Why would I want to add preload to a ball joint?

 

You mean like in the preloaded moog ball joints? The preload keeps the joints tight even when they wear, they will last a lot longer that way.

 

like this one?
1210-106 - QA1 Adjustable Ball Joints

 

It's like a Mack kingpin. Mack uses tapered kingpins with an adjuster at the bottom. As the kingpin wears, it becomes sloppy. By tightening the adjuster, you can keep taking out the slop until there's no more adjustment left over. The kingpin then needs to be replaced.

The adjustable balljoint is the same way. You can put a new balljoint in you car and within a year, there will probably be a little bit of play in it. You can use the adjuster to add preload to remove that play. Eventually, there won't be any adjustment left and those QA1 balljoints are rebuildable.

For the average person, it's easier to just replace the balljoint when it becomes excessively worn. About the only people I could see who would benefit from this QA1 balljoint would be stock car racers. They could adjust the preload after every race and if they get damaged, they can be rebuilt.

Even QA1 says these are not for street use.