Setting caster angle

I did a search, but it didn't come up with anything... This is probably a stupid question, but how do I measure my caster angle? I know how to set it, just don't know where to measure it. Almost time to put my axles back in

 

I'm imagining you could do it with a plumb bob and protractor... never actually measured caster though.

 

To me the best way to do it would be to put the axle in and take it to an alignment shop and let them check it, then determine which shims you need, and simply slide them in under the springs. When I had mine done the 1st time years ago they put it on the rack and set the toe in, checked the rest and charged me $20. At least to me it was worth the lack of headache. If you haven't changed the shackles for longer than stock it probably won't change, my 2 1/2" lift kit didn't hardly effect mine at all, something like 1/2* IIRC.

 

Caster - tire doesn't move, pivot angle (wrt vertical) of the spindle rotates.
Camber - pivot angle doesn't move, top of tire tips in or out.
Does that help?

 

Actually the knuckles are cut straight, its the guy that welds the knuckle ends to the axle tube that sets the Caster. You can set caster one of 2 ways, with shims to rotate the caster backward to effect the relationship to the ground or you can cut the welds on the knuckles and rotate them backward and reweld them, either way you are changing the vertical pivot angle in relationship to the ground.